Diversity and spatio-temporal dynamics of Caulerpa J.V.Lamouroux, with two new locality records from Northern Sri Lanka

MARINE REPORTS 4(2) (2025) : 80-102 DOI: 10.5281/zenodo.18001560 [email protected] MARINE REPORTS e-ISSN: 2822-5155 Journal homepage: https://scopesscience.com/index.php/marep/ Received: 18 November 2025; Received in revised form: 17 December 2025 Accepted: 17 December 2025; Available online: 21 December 2025 RESEARCH PAPER Citation: Anjalie, N.M.I., Nahmagal, K., & Sivashanthini, K. (2025). Diversity and spatio-temporal dynamics of Caulerpa J.V.Lamouroux, with two new locality records from Northern Sri Lanka. Marine Reports, 4(2), 80-102. https://doi.org/10.5281/zenodo.18001560 DIVERSITY AND SPATIO-TEMPORAL DYNAMICS OF Caulerpa J.V.Lamouroux, WITH TWO NEW LOCALITY RECORDS FROM NORTHERN SRI LANKA Narasinghe Mudiyanselage Ishara ANJALIE1, Nahmagal KRISHNAPILLAI2, Sivashanthini KUGANATHAN1* 1Department of Fisheries, Faculty of Science, University of Jaffna, Jaffna 40000, Sri Lanka 2Department of Botany, Faculty of Science, University of Jaffna, Jaffna 40000, Sri Lanka Anjalie, N.M.I.: [email protected], https://orcid.org/0009-0008-3138-2159 Nahmagal, K.: [email protected], https://orcid.org/0000-0001-6609-5055 Sivashanthini, K.: [email protected], https://orcid.org/0000-0002-3005-2325 *Corresponding author: Sivashanthini KUGANATHAN, [email protected], +94 77 400 6669 Abstract The genus Caulerpa is a key component as a primary producer of tropical and subtropical marine ecosystems, yet its diversity and distribution along the northern coast of Sri Lanka remain poorly documented. This study provides a comprehensive assessment of the diversity and spatio-temporal dynamics of Caulerpa species at four coastal sites, Mandaitivu, Mathagal, Kankesanthurai, and Point Pedro from July 2024 to June 2025. Monthly sampling using the quadrat-transect method revealed six species: Caulerpa lentillifera, Caulerpa racemosa, Caulerpa sertularioides, Caulerpa taxifolia, Caulerpa serrulata, and Caulerpa peltata. Among these, C. lentillifera and C. serrulata are reported as new locality records for the Jaffna region, while the distributions of C. taxifolia and C. peltata are significantly expanded. C. sertularioides was the most widespread and dominant species, occurring at all sites, whereas C. lentillifera and C. taxifolia were rare, each found at only one location. Distinct seasonal patterns were observed, with a complete absence of all species during the northeast monsoon (November–December). Mandaitivu exhibited the highest diversity overall, with peak Simpson diversity (0.375) and evenness (0.946) in March and maximum Shannon diversity (1.121) and species richness (4) in June. Canonical Correspondence Analysis indicated that the growth and distribution of Caulerpa species along the northern coast of Sri Lanka are mainly influenced by salinity, temperature, nutrient availability, dissolved oxygen, turbidity, and seasonal monsoonal variability. This study establishes the first detailed baseline of Caulerpa diversity in northern Sri Lanka, highlighting its ecological significance and providing a foundation for future conservation and bioprospecting efforts. Keywords: Caulerpa diversity, diversity indices, new records, Northern Sri Lanka, seaweed, spatio-temporal dynamics Anjalie et al. Spatio-temporal dynamics of Caulerpa from Sri Lanka Marine Reports 4(2) (2025) 80-102 81 Introduction The genus Caulerpa J.V.Lamouroux, a prominent group of green macroalgae (seaweed), is a critical component of tropical and subtropical marine ecosystems. Recognized for its rapid growth, invasive potential (e.g., Caulerpa taxifolia in the Mediterranean), and morphological plasticity (feathery, grape-like, etc). Caulerpa contributes significantly to primary production, provides habitat and food for marine organisms, and serves as a bioindicator for environmental health (Guiry & Guiry, 2020). Environmental factors, including seawater temperature, salinity fluctuations, light availability, and hydrodynamic conditions, strongly influence the distribution, productivity, and seasonal variability of Caulerpa species. Several studies have emphasized how climate-driven changes, such as warming seas and shifts in monsoon patterns, will likely further impact their abundance and spatial distribution (Meinesz, 2007). C. lentillifera is known for its rapid vegetative expansion in shallow tropical lagoons, which is greatly determined by external factors such as water temperature, seasonality, weather, and geographical location (Syakilla et al., 2022). Caulerpa racemosa occurs pantropically, and its growth and dispersal are greatly affected by the temperature of seawater, which plays an important role in controlling its seasonal growth and physiological functions (Verlaque et al., 2000). Salinity, water temperature, seasonal cycles, and nutrient availability due to upwelling events are major factors affecting growth and distribution patterns of Caulerpa sertularioides (Fernández-García et al., 2012). Caulerpa taxifolia generally occurs over soft sediment grounds (McKinnon et al., 2009), its growth and distribution strongly dependent upon the temperature of the water (Phillips et al., 2002). The shallow, warm, and sunlit lagoons, sandy-muddy bottoms, and seagrass beds of the Jaffna Peninsula and the Palk Strait are ideal habitats for many Caulerpa species. Caulerpa species are well-documented in the Gulf of Mannar (to the northwest) and along the southern coast of India, just miles away across the Palk Strait. It is highly