Diversity and distribution of gastropods in the seaweed-dominated rocky intertidal area of Thikkodi, southwest coast of India

Gastropods are a diversified class of the phylum Mollusca, and one of the most represented taxa associated with seaweeds. Studies on the gastropod diversity along the rocky intertidal area are extensive, however, the distribution of gastropods in the seaweed-dominated intertidal area specifically is limited. This study was conducted to know the influence of seaweed abundance on the spatial and seasonal diversity of gastropod species and the zonal preference of individuals of gastropods in littoral, eulittoral and sublittoral areas. The littoral zone showed an abundance of Littoraria undulata and Cellana radiata and the seaweed species viz., Ulva intestinalis, Ulva compressa and Cladophora vagabunda. The eulittoral zone showed higher abundance (263 Ind./m²) and diversity (H’=2.54) of gastropods, mostly comprised of species of Clypeomorus batillariaeformis, Planaxis sulcatus, Turbo intercostalis, Trochus radiatus, Nerita albicilla and seaweed species like Valoniopsis pachynema, Lychaete herpestica, Acrosiphonia orientalis, Chondracanthus acicularis and Gelidium pusillum. The sublittoral zone recorded more species of the family Muricidae and an abundance of Anachis terpsichore from the family Columellidae. The highest biomass of seaweed in the post-monsoon season supports a higher numerical abundance of gastropod individuals during this study. The pre-monsoon season was characterized by higher species number and diversity (H’ = 2.79; d =5.11) followed by the monsoon season (H’ = 2.77; d =4.73). The abundance of gastropod species is more correlated to a combination of seaweeds, indicating that, structurally complex types, with high biomass of seaweeds are more preferred for assemblages of gastropods than species specificity. Overall, this study revealed that spatial heterogeneity in terms of the abundance of seaweed is one of the factors for the difference in gastropod species distribution and diversity in this study area.