Malacofauna associada a rodolitos da plataforma Continental na Costa Oeste do Estado do Ceará, Brasil

Abstract

Rhodoliths are free-living nodules formed by non-geniculate coralline algae (Corallinophycidae) characterized by a calcified thallus, not adhered to the substrate. Among the invertebrate taxa which are associated to them, the Mollusca phylum stands out because of the large supply of shelters provided by the algae. The goal of the present study is to describe the malacofauna associated to the rhodoliths of the west coast’s continental shelf of the Ceará State, Northeast Brazil. The volume, porosity, mean diameter, sphericity, number of branches and sediment cover percentage over the rhodoliths were quantified in order to determine the relationship between these variables and the richness and abundance of the malacofauna. In 76 rhodoliths analyzed, a total of 895 individuals were found, distributed in 3 classes, 33 families and 45 species. The species of coralline algae found in greatest abundance was Mesophyllum cf. macedonis Athanasiadis, 1999. The most abundant gastropod species was Caecum achirona (de Folin, 1867), the most abundant bivalve species was Chama sinuosa Broderip, 1835, and the only species of polyplacophora found was Ischnochiton striolatus (Gray, 1828). The gastropod Nanaphora verbernei (Moolenbeek & Faber, 1989) and the bivalve Kellia sp. correspond to new records for the Ceará State and the Northeast region of Brazil, respectively. The molluscs found were categorized as herbivores, carnivores, parasites or filter feeders. The rhodoliths with bigger diameters were also the most porous and the least spherical ones. The rhodoliths with the greatest number of branches were also the ones with the lowest values of sediment cover, and vice versa. The Redundancy Analysis (RDA) showed that the majority of the molluscan species found were associated to the more porous rhodoliths and with higher average diameter values (F: 1.235, p <0.05). The polyplacophora I. striolatus, the gastropod Caecum floridanum Stimpson, 1851 and the bivalve species 1 were more associated to rhodoliths with higher numbers of branches. C. sinuosa were more associated with spherical rhodoliths. The species Alaba incerta (d'Orbigny, 1841), Caecum ryssotitum de Folin, 1867, Cumingia lamellosa G. B. Sowerby, 1833, Pinctada imbricata Röding, 1798 and Semele proficua (Pulteney, 1799) were more associated with rhodoliths with higher coverage values of sediment. The Generalized Linear Model (GLM) showed that porosity was the variable that had a direct influence on richness and abundance of the associated malacofauna (p <0.001). This study provides new knowledge about the ecology of rhodoliths and their associated fauna.