unlikely that these algae, which can spread via currents or human activity, would completely skip over the northern coast of Sri Lanka, is almost certainly a reflection of the lack of research and not proof of their actual absence. They are very likely present but have not been formally documented, identified, and published in scientific literature due to the historical constraints prevailed in the country. There has been a slow but steady resurgence of marine research in these previously inaccessible areas. In Sri Lanka, historical biodiversity assessments, such as the one by Silva et al. (1996) documented a rich diversity of 19 Caulerpa species. However, subsequent studies have suggested a potential decline, with a comprehensive national survey by Coppejans et al. (2009) reporting only 12 species, and a more recent, localized study in the southern coastal waters by Premarathna et al. (2020a) finding just three: C. racemosa, C. imbricata, and C. sertularioides. Nevertheless, details on precise distribution, exactly where and how abundantly it is distributed is lacking. This apparent contraction in species reports underscores a critical gap in our current understanding: the lack of comprehensive, region-specific diversity studies. While the south has been surveyed, the northern coastal waters of Sri Lanka, particularly around the Jaffna Peninsula, remain relatively under-explored in this context. It is evident from a study by Premarathna et al. (2020b) that he collected C. racemosa and C. sertularioides from Point Pedro and Kankesanthurai, Jaffna, respectively to examine the cytotoxic effect and wound healing ability of different seaweed species available in Sri Lanka. This research was primarily a pharmacological and biochemical investigation, not a broad ecological survey. According to an incidental study by Abirami et al. (2024), the marine algae C. peltata, C. racemosa, and C. taxifolia were reported only in Kankesanthurai waters. It is important to note that this was not a comprehensive study, and the finding was made incidentally highlighting the need for more systematic surveys in the region. The recent collection of C. racemosa in Jaffna for a study on antioxidant properties (Silva et al., 2025) confirms the genus's presence but does not address the full scope of its diversity in the region. A focused study on the diversity and distribution of the green macroalgal Anjalie et al. Spatio-temporal dynamics of Caulerpa from Sri Lanka Marine Reports 4(2) (2025) 80-102 82 genus Caulerpa in Northern Sri Lanka is critically important for ecological, economic, and conservation reasons. The overall objective of the study is to document the previously unreported new Caulerpa species to the locality and to comprehensively assess the current diversity, distribution, and abundance of the macroalgal genus Caulerpa along the northern coastal waters of Sri Lanka, in order to establish a critical baseline for conservation, sustainable management, and future bioprospecting efforts. Materials and Methods Study area The survey was conducted at four locations along the northern coast of Sri Lanka, namely Mandaitivu, Mathagal, Kankesanthurai (KKS), and Point Pedro. At each location, three replicate sites were selected for sampling, and their precise geographic coordinates were recorded using a handheld Geographical Positioning System (GPS) (Garmin Oregon 750, USA). The sites at Mandaitivu were located at N 09.603067°, E 079.999170°; N 09.603035°, E 079.998891°; and N 09.603331°, E 079.999386°. At Mathagal, the sampling sites were positioned at N 09.603336°, E 079.999372°; N 09.799914°, E 079.959926°; and N 09.800058°, E 079.960654°. The sites at KKS were located at N 09.799610°, E 079.960669°; N 09.816769°, E 080.045700°; and N 09.816479°, E 080.046209°, while those at Point Pedro were situated at N 09.825647°, E 080.251704°; N 09.825897°, E 080.251441°; and N 09.825300°, E 080.251944°. Figure 1 shows the selected areas along the Northern coast of Sri Lanka. Figure 1. Map showing the study locations in Northern Sri Lanka Sampling design and sample collection Monthly field surveys were carried out from July 2024 to June 2025 to collect and identify seaweed samples. At each study location, three permanent sampling sites were established, spaced roughly 50 meters apart. From each site, a 25-meter transect was laid perpendicular to the shoreline, and five quadrats measuring 0.5 m² were placed at 5-meter intervals along the transect line. Table 1 shows the sampling frequency. Seaweed specimens present within each quadrat were gently hand-collected to avoid physical damage, following the standard quadrat– transect sampling technique, and placed in seawater-filled containers for transport to the laboratory at the University of Jaffna for identification (Abirami et al., 2024). Since the objective of the study was to record the diversity of seaweeds, all individuals occurring within the quadrats were collected rather than a predetermined number of specimens. During the study, a total of six distinct species belonging to the genus Caulerpa were identified from the selected coastal areas of Jaffna. Anjalie et al. Spatio-temporal dynamics of Caulerpa from Sri Lanka Marine Reports 4(2) (2025) 80-102 83 Table 1. Sampling design to show the number of sampling sites in each location and sampling frequency. Location No. of Sites Quadrats per Site Sampling Frequency Total Quadrats per Site Mandaitivu 3 5 Monthly × 12 months 60 Mathagal 3 5 Monthly × 12 months 60 Kankesanthurai 3 5 Monthly × 12 months 60 Point Pedro 3 5 