Biology of deep-sea calanoid copepod genus Pleuromamma with particular references to phylogeny, pore signatures, moulting and life history.

Abstract

The pore signature of calanoid copepods is of increasing interest in phylogenetic and biogeographical studies. Some recent studies have been restricted to the urosome on the assumption that most of the species-specific information resides there. The present study tests that assumption in eight Pleuromamma species by assessing the signatures of the cephalosome, metasome and urosome separately in each species. Most of the species- specific information is in the urosome, but a significant proportion also resides in the cephalosome and a lesser component in the metasome. Changes in the pore signatures between the species parallel changes in other morphological characters. The pore signatures shed further light on the phylogeny of the Pleuromamma species which had been diffuse in studies of conventional morphology of this genus. Interspecific differences in the pore signatures occur as early as copepodid III. Sexual differences in the pore signature are found in copepodid IV and are primarily evident in the urosome of the adult. The species-specific components of the urosomal signature are greater in the female than in the male. In general, the degree of the intraspecific variation in the pore signature increases with increasing number of the total integumental pores and is limited to 10% or less of the total. Inter- and intraspecific variation are an expression of the phylogenetic grouping of species, or generic similarities between the species within the genus. In calanoid copepods, external and internal morphological changes associated with the intermoult cycle in Pleuromamma robusta conform with those in other crustaceans. Early stages of the intermoult cycle are the completion of the integumental structure and somatic tissue after the previous ecdysis and late stages are in preparation for the next ecdysis. Adult copepods do not moult, but progress through part of the moult cycle exhibited by earlier copepodid stages. Changes in the integumental structure were evident as increases in the thickness of the cuticle throughout the period of the intermoult cycle, which also affect the body appearance of copepodids such as soft and hard bodies. The different body appearance is of great use for studies of population dynamics, e.g. detection of the timing of the population reconstruction, overlapping generations, the identifjcation of the newly recruited stock and the seasonal changes of the recruitment rate. The Rockall Trough population of P. robusta produces three generations per year. The peak of reproduction of this species was in September and was followed by a hiatus of reproduction in November. In addition, significant relationships between day and night vertical distribution of individuals of copepodid V and adults at different stages of the intermoult cycle In P. robusta were found in relation to their mating behaviour.