Artificial coastal defence structure as a survival tool for the shallow water sponges

Artificial coastal defence structures were used to stabilize eroding shorelines for centuries, although limited knowledge documented about the associated flora and fauna, especially regarding inhabited sponges. The current underwater survey along the 1076 km coastal stretch of Tamil Nadu from Chennai to Thengapatinam (Classified as Zones), was proposed to study the diversity of the sponge species associated with the artificial coastal defence structures. During this study, a total of 18 species of sponges recorded, which belongs to the 13 genera, 10 families, and 8 orders. Among the Zones, maximum species diversity was documented in Zone 4 (15 species), followed by Zone 5 (9 species) and Zone 6 (5 species), whereas the Zone 1 recorded nil species. In terms of artificial structure types, the fishing harbour structure recorded a maximum of 18 species, followed by recreational (4 species), protection (2 species), and commercial (1 species). Sponges being a vital component of coral reefs have both ecological and commercial significance likewise assembles on artificial coastal structures. The sponge species Callyspongia pambanensis and Biemna fortis dominated over other species by occupying most of the structures and zones. The three primary criteria; temperature, geographical proximity between zones, and sources of larval supply determining the sponge population dynamics. However, proximity to a source of larval supply creates high diversity on artificial structures along the Tamil Nadu coast. Further, diversity reduction correspondingly noted during summer than in winter and rainy seasons, which annulets thermal impact. This ability to provide the refugee for the sponges will craft the artificial coastal defence structures as a novel habitat for the sponge population.