Choosing settlement substrate: sponges in soft-bottom assemblages at the shelf-break frontal area, Argentine Sea

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

Girona

Conferencia; VIII World Sponge Conference; 2010

The shelf-break frontal area of the Argentine Sea supports a high biological production as a result of high input of nutrients, consequently influencing higher trophic levels. Patagonian scallop fishing grounds (between 37º and 45ºS, at approximately 100m depth) are located in the mentioned area and are characterized by the presence of soft bottoms, mainly composed by sand, as the majority of the Argentinean shelf. Given the absence of hard substrates like rocks, sessile species of this ecosystem colonize secondary hard bottoms composed by parts of living and non-living organisms. Sponges at Patagonian scallop commercial fishing grounds represent 5-10% of the total epibenthic community biomass, although at specific areas with low scallop biomass (i.e. ~40ºS) they can reach higher values. Annual monitoring of the benthic assemblage associated to the scallop fishery is developed as part of the management strategy. The results here presented are a compilation of data collected in the past decade during the monitoring cruises carried out by INIDEP from selected and preserved sponge samples in shelf areas subjected to scallop trawling. More than 50% of the benthic species richness of the Patagonian scallop fishing grounds corresponds to epibiotic sessile taxa, and from those species, 40% (47 species until present) is represented by sponges. Settlement substrate (at least one possible) was successfully recorded for more than 80% of the identified species. In all the registered cases mollusks or their empty shells were found to be the substrate for the following species: Iophon proximum, Clathria microxa, Clathria discreta, Clathria (Microciona) antarctica, Clathria spp.; Dasychalina validissima, Isodyctia setifera, Hymedesmia (Stylopus) antarctica, Calyx kerguelensis, Tedania mucosa, Tedania massa, Tedania spinifera, Tedanis sp., Mycale doellojuradoi, Mycale magellanica, Amphilectus fucorum, Dictyonella sp., Stelodoryx argentinae, Stelodoryx cribrigera, Callyspongia ramosa, Pachychalina sp., Phorbas sp., Ulosa sp., Halichondria sp., Hymeniacidon sp., Lyssodendoryx (Ectodoryx) spp., Haliclona spp., Sycon sp., Oceanapia sp., Eurypon sp., Chalinula sp., Suberitidae and Clionaidae (holes). Among these species, 14 were registered on Zygochlamys patagonica (mainly empty shells) and 33 on Fusitriton magellanicus (see Schejter et al., poster presentation). Some of these species (Halichondria sp., Phorbas sp., Haliclona cf. bilamellata, Haliclona (Reniera) sp. and Haliclona (Haliclona) sp.) were registered in this benthic community using other substrates like crab carapaces, dead coral, Rajoidea egg capsules and polychaete tubes. Due to sampling procedure (dredges and trawls), sponges (and other benthic organisms) were frequently damaged or fragmented and the recognition of the original settlement substrata was not always possible. The commercial species Zygochlamys patagonica and the hairy snail Fusitriton magellanicus allow the development of highly diversified sponge gardens, a peculiar habitat that can host a wide variety of endobionts, mainly microcrustaceans and polychaetes (see Schejter et al. (2), poster presentation). The risk of overexploitation of scallop beds could lead to a general loss of biodiversity, or at least to a transformation, because living molluscs are able to host the majority of the species, but their dead valves were not preferred by the same species. This study was partially supported by INIDEP, CONICET, PICT 2007 2200 and PICT 2008 1119.