The gastropod Fusitriton magellanicus: a very suitable substrate for the settlement of sponges in soft bottoms of the Argentine Sea.

LAURA SCHEJTER; BERTOLINO MARCO; CALCINAI BARBARA; CERRANO CARLO; BREMEC CLAUDIA

Conferencia; VIII World Sponge Conference; 2010

Fusitriton magellanicus is frequently collected in the by-catch at Zygochlamys patagonica commercial fishing grounds in the Argentine Sea, located in the shelf-break frontal area, one of the most productive ecosystems in the SW Atlantic Ocean. Preliminary results showed that F. magellanicus would be the second most important living substrate colonized by encrusting organisms in this community, fact that highlighted its importance in the maintenance of the species richness in the exploited bottoms influenced by the shelf-break front in Argentine waters. However, sponges using this gastropod shell as settlement substrate deserve more detailed studies. In this sense, we studied living F. magellanicus, empty shells and pagurized shells that were collected between 37º00,27’ and 45º01,70’ S and 54º40,46’ and 60º25,62’ W, along the 100m isobath and between 81-150m, during 3 research cruises in 2007 and 2008. In total, 556 shells were studied (443 living Fusitriton magellanicus, 86 empty shells and 27 pagurized shells). More than 70% of all the studied shells presented encrusting organisms. More than 20% of the living studied shells was encrusted by sponges (only 8.14 and 7.40% were encrusted with sponges, for empty and pagurized shells, respectively). In total, 25 species were identified by means of the classic methodology described by Rützler (1972). The most frequent sponge species encrusting living individuals of Fusitriton magellanicus were Hymedesmia (Stylopus) antactica (20%), Tedania spp. (9%) and Dictyonella sp. (7%). These species were often found heavily fouling the shells. Moreover, Tedania mucosa can reach twice or three times the volume of the living gastropod, resembling a “motile sponge”. Other sponges less frequently registered on living gastropods and empty shells (£ 5%) were Clathria (Microciona) antarctica, Callyspongia ramosa, Stelodoryx argentinae, Stelodoryx cribrigera, Amphilectus fucorum, Iophon proximum, Clathria (Microciona) spp., Lissodendoryx (Ectodoryx) spp., Ulosa sp., Hymeniacidon sp., Haliclona spp., Halichondria sp.,  Chalinula sp., Eurypon sp., Oceanapia sp. and Phorbas sp.. We also found other species belonging to Suberitidae. More than 70% of the empty shells (but only 12% of the living organisms) presented holes, possibly bored by Clionaidae sponges. Only 6 (6%) of the living gastropods were encrusted simultaneously by 2 sponge species while monospecific sponge colonies were registered in the majority of the individuals. Even so, sponges could share the shells with other epibionts, sometimes competing for the free space; the sponges overgrowth and cover other epibionts that could eventually die (i.e. bryozoan colonies).     Considering that the study area is a soft bottom and the only available substrates for settlement of sessile species are hard parts of other living organism (or empty shells or carapaces), we confirm that shells of the gastropod Fusitriton magellanicus play a very important role for settlement of sessile species, and that they represent the substrate hosting the highest species richness of sponges in the area.  This study was partially supported by INIDEP, CONICET, PICT 2007 2200 and PICT 2008 1119.