Contribution of different seston components to ascidian food in

MARCOS TATIÁN; CAROLA MILOZZI; RICARDO SAHADE; GUILLERMO MERCURI; CRISTIAN LAGGER

POLARFORSCHUNG

Año: 2008 vol. 571 p. 128 - 134

0032-2490

Benthic suspension-feeders typically feed on phytoplankton and conse­quently play an important role in the coupling of pelagic and benthic systems. In Potter Cove, suspension-feeders reach a high biomass and some taxa, par­ticularly ascidians, demonstrate one of the highest species richness values reported in coastal Antarctic areas (Sahade et al., 1998). This is unexpected because of the scarcity of fresh phytoplankton (Schloss and Ferreyra, 2002) and the high input of inorganic material within inner Potter Cove (Schloss et al., 1999). Particles are also frequently resuspended from the fine muddy sedi­ments that characterize the bottom. According to Robbins (1985) ascidians cannot survive in environments affected by heavy inorganic sedimentation since  the particles increase the energy required to extract food items and also clog the filter system (branchial sac and associated mucous layer). Surprisingly, studies performed with sediment traps in the inner cove revealed a relatively high percentage of organic particles in the water column-bottom layer, at 25-50 cm from the bottom (Schloss et al., 1999). This suggests  the presence of food and potential differences in the capability to retain particles from the water col­umn in the area according to body length and distance in relation to the bottom. In order to obtain insights about the origin and composition of food items in the benthic filter-feeding community through the year, a number of ascidians were selected for our investigation. Monthly gut content analysis of the erect species Cnemidocarpa verrucosa (Lesson, 1830) in Potter Cove, revealed organic material decreasing along the gut. This suggested an intake and assimilation of organic particles throughout the year  (Tatián et al., 2002). In contrast, micro­scopic analyses carried out in these gut contents revealed different degrees of stomach repletion (fullness expresed as percentage of the total stomach vol­ume). A wide variety of particles mainly composed of detritus and particles less than 5 µm in size were found (Tatián et al., 2004). The ascidian Corella eumyota Traustedt, 1882 is one of the dominant macrobenthic species within inner Potter Cove, and can reach an abundance of 2.2 individuals m-2 (Kowalke, 1999). Despite its large size (up to 20 cm in length) it shows a depressed shape or “a body more or less compressed laterally” (Van Name, 1945) and it is attached to the bottom by its ventral side. It is commonly covered by mud with the siphons close to the bottom surface. This ascidian was selected for microscopical gut contents analysis to determine its diet throughout the year. Comparison between the erect C. verrucosa and the depressed C. eumyota will help us to understand the structure of the diverse ascidian community that is affected by short-term changes characteristic of  Antarctic-coastal ecosystems (Tatián et al., 2002).