Monthly × 12 months 60 Morphological analysis In the laboratory, specimens were carefully rinsed with fresh water to remove attached sediments, epiphytes, and debris. Morphological characteristics were examined through visual observation, focusing on the overall thallus structure, branching pattern, stolon and rhizoid development, and the shape, size, and arrangement of ramuli (assimilatory branches). Additional attention was given to variations in color and texture. Taxonomic identification Species identification was carried out using standard taxonomic keys and descriptions provided in Coppejans et al. (2009): Sri Lankan Seaweeds – Methodologies and Field Guide to the Dominant Species, and in The Living Marine Resources of the Western Central Pacific, Volume 1: Seaweeds, Corals, Bivalves and Gastropods (Carpenter & Niem, 1998). Identification details were further verified with updated nomenclature and distributional information available in AlgaeBase (Guiry & Guiry, 2020). Diversity indices calculation Diversity indices, the Shannon–Wiener diversity index (H′), Simpson’s index (D), species richness, and evenness index (J′), were computed using the PAST software package (Paleontological Statistics, version 3.0). The species abundance data obtained from each sampling site and month were entered into the program to derive the corresponding diversity values. Statistical analysis Canonical Correspondence Analysis (CCA) was performed separately for each location to examine the relationship between water quality parameters (wind speed, water temperature, air temperature, salinity, pH, turbidity, phosphate, and dissolved oxygen) in different seasons and species abundance. Results Habitat diversity of Caulerpa The selection of Mathagal, Mandaitivu, Kankesanthurai, and Point Pedro along the northern coast of Sri Lanka was based on ecological and geographical diversity to capture spatial and temporal variations in seaweed distribution. These sites collectively represent a range of habitat types, including sandy, rocky, and muddy substrates, and encompass important environmental gradients such as salinity, temperature, depth, and nutrient levels. The areas differ in exposure Anjalie et al. Spatio-temporal dynamics of Caulerpa from Sri Lanka Marine Reports 4(2) (2025) 80-102 84 and anthropogenic influence, from the relatively undisturbed shores of Mathagal and Mandaitivu to the moderately impacted semi-urban zone of Kankesanthurai. For example, the Mathagal coast is characterized by a low-lying topography, with occasional rocky cliffs formed where the coast crosses crystalline rock formations. This topography is illustrated in Figure 2a. Their distribution across the coastline ensures adequate spatial coverage, while accessibility and logistical feasibility allowed consistent sampling. Overall, the selected locations provided representative conditions for examining seaweed diversity, abundance, and environmental relationships along the northern coastal ecosystems of Sri Lanka. Sample collection and identification A total of six distinct species were i.e. C. lentillifera, C. racemosa, C. sertularioides, C. taxifolia, C. serrulata and C. peltata were confirmed. Of these C. lentillifera and C. serrulata are new locality records for Jaffna, Northern Sri Lanka. Subsequent to their initial identification in Kankesanthurai, C. taxifolia and C. peltata were in fact constitute new locality records for a broader area of the Jaffna Peninsula, with confirmed populations in Mathagal, Mandaitivu, and Point Pedro. The specimens were found colonizing both sandy and rocky substrates within the dynamic shallow waters, inhabiting both intertidal and subtidal zones. Occurrence of Caulerpa racemosa in the infralittoral fringe, partly air-exposed at extreme low water in association with Caulerpa sertularioides and Acetabularia acetabulum is illustrated in Figure 2b. Colonies of Caulerpa lentillifera and Caulerpa racemosa are shown in Figure 2c and d. Taxonomical hierarchy and distinct morphological characteristics The taxonomic classification of the studied genus Caulerpa is: Kingdom : Plantae Phylum : Chlorophyta Class : Ulvophyceae Order : Bryopsidales Family : Caulerpaceae Genus : Caulerpa The following section explains the taxonomic hierarchy and distinct morphological characteristics for identifying Caulerpa species. Caulerpa lentillifera Species : Caulerpa lentillifera J. Agardh The thallus is composed of a horizontally branched stolon that gives rise to numerous erect branches. These erect branches are densely covered with short, club-shaped ramuli along most of their length. Each ramulus has a distinct constriction between its stalk and the rounded tip, which serves as a distinguishing feature of this species (Figure 2b and d). C. lentillifera showed a stolon diameter of 1.14 ± 0.18 mm, a mean length of 102.06 ± 13.95 mm, and a ramuli diameter of 1.00 ± 0.18 mm. This species shows a broad distribution across tropical marine environments in both the Indian and Pacific Oceans (Coppejans et al., 2009). Anjalie et al. Spatio-temporal dynamics of Caulerpa from Sri Lanka Marine Reports 4(2) (2025) 80-102 85 FMgure 2. a) Mathagal coast characterozed by occasoonal rocky cloffs formed where the coast crosses crystallone rock formatoons; b) Caulerpa racemosa on the onfralottoral fronge, partly aorexposed at extreme low water (Mathagal) on assocoatoon woth Caulerpa sertularFoFdes and AcetabularFa acetabulum; c) Caulerpa lentFllFfera colony; d) Caulerpa racemosa colony. Caulerpa racemosa Species : Caulerpa racemosa J. Agardh