Population and individual energetics of Ligia oceanica (L.) (Crustacea:Isopoda) in the rocky supralittoral

Abstract

An energy budget was constructed on a monthly basis for a population of the supralittoral isopod Ligia oceanica (L.) studied over 3 yr on an exposed sea wall. Animals fed mostly on encrusting diatoms. Production due to growth ( P g ) and reproduction ( P r ) were determined from field data to be 46.4 and 3.8 kJ·m −2·yr −1, respectively. Moulting energy loss ( E x ) was estimated from the incidence of moulting animals in field samples, and laboratory estimates of moulting stage duration and moulting weight loss. E x was found to be a significant component of the energy budget (13.3kJ·m −2·yr −1). Absorption efficiency was determined in the laboratory for groups of animals fed on Enteromorpha to be 44.7±4.4% (gravimetric) and 58.0 ± 3.3% (energetic). Total population assimilated energy was then estimated to be 160.2 kJ·m −2·yr −1, but was highly seasonal, varying by 13 times between months. Average population biomass ( B ̄ ) was 23.2kJ·m −2, P B ̄ = 2.73 and P R ̄ = 0.656 . Production by this supralittoral population was found to be less than, but of the same order as, other populations of major intertidal herbivores. Compared with other isopods, marine species were found to be more important in community energy transfer. Variation in absorption efficiency between isopod species was found to be significantly correlated with trophic mode, and probably reflects the quality of the available food. The importance of knowing the long-term dynamic state of the population before interpreting population energy budgets is stressed. Individual energy budgets were constructed for fast-growing, early breeding (LATE group) and slow-growing, delayed breeding (EARLY group) females. Small differences in reproductive effort calculated from the individual energy budgets were due to differences in tissue energy content.