The breeding biology of Euchaeta norvegica (Copepoda : Calanoida)

Abstract

First few paragraphs from General Introduction GENERAL INTRODUCTION OBJECTIVES: Euchaeta norvegica (Boeck, 1872) is one of the larger species of calanoid copepod (adult females = 7-llmm total length, adult males 5.5-7mm total length) and has a widespread distribution in northern waters. Details of its distribution provided from the records of the Continuous Plankton Recorder Survey (Edinburgh Oceanographic Laboratory, 1973) show that it is present throughout the northern temperate waters of the north Atlantic Ocean, with its centre of abundance situated in the boreal waters of the Greenland-Labrador Current Complex. Hedgpeth (1957), in his classification of marine environments, showed the boundary between oceanic and neritic as approximating to the position of the 100-fathom (ca.182 metre) depth contour. Colebrook (1964, 1972) has since found that further divisions in the classification of geographic areas and water types are possible and has classified E. norvegica as being a typical component of the "north-west oceanic" plankton community. Relatively few specimens of E. norvegica are found in neritic seas although it is often found in abundance in Norwegian fjords and Scottish west coast sea lochs, and the deep waters associated with them. In its typical oceanic environment the adults occupy an essentially mesopelagic depth distribution whilst the younger copepodite stages are found in the more surface waters. A large body size and its often high abundance makes E. norve£ica one of the dominant biomass copepods of the northern temperate and boreal plankton communities. This, coupled with the fact that it appears to be wholly carnivorous (see Harding, 1974), emphasises the importance of this copepod in marine ecosystems. It is thus surprising that little published information is available about the biology and ecology of this species. The necessity to mate and breed is a dominant factor in an animal's life. The research presented in this thesis was carried out in order to examine these important processes in E. norvegica with particular reference to those aspects which are likely to determine the breeding intensity of a population. This study has, therefore, concentrated on an examination of sperm and spermatophore production in relation to fertilization and subsequent egg-laying, rather than the development of generation cycles after the eggs have hatched. The female reproductive system has been described, with an emphasis on its function during mating and egg-laying. The male reproductive system has been described and its histology and ultrastructure have been investigated with regard to secretion of the various components of the spermatophore. The mating process and the inseminatory position adopted by the male has been predicted. Analyses have been made of the frequency distributions of spermatophores found on adult females as well as the patterns of distribution of these spermatophores on the different areas of the female genital segment. The significance of such distributions have been discussed in terms of mating behaviour. The relationship between maternal body size and the size of the egg- clutch produced has been quantified. Seasonal variations in the sex- ratio of adults, as well as stage IV and stage V copepodites, have been followed and possible factors responsible for these changes have been discussed. Finally, the proportion of males present in the adult population has been examined as a factor determining the numbers of spermatophores attached to adult females and the quantity of egg-sacs subsequently produced by the population.