Diet of two sea urchins and three sea stars from shallow (intertidal to 6 m depth), north Patagonia (42° 43´S)

PABLO E. PENCHASZADEH, MARTIN BROGGER, VICTORIA ZAVATTIERI Y GREGORIO BIGATTI.

Kobe, Japan

Simposio; 27- An international Nearshore Biodiversity symposium. NaGISA World Conference; 2006

Census of Marine Life

Many authors have reported sea urchin and star fish regulation on the community structure principally by predation on the macrobenthos. The most common sea urchins species from shallow Argentina are Arbacia dufresnii and Pseudechinus magellanicus. At the same substrates both species were eating similar prey types and both changed their diet according to variations on the biodiversity of the associated community. One morphological difference between this two species is that A. dufresnii lack tube feet adapted for capture of drift algae and algal manipulation; consumption may be restricted to grazing on plants and sessile animals. Both species show a high degree of carnivory. Thus, prey are variable according to habitat and feeding basically depends on availability, potential for capture and ingestion and palatability. The diet of three Asteriinae, the brooding Anasterias minuta, the fissiparous  Allostichaster inaequalis, and the broadcast-spawner Cosmasterias lurida, was studied. Sea stars were sampled in intertidal and subtidal environments to determine proportion of foragers and diet composition. Foraging occurs by stomach eversion in the three studied species. A. inaequalis (radius range 0.5-6.7; n = 208) was found manly in subtidal habitats, A. minuta (radius range  0.2-7.2; n = 1,125) in intertidal and subtidal areas, and C. lurida (radius range 3.6-19.4; n = 424) was exclusively subtidal. Foraging individuals made 20-40% of the sample. A. inaequalis fed mostly pelecypods (89% of the prey items). Individuals of all sizes preyed on the mytilid Aulacomya atra atra; the burrowing bivalve Lithophaga patagonica was preyed by the largest sea stars. Other prey were gastropods and clams. A. minuta in subtidal stations preyed mainly on the snail Tegula patagonica (75%), on small bivalves (A. atra atra) and, rarely, on chitons. Intertidal individuals had a broader diet. The largest of the studied species, C. lurida, fed mostly A. atra atra (51%), with a positive and significant correlation between predator and prey sizes (R = 0.65). The smaller sizes preyed upon T. patagonica (36%) while medium size starfish forage on L. patagonica.