Sexual dimorphism in calanoid copepods: morphology and function

Mate location and recognition are essentially asymmetrical processes in the reproductive biology of calanoid copepods with the active partner (the mal e) locating and catching the largely passive partner (the female). This beh avioural asymmetry has led to the evolution of sexual dimorphism in copepod s, playing many pivotal roles during the various successive phases of copul atory and post-copulatory behaviour. Sexually dimorphic appendages and stru ctures are engaged in (1) mate recognition by the male; (2) capture of the female by the male; (3) transfer and attachment of a spermatophore to the f emale by the male; (4) removal of discharged spermatophore(s) by the female ; and (5) fertilization and release of the eggs by the female. In many male calanoids, the antennulary chemosensory system is enhanced at the final mo ult and this enhancement appears to be strongly linked to their mate-locati ng role, i.e. detection of sex pheromones released by the female. It can be extreme in calanoids inhabiting oceanic waters, taking the form of a doubl ing in the number of aesthetascs on almost every segment, and is less expre ssed in forms residing in turbulent, neritic waters. Mate recognition is a process where chemoreception and mechanoreception presumably work in conjun ction. The less elaborate male chemosensory system in the Centropagoidea is counterbalanced by females playing a more active role in generating hydrom echanical cues. This is reflected in females in the shape of the posterior prosomal margin, the complexity of urosomal morphology and the size of the caudal setae. Visual mate recognition may be important in the Pontellidae, which typically show sexual dimorphism in eye design. The most distinctive sexual dimorphism is the atrophy of the mouthparts of non-feeding males, il lustrating how copepod detection systems can be shifted to a new modality a t the final moult. In the next phase, the male captures the female using th e geniculate antennule and/or other appendages. Three types of antennulary geniculations are recognized, and their detailed morphology suggests that t hey have originated independently. Grasping efficiency can be enhanced by t he development of supplemental hinges. The scanty data on capture mechanism s in males lacking geniculate antennules are reviewed. It is suggested that the loss of the antennulary geniculation in many non-centropagoidean calan oids has evolved in response to increasing predator pressure imposed on pai rs in amplexus. Spermatophore transfer and placement are generally accompli shed by the modified leg 5 of the male. In some males, leg 5 consists of bo th a chelate grasping leg and a spermatophore-transferring leg, whereas in others, only the latter is developed. Tufts of fine setules/spinules and/or sclerotized elements on the terminal portion of the leg are involved in th e transfer and attachment of the spermatophore. The configuration of gonopo res, copulatory pores and their connecting ducts in the female genital doub le-somite is diversified in the early calanoid offshoots such as Arietellid ae and Metridinidae, whereas in more derived groups, it is constant and inv ariable, with paired gonopores and copulatory pores located beneath a singl e genital operculum. The absence of seminal receptacles in most Centropagoi dea limits the female's ability to store sufficient sperm for multiple egg batches, suggesting that repeated mating is necessary for sustained egg pro duction. Discharged spermatophores are usually removed by the female leg 5 and/or specialized elements on other legs. In Tortanus (Atortus) Ohtsuka, which has rudimentary fifth legs in the fema le and complex coupling devices in the male, a spermatophore supposedly rem ains on the female urosome, since eggs appear to be released from a ventral opening of the spermatophore. The type of sexual dimorphism is closely rel ated to habitat and biology. Some hyperbenthic families never show multipli cation of aesthetascs on the male antennule, whereas families of the open p elagic realm such as the Aetideidae always have non-feeding males exhibitin g secondary multiplication of antennulary aesthetascs. The various aspects and diversity of calanoid sexual dimorphism are herein considered in an evo lutionary context.