GROWTH AND ENERGY-UTILIZATION OF THE INTER-TIDAL ISOPOD IDOTEA-BALTICA (PALLAS) (CRUSTACEA, ISOPODA)

Growth and energy relationships of an intertidal population of Idotea baltica (Pallas) (Crustacea: Isopoda) at Cape Blomidon, Nova Scotia, were examined in 1977 and 1978. Most juveniles were released from female brood pouches in mid-summer. Subsequent growth of both sexes followed sigmoid patterns between 11th July and 15th November 1977, which were best described by logistic (males) and Gompertz (females) formulae. In 1978 growth of both sexes was apparently linear. Reasons for the choices of these equations are presented. The first six instars of I. baltica were identified, and, from the relationships between length and weight of successive instars, we deduced that males moult an average of nine times compared to eight for females. From field collections the average life span of males was calculated to be 374 days, whereas females survived 415 days. Measurements of respiration and ingestion at regular intervals throughout the life cycle allowed construction of a cumulative budget outlining the major pathways of energy flow through I. baltica. Males assimilated considerably more ingested food than females (66.1% compared with 46.2%), although both sexes incurred similar metabolic losses of assimilated energy (56.7% males, 57.7% females). Moulting accounted for 5.2-7.9%, and a maximum value of 2.1% was estimated for non-faecal (nitrogen) excretion. Males attained a larger size than females, the total cost of growth (33.4%) being similar to the sum of production costs in females: 26.5% for somatic growth and 8.5% for reproduction. The higher assimilation efficiency of males may be associated with increased retention of ingested food to allow for recovery of minerals lost during moulting, and it may also represent a mechanism ensuring that males grow to a greater length than females, which is a prerequisite for successful copulation. © 1979.