Nutrient assimilation and distribution pathways in planktotrophic larvae of echinoid echinoderms
Jaeckle, W.; Twanow, C.; Balser, E.
American Zoologist 41(6): 1483
Ruppert and Balser (1986) proposed that the axocoel-pore canal complex ("kidney") in planktotrophic larvae of echinoderms and hemichordates functions to regulate body water volume and distribute nutrients. We tested the nutrient translocation aspect of this hypothesis by tracing the distribution of the protein ferritin in larvae of the echinoids Lytechinus variegatus and Arbacia punctulata. Larvae were exposed to ferritin (1-2 mg/ml in seawater) for varying periods of time and the presence of this compound in tissue was detected using the ferrocyanide reaction. Dissolved ferritin was ingested and assimilated materials were first detected in stomach cells. With increased exposure, the label was detected in cells of the esophagus, intestine, and larval kidney. Cells of the pore canal and the axocoel became intensely labeled. Label also appeared in the left larval hydrocoel, a coelom confluent with the left axocoel that develops into the adult water vascular system. Blastocoelic mesenchyme cells and cells of the ectoderm (rarely) were also labeled. In contrast, no label was detected in cells of coeloms not connected to the larval kidney. Thus, proteins initially assimilated from seawater by the digestive system are proposed to be delivered to the developing juvenile rudiment via connections to the larval kidney. These data support Ruppert and Balser's hypothesis that organic materials within the larval blastocoelic compartment pass through a fenestrated epithelium of the left axocoel and may be distributed to distant cells. Our results provide a refinement to their hypothesis by demonstrating that materials passing into the larval axocoel enter a coelomic circulation system that delivers them to the developing juvenile.