Bryozoan assemblages on gastropod shells occupied by the hermit crab Pagurus comptus

Abstract

Bryozoans are common on gastropod shells inhabited by the hermit crab Pagurus comptus in the southern southwest Atlantic. The aim of this study was to test whether bryozoan biodiversity was higher on larger and more complexly sculptured gastropod shells, as well as to analyze spatial and bathymetric changes in bryozoan assemblages. Forty-four bryozoan species were recorded on 59 gastropod (morpho) species. Larger shells tended to host a higher number of bryozoan species than smaller ones. Bryozoan species richness differed significantly among gastropods with different shell ornamentation, but richness did not increase along a predicted qualitative scale of increasing ornamentation complexity. Almost smooth but large shells had a much higher bryozoan richness than expected, and small shells with prominent ornamentation had unexpectedly the lowest richness, suggesting that shell size may be more important than ornamentation in determining bryozoan richness. Most of the shells hosted two bryozoan species, although up to 11 species per shell were observed. Maximum bryozoan richness occurred between 99 and 137 m. Bryozoan species richness on gastropod shells occupied by P. comptus was higher in the Burdwood Bank than around Tierra del Fuego and Isla de los Estados. Epibiont assemblage composition and species richness differed among the six most abundant gastropods (Trophon ohlini, Pareuthria atrata, Cerithiopsis caelatum, Fuegotrophon pallidus, P. fuscata, Argeneuthria cerealis). The two most abundant bryozoans, Burdwoodipora paguricola and Odontoporella adpressa, do not overlap in their occupation of shell surfaces. This is the first study analyzing the relationships among bryozoans, gastropod shells, and hermit crabs in subantarctic waters.