The thallus is composed of a creeping stolon system that gives rise to erect branches. These branches bear numerous short-stalked ramuli, which range from oval to spherical in shape. The ramuli exhibit variable phyllotaxy, being arranged in radial, alternate, pinnate, or irregular patterns along the erect axes, with densities ranging from sparse to dense. A key morphological feature is the diverse form of the ramuli apices, which can appear clavate (club-shaped), turbinate (top-shaped), globose-truncate (spherical with a flattened end), or occasionally discoid (disc-shaped) (Figure 2c). C. racemosa had a stolon diameter of 1.14 ± 0.27 mm, with an average length of 62.58 ± 18.00 mm and a ramuli diameter of 1.26 ± 0.33 mm. The species has a pantropical distribution (Coppejans et al., 2009). Anjalie et al. Spatio-temporal dynamics of Caulerpa from Sri Lanka Marine Reports 4(2) (2025) 80-102 86 Caulerpa sertularioides Species : Caulerpa sertularioides (S.G Gmelin) M.A Howe The thallus is composed of erect, feather-like branches bearing pinnately arranged, cylindrical ramuli, each terminating in a mucronate tip (Fig. 3a and b). C. sertularioides presented a stolon diameter of 0.20 ± 0.058 mm, a mean length of 15.04 ± 1.90 mm, and a branchlet length of 12.22 ± 1.44 mm. This species is distributed across the tropical Indo-Pacific and the tropical eastern Atlantic regions (Coppejans et al. 2009). Caulerpa taxifolia Species : Caulerpa taxifolia (Vahl) C. Agardh Thallus consists of narrow, erect branches that are generally unbranched or branched and feather-like. The pinnules (ramuli) are arranged pinnately, distinctly flattened, and have a curved, sickle-like shape (falcate) (Figure 3c). C. taxifolia showed a stolon diameter of 1.00 ± 0.22 mm, an average length of 38.48 ± 11.25 mm, and a branchlet length of 2.88 ± 0.38 mm. The species occurs throughout tropical oceans worldwide and has been introduced into the Mediterranean Sea (Coppejans et al., 2009). Caulerpa serrulata Species : Caulerpa serrulata (Forssekal) J. Agardh The thallus is composed of erect, cylindrical, and generally unbranched axes that arise from a creeping stolon. These axes are densely covered with small, serrated ramuli, arranged in either an alternate or opposite pattern. The ramuli possess pointed or slightly mucronate tips, and this overall morphology confers a distinctive bushy, feathery appearance to the thallus (Figure 3d and e). C. serrulata had a stolon diameter of 1.62 ± 0.19 mm, with an average length of 20.52 ± 5.04 mm and a branchlet diameter of 2.20 ± 0.28 mm. This species is distributed in the tropical Indo-Pacific Ocean (Coppejans et al., 2009). Caulerpa peltata Species : Caulerpa peltata var. The thallus is composed of a horizontally creeping, branched stolon that gives rise to erect axes. These axes bear numerous short-stalked ramuli, each of which terminates in a flattened disc (Figure 3f and g). C. peltata exhibited a stolon diameter of 0.84 ± 0.31 mm, a mean length of 70.34 ± 15.25 mm, and a ramuli (disc) diameter of 1.20 ± 0.22 mm. Based on the above distinct morphometric characteristic features, the collected species were further authenticated as C. lentillifera, C. racemosa, C. sertularioides, C. taxifolia, C. serrulata and C. peltata. The earlier study, record of Caulerpa sp. (Coppejans et al., 2009) in Sri Lanka was confined to a solitary notification which did not elaborate on its specific distribution within Jaffna. The present study establishes C. lentillifera and C. serrulata as new records for the Jaffna region while significantly expanding their known distribution for C. taxifolia and C. peltata in the Jaffna peninsula, Sri Lanka. Anjalie et al. Spatio-temporal dynamics of Caulerpa from Sri Lanka Marine Reports 4(2) (2025) 80-102 87 FMgure 3. a) Moxed seaweed vegetatoon under low water level, domonance of Caulerpa sertularFoFdes on assocoatoon woth HalFmeda opuntFa; b) Caulerpa sertularFoFdes woth prostrate rhozomes attached by many rhozoods; c) Caulerpa taxFfolFa; d) Caulerpa serrulata on the onfralottoral fronge, partly aor-exposed at extreme low water (Mathagal) on assocoatoon woth Caulerpa sertularFoFdes; e) Caulerpa serrulata. f) Caulerpa peltata; g) a portoon of Caulerpa peltata. Anjalie et al. Spatio-temporal dynamics of Caulerpa from Sri Lanka Marine Reports 4(2) (2025) 80-102 88 Spatial distribution of identified species Analysis of their spatial distribution revealed distinct patterns across the four sampling locations. C. sertularioides was the most widespread species, being present at all four sites. This was followed by C. racemosa and C. peltata, which were each recorded from three locations. C. serrulata demonstrated a more restricted presence, collected from only two locations. In contrast, C. lentillifera and C. taxifolia appeared to be the least common within the study area, with each found at a single location, suggesting a more niche or limited distribution. A summary of these distributional records is provided in Table 2, offering a clear quantitative overview of species occurrence. Table 2. The presence (+) or absence (-) of different Caulerpa species across four locations in Sri Lanka: Mandativu, Mathagal, KKS, and Point Pedro. Location Caulerpa lentillifera Caulerpa racemosa Caulerpa sertularioides Caulerpa taxifolia Caulerpa serrulata Caulerpa peltata Mandaitivu + - + - + + Mathagal - + + - + + KKS - + + + - + Point Pedro - + + - - - Categorization of Caulerpa species by abundance The relative abundance of the collected Caulerpa species was assessed using two distinct