Abundance of the sand dollar, Mellita quinquiesperforata (Mellitidae) in La Boquilla, Cartagena de Indias, Colombian Caribbean Sea

The sand dollar Mellita quinquiesperforata (Leske, 1778) is a lunulate echinoderm inhabiting sandy substrates in the Caribbean Sea's shallow coastal waters. This species functions as an environmental engineer, altering substrate characteristics, yet few studies have examined its abundance, size distribution, or relationship with substrate features. This study examines the impact of climate seasons, months, and substrate features on the relative abundance and size of M. quinquiesperforata, based on monthly observations conducted at La Boquilla beach (northern Cartagena de Indias, Colombia) from January to September 2023. According to our observations, aggregations of M. quinquiesperforata were observed, with densities ranging from 5 to 87 ind per m2. Relative abundance varied significantly with climatic seasons (Kruskal-Wallis test chi-squared = 42.041, P-value = 7.427e-10), with the lowest densities recorded in July (transition season, 5-20 ind m-2) and the highest in April (dry season, 10-45 ind m-2). No individuals were observed during January and February. Aggregations were most prevalent 31-40 m from the low-tide line, where the highest densities were recorded. Size measurements ranged from 0.7 to 9 cm, with an average of 5.76 ± 0.67 cm (n = 2.526), while weights averaged 17.3 ± 5.6 g. Significant differences in size were observed between climatic seasons (chi-squared = 9.4294, P-value = 0.008962), months (chi-squared = 58.891, df = 5, P-value = 2.06e-11), and spatial distribution (distance from the low-tide line) (chi-squared = 13.899, P-value = 0.0009591). No significant differences were detected in weight. Most individuals collected were adults, suggesting limited recruitment during the study period. Abundance estimates for La Boquilla beach were lower than reported for other South American localities, highlighting potential site-specific differences in environmental or ecological factors affecting M. quinquiesperforata populations. These findings contribute to a better understanding of the population dynamics and habitat preferences of this key benthic species in the Caribbean region.