Male Reproductive Characteristics of Two Parasitic Copepods, Lernaeocera Branchialis (L.) (Pennellidae) and Lepeophtheirus Pectoralis (Müller) (Caligidae)

Abstract

On an average, of Lernaeocera branchialis (L.) needed a shorter time to inseminate a group of females than of Lepeophtheirus pectoralis (Muller). The higher insemination rate of L. branchialis may represent adaptations to the two-host life cycle of the female, and is also partly achieved because most invest less in females inseminated after the first successful mating. Laboratory experiments also showed that of L. branchialis did increase their rate of spermatophore production in the presence of a possible competitor. The size of the first pair of spermatophores was correlated with the size of the male in either copepod species, but a comparison between L. branchialis and L. pectoralis showed that of the former species produced the largest spermatophores relative to their size. Females of L. branchialis did benefit nutritionally from relatively large ejaculate volumes by increased survival in their second pelagic phase. However, production of relatively large ejaculate volumes may be beneficial to the male in that it may reduce the time needed to effectively inseminate a female. The relatively large spermatophores of of L. branchialis may thus represent an indirect consequence of the female's two-host life cycle. In both copepod species most matings resulted in insemination of the female. In L. branchialis this success depends mainly on the virility of the male, whereas in L. pectoralis the placement of the spermatophores is more haphazard, and successful mating depends both on the virility of the male and the precision of the placement of the spermatophore pairs. In recent years, several publications have demonstrated a cost of reproduction in (Bell and Koufopanou, 1986; Partridge, 1987; and references therein). Dewsbury (1982) stresses that males are limited with respect to the number of ejaculates they can deliver and the time required to restore depleted reserves. Thus, selection pressures associated with costly ejaculates and sperm competition should lead not to inseminate as many females as possible but to ensure that the number of ejaculates with each female ensures effective paternity (Dewsbury, 1982). In parasitic copepods on fishes two main categories of life cycles are found. In most families and in the majority of species, only a single host is included in the life cycle, whereas in the family Pennellidae, the females change host species (Ho, 1966; Kabata, 1979). Because of their change of host, pennellid females probably have a higher t This work was done at the Section of Marine Zoology and Marine Chemistry, Department of Biology, University of Oslo, P.O. Box 1064, Blindern, 0316 Oslo 3, Norway, and is published posthumously (see p. 751). Reprint requests may be sent to Dr. Thomas Schram at the same address. postmating mortality and are available for insemination during a shorter time (Anstensrud, in press) than females with only one host in the life cycle. Thus, in parasitic copepods male reproductive biology may represent adaptations only to sperm competition, female choice, and operational sex ratio as generalized by Dewsbury (1982), but also to the life cycle of the female. In the present study, male reproductive characteristics of the two parasitic copepods Lernaeocera branchialis (L.) and L. pectoralis (Miiller) are compared and discussed. The two parasitic copepod species have several life-history traits in common, and, after three pelagic larval stages, two nauplius stages and one copepodite stage, both L. branchialis and L. pectoralis infect a host, often flounders, Platichthys flesus (L.) (Boxshall, 1976; Kabata, 1979), where the parasites reach maturity and mate. Males of both copepod species are polygamous and each ejaculation results in simultaneous transfer of two spermatophores to the female (Anstensrud, 1989a, 1990a, b). On the other hand, females of the two copepod species have both different life cy-