metrics. The first was the raw frequency of occurrence, which represents the number of sampling locations where a particular species was present. The second was the percentage frequency, calculated using the formula: (Number of individuals in each species / Total number of individuals in all Caulerpa species). This standardized metric allows for a direct comparison of how widespread each species is across the study area (Magurran,2021). Table 3 summarizes the abundance of the six Caulerpa species recorded during the study. Table 3. Categorization of Caulerpa Species by Abundance. Caulerpa Species Presences (out of 4) Percentage Frequency Abundance Category Justification Caulerpa lentillifera 1 25% Rare Found at only one location (Mandaitivu). Caulerpa racemosa 3 75% Common Also found at three out of four locations (Mathagal, KKS, Point Pedro). Its distribution is very similar to C. peltata. Caulerpa sertularioides 4 100% Very Common / Dominant Found at every single location. This is the most widespread and Anjalie et al. Spatio-temporal dynamics of Caulerpa from Sri Lanka Marine Reports 4(2) (2025) 80-102 95 Figure 8. CCA ordination of seaweed species and environmental variables at Kankesanthurai Anjalie et al. Spatio-temporal dynamics of Caulerpa from Sri Lanka Marine Reports 4(2) (2025) 80-102 96 Figure 9. CCA ordination of seaweed species and environmental variables at Point Pedro Anjalie et al. Spatio-temporal dynamics of Caulerpa from Sri Lanka Marine Reports 4(2) (2025) 80-102 97 Discussion The present study provides a critical, detailed inventory of the green macroalgal genus Caulerpa along the northern coast of Sri Lanka, revealing a previously underdocumented marine botanical landscape. Across the four surveyed locations—Mandaitivu, Mathagal, Kankesanthurai, and Point Pedro—six distinct species were meticulously identified, painting a picture of moderate but ecologically vibrant diversity. The documented species—C. lentillifera, C. racemosq, C. sertularioides, C. taxifolia, C. serrulata, and the notable C. peltata —represent a fascinating array of forms and adaptations. In the present study, species identification was based on detailed morphological characters, as Caulerpa species possess distinctive and well-documented diagnostic features that allow reliable identification at the species level. Each species presented a unique morphological character within the seascape. C. lentillifera, with its delicate, bead-like filaments, offered a more fragile and intricate structure, often swaying in the subtidal currents. C. racemosa, often found in dense, verdant mats, was characterized by its grape-like clusters of spherical branchlets, creating a textured carpet over the substrate. The feather-like fronds of C. sertularioides closely resembled miniature underwater ferns, while C. serrulata displayed a distinctive, stiff morphology with saw-toothed edges along its upright branches. C. peltata was identified by its unique, shield-shaped appendages that terminate its stalks, and the presence of C. taxifolia, with its flat, feather-shaped fronds resembling the fronds of a yew tree, was of particular interest given its reputation as one of the world's most invasive marine algae (Coppejans et al., 2009). The distribution and dominance of these species were far from uniform, revealing a complex interplay of environmental factors. The research uncovered significant spatial and temporal variations. Spatially, the composition of Caulerpa communities shifted from one location to another. For instance, some coves exhibited a dominance of C. racemosa, blanketing the sandyrocky bottoms, while other sites, perhaps with stronger water movement, were populated by more robust species like C. serrulata or C. taxifolia. These patterns were heavily influenced by local environmental conditions and substrate characteristics; the algae's attachment to rocky outcrops, sandy patches, or within seagrass meadows dictated which species could successfully establish and thrive. Temporally, the populations were dynamic, fluctuating with the seasonal hydrodynamics of the region. The monsoon cycles, with their attendant changes in water temperature, salinity, turbidity, and wave energy, likely governed the growth, reproduction, and seasonal die-back of these algae. A species abundant in the calm, clear waters of one season might recede during periods of heightened turbulence and sedimentation. The present study is more than a simple checklist; it contributes valuable baseline information to the severely limited body of research on Sri Lankan seaweed flora. By establishing this foundational record, it provides a critical reference point for future ecological monitoring. Understanding the current diversity and distribution of native species like C. lentillifera is essential, especially in the context of potential shifts caused by climate change or the spread of non-native species such as C. taxifolia. Ultimately, these findings underscore that the northern coast of Sri Lanka hosts an ecologically significant and dynamic assemblage of Caulerpa species, whose presence and interactions are key to the health and complexity of its coastal ecosystems. Anjalie et al. Spatio-temporal dynamics of Caulerpa from Sri Lanka Marine Reports 4(2) (2025) 80-102 98 Spatial Distribution and Habitat Influence The investigation into the spatial distribution of Caulerpa species along the northern Sri Lankan coast reveals a compelling narrative of ecological specialization and adaptation, where the success of each species is intricately dictated