Biodiversity and distribution of marine Gastrotricha along the Pacific cost of Costa Rica

Gastrotricha is a minor but stimulating phylum of small-sized invertebrates that, in marine settings, appear particularly speciose in clean sandy sediments. However, not much is known about their biodiversity and ecology on the beaches of the Neotropical region. Therefore, this study aimed to determine the biodiversity of these small metazoans, their distribution on the Pacific Coast of Costa Rica, and their relationships with local environmental variables. We collected sediment samples from 27 sandy beaches and recorded various factors like granulometric characteristics, organic matter, carbonate content, water salinity, and GPS geographical data. We found a total of 50 species of Gastrotricha, most of which are likely new to science. Although the overall diversity was high, individual beaches had low numbers of species. The main environmental factors that produced the change in species identity between samples were sediment grain size and calcium carbonate content. The study found that species of the Order Chaetonotida were mainly present in the littoral zone, while those of the Order Macrodasyida were found in both the littoral and sublittoral zones. Additionally, Chaetonotida species showed a preference for finer sediments. We also observed a slight change in species composition from North to South along the coastline, which may be attributed to other environmental and regional differences that need further evaluation, like the upwelling effect, estuarine dynamics, and exposure to oceanic influence.