Life history and energy relations of Idotea baltica (Pallas) (Crustacea : Isopoda)

The life cycle and general biology of an intertidal population of Idotea baltiea (Pallas) at Cape Blomidon, N.S., was examined from April 1977 to September 1978. In 1977, breeding activity was greatest at the beginning of June. Growth of the new cohort until 15 November was rapid, and the mean lengths of males and females from each field collection fitted rather well to logistic and Gompertz growth formulae, respectively. The maximum estimated rate of growth of males during this period was 0.87 mm/day compared to 0.30 mm/day for females. Maximum recorded abundance was 87 isopods/ 10 kg Asecophyllum on 15 November 1977, but shortly thereafter the isopods left the intertidal zone and were not found again until 24 April 1978. Growth in 1978 was best described by linear equations, and males again grew more rapidly than females. No definitive size was identified. At maturity, an average male (24.7 mm) had moulted 9 times compared to 8 for a female (17.6 mm). Fecundity was at least partially related to body length, and in 1978 an average female produced 182 embryos. The reproductive output corresponded to 15.1% of her body weight. Both sexes died after their respective roles in the breeding cycle were completed, but because of the long brood carrying period, females lived 1.5 months longer than males. Measurements of daily oxygen consumption and ingestion at regular intervals over the life cycle were conducted to allow construction of an energy budget. Over the life cycle, males and females ingested similar quantities of food; however, males assimilated more (66.1%) of this than females (46.2%). Of the energy assimilated, males invested 33.4% in growth, whereas females deposited only 21.5% as new tissue. The difference (6.9%) corresponded closely to the energy of egg production (8.5%). Maintenance costs were similar in both cases. Because males moulted Once more than females over a shorter life cycle, evidence suggested a differential absorption rate of minerals from the food supply to account for minerals lost during ecdysis. For a given weight of food consumed, male faecal pellets had significantly lower ash contents and weights than pellets collected from females. This fact, compared with the extra moult occurring in males, suggests that males retain a higher proportion of minerals from their food than do females to compensate for greater mineral losses in ecdysis.