by the physical and dynamic character of its habitat. The study revealed that C. sertularioides was the most widely distributed species, a true generalist, occurring in all sampled locations and seasons, demonstrating remarkable ecological plasticity. Its presence across all sampled locations and seasons is a direct testament to its broad tolerance for environmental variables, as supported by the work of Coppejans (1992). Its creeping habit allows effective colonization of both sandy and rocky substrates, explaining its abundance in both sheltered and moderately exposed habitats. In contrast, C. lentillifera and C. taxifolia exhibited restricted distributions, each being recorded only at specific sites with suitable habitat conditions. The presence of C. racemosa and C. peltata was more localized but showed seasonal fluctuations, suggesting a preference for intermediate environmental stability. Habitat diversity among the four sites strongly influenced species composition. Mathagal and Kankesanthurai supported greater species richness due to the presence of mixed rocky and sandy substrates, which provide firm attachment points and favorable hydrodynamics. These areas also experience moderate wave action, promoting nutrient exchange without excessive thallus damage. In contrast, Mandaitivu, characterized by muddy and sandy substrates, supported fewer Caulerpa species. The soft sediment may hinder proper holdfast attachment and limit light penetration, reducing diversity. Point Pedro, with its exposed rocky habitat dominated by the forces and currents, occasionally supported C. sertularioides and C. racemosa, both of which exhibit phenotypic plasticity suited for turbulent waters. Leliaert and Coppejans (2006) showed that species of Caulerpa commonly occur in shallow tropical and subtropical marine environments, inhabiting a variety of substrates such as sand, rock, and mud. They also reported that many species are associated with seagrass meadows, reef flats, and lagoonal habitats, where light availability and substrate type strongly influence their distribution. Temporal Variations and Seasonal Trends Temporal analysis across twelve months revealed marked fluctuations in Caulerpa abundance. The months of November and December showed minimal or no growth, coinciding with the northeast monsoon period. During this season, heavy rainfall and runoff increase turbidity and nutrient loading while reducing light penetration, salinity, and water clarity, conditions generally unfavorable for photosynthetic macroalgae (Jargal et al., 2023). The near-total absence of Caulerpa during the Northeast Monsoon (November-December) represents a period of extreme environmental stress that pushes these algae beyond their physiological limits. This "bottleneck" effect is caused by a synergistic confluence of adverse factors such as light deprivation and osmotic shock and nutrient overload and physical scour. Conversely, from January to October, seaweed abundance progressively increased, particularly from February onward, when hydrological conditions stabilized. The highest abundance was observed in September and October, indicating post-monsoonal recovery and optimal growth conditions. These findings align with Williams and Grosholz (2002), who observed that most Caulerpa species exhibit optimal growth at temperatures between 26 – 29 °C with high irradiance and moderate water motion. Recorded water temperatures during the present study (27 – 28 °C) fell within this favorable range, supporting enhanced metabolic and photosynthetic activity. Anjalie et al. Spatio-temporal dynamics of Caulerpa from Sri Lanka Marine Reports 4(2) (2025) 80-102 99 Distinct seasonal peaks for different Caulerpa species were evident. C. lentillifera showed maximum abundance in September, while C. racemosa and C. sertularioides peaked in October. The timing of these peaks likely reflects differences in species’ growth and tolerance to environmental conditions. C. racemosa, known for its rapid vegetative propagation and adaptability, often dominates following periods of disturbance, while C. lentillifera prefers more stable and nutrient-rich conditions. Similar seasonal dynamics were reported by Paul et al. (2014) in Australia, where C. racemosa proliferated after monsoon disturbances while C. lentillifera dominated in calmer seasons. Morphological Adaptations and Ecological Significance The six recorded Caulerpa species exhibit diverse morphological traits that enhance their ecological success under varying hydrodynamic regimes. For instance, C. sertularioides possesses erect, feather-like fronds that maximize light capture, while its creeping stolons help stabilize sediments. C. peltata and C. serrulata have flattened, disc-like ramuli that resist strong water movement, allowing persistence in exposed environments. C. racemosa, with its characteristic grape-like ramuli, exhibits efficient surface area for photosynthesis and nutrient uptake. These morphological adaptations reflect ecological specialization and plasticity, enabling different Caulerpa species to occupy distinct ecological niches. Morphological plasticity within the genus has been widely documented in Coppejans et al. (2009), allowing Caulerpa to colonize habitats ranging from shallow to deeper subtidal zones. Morphological measurements of the collected specimens align well with those documented by Coppejans et al. (2009), supporting the identification and confirming the typical species characteristics. Environmental,Drivers,and,Anthropogenic,Factors Environmental