Feeding, Growth, Respiration, and Carbon Utilization of a Euphausiid Crustacean

Abstract

In the euphausiid shrimp Euphausia pacifica, a member of the oceanic plankton, growth was as rapid as 0.048 mm/day in juveniles in the laboratory, more than twice as fast as that observed for oceanic populations (0.02 mm/day) by other workers. Periods between molting, which occurs throughout the life span, varied between 3 and 8 days depending on the temperature but not on the amount of food eaten. Respiration accounted for the major portion of the assimilation of carbon (62–87%) and molting, growth, and reproduction accounted for the remainder. In mature animals, 9% of the assimilated carbon was released as the organic portion of the eggs. Rapidly growing animals incorporated as much as 30% of the assimilated carbon, but calculations for an oceanic population gave 9% as the incorporation of organic carbon into tissue (excluding eggs and molts) over the life span of the animal. Assimilation of ingested carbon (digestion) appeared to be high, usually over 80%, as judged from tracer experiments. In the laboratory, crustacean nauplii seemed to be a preferred food over unicellular algae but both were eaten when available; one alga was rejected. Nauplii must be smaller than 0.8 mm for many to be ingested by adult E. pacifica. Molting, which mainly occurred at night, depressed feeding. The filtering rates for the animal on unicellular algae are sufficient for growth and metabolism if the carbon available to the animal in the sea is equivalent to that contributed by both detritus and phytoplankton standing crop in the area of the northeastern Pacific Ocean where E. pacifica is found. It is suggested that adult E. pacifica must capture between 100 and 200 nauplii per day as 1-year-olds and 200–300 per day as 2-year-olds to satisfy all of its carbon requirements. The mean density of small crustaceans in nature is probably too low for this animal to fulfill its needs. However, concentrations may be high enough in local aggregations.