Discrimination in ingestion of protistan prey by larval crabs

We determined the incidence of ingestion of 4 autotrophic dinoflagellates a nd 1 heterotrophic dinoflagellate by first stage larvae of 4 species of cra bs. Crab species were 2 winter spawning brachyurans Cancer magister and C. oregonensis, 1 summer spawning brachyuran Hemigrapsus oregonensis, and 1 an omuran Rhinolithodes wosnessenskii. Autotrophic dinoflagellate prey were Pr orocentrum micans, which sustain survival of crab larvae in laboratory cult ure, and 2 species of Alexandrium spp. that do not. P. micans were ingested by virtually all larvae of all 4 crab species, while both toxic and non-to xic strains of Alexandrium were almost never ingested. Results of rearing e xperiments generally confirmed that larvae were receiving no nutritional co ntribution from Alexandrium spp. prey. When brachyuran larvae were presente d with mixtures of P. Micans and Alexandrium spp. in defined ratios, virtua lly all larvae ingested both types of algal prey, Suspending Alexandrium ce lls in P micans exudate did not enhance their ingestion nor did suspending R micans in Alexandrium exudate reduce ingestion. Ingestion of plastic bead s was low (< 12%) except when offered in combination with P. micans cells ( 58%). H. oregonensis larvae ingested the heterotrophic dinoflagellate Nocti luca scintillans that had previously fed on either P. micans or one of the toxic Alexandrium strains, with no apparent preference. Results suggest the presence of a positive ingestion stimulus provided by P. micans and N. sci ntillans, but its absence in Alexandrium spp. Absence of ingestion of Alexa ndrium was not related to the presence of toxins, The ingestion stimulus ap pears to reside on the prey cell surface. Although crab larvae appear able to discriminate among algal prey, non-dis criminate feeding seems likely to occur in mixed prey assemblages in which at least some prey possess the po sitive ingestion cue, perhaps permitting rapid ingestion of available parti cles when dense prey patches are encountered in an otherwise sparse prey en vironment.