parameters such as temperature, salinity, and nutrient levels played pivotal roles in shaping seaweed distribution. The coastal waters of northern Sri Lanka are subject to monsoonal fluctuations, which significantly alter salinity and turbidity. Sites like Kankesanthurai and Mathagal, being more open to oceanic influence, maintained relatively stable salinity levels, favoring Caulerpa growth. In contrast, Mandaitivu, with its lagoonal influence, experienced fluctuating salinity and higher turbidity, reducing overall seaweed abundance. Anthropogenic influences were moderate but noticeable in semi-urban locations such as Kankesanthurai, where occasional wastewater discharge and fishing activities (in Mandaitivu and Mathagal) may contribute to nutrient enrichment, indirectly enhancing algal biomass. However, excessive nutrient input can also lead to epiphytic overgrowth and competition, underscoring the need for balanced nutrient levels for healthy seaweed communities. Site-specific patterns of separation with respect to environmental drivers discerned in the CCA indicate the prominent role of local hydrographic factors and water quality in determining seaweed assemblage patterns in the area. While the role of salinity as a structuring driver for the seaweed assemblage was universal among the surveyed areas, the role of other drivers, like temperature, nutrients, oxygen, and turbidity, varied significantly among the areas. Such variability in the area of study is consistent with previous research on tropical shallow coastal ecosystems, which indicates that small variations in physicochemical factors can influence the species dominance pattern. Conversely, the association of C. racemosa with high temperature and high levels of dissolved oxygen suggests that it prefers warmer, better oxygenated waters, as would be expected for a species with a broad tropical distribution. The distinct seasonal pattern, with the importance of the northeast monsoon in determining levels of nutrients and turbid conditions, further illustrates the importance of monsoonal processes in speciesenvironment interactions at Point Pedro. In total, the CCA analysis reveals that seaweed Anjalie et al. Spatio-temporal dynamics of Caulerpa from Sri Lanka Marine Reports 4(2) (2025) 80-102 100 assemblages in the study area are determined by a combination of spatially discriminable environmental factors, as well as seasonal variability, with differing responses of Caulerpa species to gradients of nutrients, temperature, and turbidity. This reveals that it is important to examine both local factors and monsoonal effects when determining the ecology and distribution of tropical coastal seaweeds. Comparative and Biogeographical Insights When compared with historical records by Silva et al. (1996), who reported 19 Caulerpa species across Sri Lanka, the present study captures a smaller but regionally significant assemblage. The moderate diversity recorded here could reflect localized environmental constraints, limited sampling range, or recent ecological shifts due to climatic and anthropogenic pressures. The continuity underscores the importance of transboundary marine conservation and monitoring efforts in the Indian Ocean region. Conclusion Overall, the study reveals that Caulerpa diversity and abundance along the northern coast of Sri Lanka are governed by complex interactions between habitat type, environmental parameters, and seasonal dynamics. The observed species distribution reflects the adaptive strategies and ecological roles of Caulerpa within these ecosystems. The data generated contribute not only to the national seaweed inventory but also to broader regional understanding of tropical macroalgal ecology in the Indian Ocean. Future research integrating molecular, physiological, and remote-sensing approaches could further reveal population connectivity, species resilience, and responses to climate-induced environmental shifts. Acknowledgements The manuscript originates from a part of the M.Phil. research of the first author. Nor-Lanka Blue Project funded the exchange program under NORPART project (DIKU Grant NORPART 2018/10045 at UiT-The Arctic University of Norway, Tromsø) to the first author and the M.Phil. Research. Special appreciation goes to the laboratory staff of Department of Fisheries for their assistance in sample processing and analysis. Ethical approval Not applicable. Informed consent Not applicable. Availability of data and material (data transparency) All relevant raw data are incorporated and presented in this paper. Conflicts of interest The authors declare that they have no conflict of interest. Funding organizations This project was supported by NORPART project (DIKU Grant NORPART 2018/10045 awarded to UiT-The Arctic University of Norway, Tromsø) in collaboration with University of Jaffna, University of Ruhuna and National Aquatic Resources Development Agency. Anjalie et al. Spatio-temporal dynamics of Caulerpa from Sri Lanka Marine Reports 4(2) (2025) 80-102 101 Authors’ Contribution Anjalie, N.M.I.: Data curation, formal analysis, investigation, methodology, validation, Writing– Original draft. Nahmagal, K.: Investigation, supervision, validation, writing – Review and editing. Sivashanthini, K.: Conceptualization, funding acquisition, investigation, methodology, project, administration, supervision, visualization, Writing – review and editing. References Abirami, R., Thakshini, H., Sivagini, K., Krishnapillai, N., Kuganathan, S., & Harichandra, K. (2024). Assessment of seaweed diversity in Kankesanthurai