Discrimination in ingestion of protistan prey by larval crabs

We determined the incidence of ingestion of 4 autotrophic dinoflagellates and 1 het- erotrophic dinoflagellate by first stage larvae of 4 species of crabs. Crab species were 2 winter spawn- ing brachyurans Cancer magister and C. oregonensis, 1 summer spawning brachyuran Hemigrapsus oregonensis, and 1 anomuran Rhinolithodes wosnessenskii. Autotrophic dinoflagellate prey were Prorocentrum micans, which sustain survival of crab larvae in laboratory culture, 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-toxic strains of Alexandrium were almost never ingested. Results of rearing experiments generally confirmed that larvae were receiving no nutritional contribution from Alexan- drium spp. prey. When brachyuran larvae were presented with mixtures of P. micans and Alexan- drium spp. in defined ratios, virtually all larvae ingested both types of algal prey. Suspending Alexan- drium cells in P. micans exudate did not enhance their ingestion nor did suspending P. micans in Alexandrium exudate reduce ingestion. Ingestion of plastic beads was low (<12%) except when offered in combination with P. micans cells (58%). H. oregonensis larvae ingested the heterotrophic dinoflagellate Noctiluca 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 inges- tion stimulus provided by P. micans and N. scintillans, but its absence in Alexandrium spp. Absence of ingestion of Alexandrium was not related to the presence of toxins. The ingestion stimulus appears to reside on the prey cell surface. Although crab larvae appear able to discriminate among algal prey, non-discriminate feeding seems likely to occur in mixed prey assemblages in which at least some prey possess the positive ingestion cue, perhaps permitting rapid ingestion of available particles when dense prey patches are encountered in an otherwise sparse prey environment.