coastal waters, Sri Lanka. In Proceedings of the NOR-LANKA BLUE Final International Conference National Aquatic Resources Research and Development Agency (NARA) (p. 31). https://blue2023.weebly.com/publications.html Carpenter, K. E., & Niem, V. H. (2001). FAO species identification guide for fishery purposes. The living marine resources of the Western Central Pacific. Volume 6. Bony fishes part 4 (Labridae to Latimeriidae), estuarine crocodiles, sea turtles, sea snakes and marine mammals (pp. v+-3381). Coppejans, E., Leliaert, F., Dargent, O., Gunasekara, R., & De Clerck, O. (2009). Sri Lankan seaweeds: Methodologies and field guide to the dominant species (Vol. 6). Belgian National Focal Point to the Global Taxonomy Initiative. Belgian Development Cooperation, Belgium, pp 265. http://www.abctaxa.be. ISSN 1784-1283, ISSN 1784-1291 (on-line pdf) Coppejans, E. (1992). Marine algae of Papua New Guinea (Madang Prov.) 2. A revised and completed list of Caulerpa (Chlorophyta—Caulerpales). Blumea: Biodiversity, Evolution and Biogeography of Plants, 36(2), 383-410. https://repository.naturalis.nl/pub/525315 Dhargalkar, V. K., & Kavlekar, D. P. (2004). Seaweeds - A field manual. http://drs.nio.org/drs/handle/2264/96 Fernández-García, C., Cortés, J., Alvarado, J. J., & Nivia-Ruiz, J. (2012). Physical factors contributing to the benthic dominance of the alga Caulerpa sertularioides (Caulerpaceae, Chlorophyta) in the upwelling Bahía Culebra, north Pacific of Costa Rica. Revista de Biología Tropical, 60, 93-107. https://doi.org/10.15517/rbt.v60i2.19970 Guiry, M. D., & Guiry, G. M. (2020, November 27). AlgaeBase. World-wide electronic publication. National University of Ireland, Galway. https://www.algaebase.org; searched on 26 October 2025. https://www.algaebase.org Jargal, N., Lee, E. H., & An, K. G. (2023). Monsoon-induced response of algal chlorophyll to trophic state, light availability, and morphometry in 293 temperate reservoirs. Journal of Environmental Management, 337, 117737. https://doi.org/10.1016/j.jenvman.2023.117737 Lamouroux, J. V. F. (1809). Mémoire sur trois nouveaux genres de la famille des Algues marines: Dictyopteris, Amansia, Bryopsis. Journal de Botanique, 2, 129135. https://www.algaebase.org/search/genus/detail/?genus_id=32944 Leliaert, F., & Coppejans, E. (2006). The genus Caulerpa (Caulerpaceae, Chlorophyta) in the Sultanate of Oman. Botanica Marina, 49(2), 147–158. https://doi.org/10.1515/BOT.2006.018 Magurran, A. E. (2021). Measuring biological diversity. Current Biology, 31(19), 1174-1177. https://doi.org/10.1016/j.cub.2021.07.049 McKinnon, J. G., Gribben, P. E., Davis, A. R., Jolley, D. F., & Wright, J. T. (2009). Differences in soft-sediment macrobenthic assemblages invaded by Caulerpa taxifolia compared to Anjalie et al. Spatio-temporal dynamics of Caulerpa from Sri Lanka Marine Reports 4(2) (2025) 80-102 102 uninvaded habitats. Marine ecology progress series, 380, 59-71. https://doi.org/10.3354/meps07926 Meinesz, A. (2007). Methods for identifying and tracking seaweed invasions. Botanica Marina, 50. https://doi.org/10.1515/BOT.2007.042 Paul, N. A., Neveux, N., Magnusson, M., & De Nys, R. (2014). Comparative production and nutritional value of “sea grapes”—the tropical green seaweeds Caulerpa lentillifera and C. racemosa. Journal of Applied Phycology, 26(4), 1833-1844. https://doi.org/10.1007/s10811-013-0227-9 Phillips, J. A., & Price, I. R. (2002). How different is Mediterranean Caulerpa taxifolia (Caulerpales: Chlorophyta) to other populations of the species?. Marine Ecology Progress Series, 238, 61-71. https://doi.org/10.3354/meps238061 Premarathna, A. D., Kumara, A. M. C. P., Jayasooriya, A. P., Jayanetti, D. E., Adhikari, R. B., Sarvananda, L., & Amarakoon, S. (2020). Distribution and diversity of seaweed species in South coastal waters in Sri Lanka. Journal of Oceanography and Marine Research, 7, 196. https://doi.org/10.35248/2572-3103.20.8.196 Premarathna, A. D., Ranahewa, T. H., Wijesekera, S. K., Harishchandra, D. L., Karunathilake, K. J. K., Waduge, R. N., ... & Rajapakse, R. P. V. J. (2020). Preliminary screening of the aqueous extracts of twenty-three different seaweed species in Sri Lanka with in-vitro and in-vivo assays. Heliyon, 6(6). https://doi.org/10.1016/j.heliyon.2020.e03918 Silva, P. C., Basson, P. W., & Moe, R. L. (1996). Catalogue of the benthic marine algae of the Indian Ocean (Vol. 79). Univ of California Press. https://www.algaebase.org/search/bibliography/detail/?biblio_id=j330af2d0e73130c3 Silva, P. C. (2003). Historical overview of the genus Caulerpa. Cryptogamie-Algologie, 24(1), 33-50. https://sciencepress.mnhn.fr/en/periodiques/algologie/24/1/historical-overviewgenus-caulerpa Silva, Y. P. N., Jayapala, H. P. S., Jayasinghe, P. S., & Lim, S. Y. (2025). Comparative Analysis of Antioxidant Properties in Selected Seaweeds of Jaffna Coastal Waters, Sri Lanka. Journal of Life Science, 35(2), 000-000. https://doi.org/10.5352/JLS.2025.35.2.000 Syakilla, N., George, R., Chye, F. Y., Pindi, W., Mantihal, S., Wahab, N. A., ... & Matanjun, P. (2022). A review on nutrients, phytochemicals, and health benefits of green seaweed, Caulerpa lentillifera. Foods, 11(18), 2832. https://doi.org/10.3390/foods11182832 Verlaque, M., Boudouresque, C. F., Meinesz, A., & Gravez, V. (2000). The Caulerpa racemosa complex (Caulerpales, Ulvophyceae) in the Mediterranean Sea. Botanica Marina, 43. https://doi.org/10.1515/BOT.2000.005 Williams, E., & Grosholz, E. (2002). International Caulerpa taxifolia Conference Proceedings. https://escholarship.org/uc/item/16c6578n