Sexual dimorphism in calanoid copepods: morphology and function

Abstract

Mate location and recognition are essentially asymmetrical processes in the reproductive biology of calanoid copepods with the active partner (the male) locating and catching the largely passive partner (the female). This behavioural asymmetry has led to the evolution of sexual dimorphism in copepods, playing many pivotal roles during the various successive phases of copulatory and post-copulatory behaviour. Sexually dimorphic appendages and structures 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 female 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 moult and this enhancement appears to be strongly linked to their mate-locating 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 doubling in the number of aesthetascs on almost every segment, and is less expressed in forms residing in turbulent, neritic waters. Mate recognition is a process where chemoreception and mechanoreception presumably work in conjunction. The less elaborate male chemosensory system in the Centropagoidea is counterbalanced by females playing a more active role in generating hydromechanical 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, illustrating how copepod detection systems can be shifted to a new modality at the final moult. In the next phase, the male captures the female using the geniculate antennule and/or other appendages. Three types of antennulary geniculations are recognized, and their detailed morphology suggests that they have originated independently. Grasping efficiency can be enhanced by the development of supplemental hinges. The scanty data on capture mechanisms in males lacking geniculate antennules are reviewed. It is suggested that the loss of the antennulary geniculation in many non-centropagoidean calanoids has evolved in response to increasing predator pressure imposed on pairs in amplexus. Spermatophore transfer and placement are generally accomplished by the modified leg 5 of the male. In some males, leg 5 consists of both 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 the transfer and attachment of the spermatophore. The configuration of gonopores, copulatory pores and their connecting ducts in the female genital double-somite is diversified in the early calanoid offshoots such as Ari...

Motility patterns and mate encounter rates in planktonic copepods

“…Given adult longevities on the order of 10 d in 1 mm-sized copepods (Hirst and Kiørboe 2002), this implies a potential for many mating events in the lifetime of an adult female. In some species, one mating is sufficient for the entire reproductive career of a copepod, while in other species, repeated mating events are required, up to one per batch of eggs produced (e.g., Berger and Maier 2001;Ohtsuka and Huys 2001). In either case, the substantial mate search capacity demonstrated in this study for a number of copepods appears to be sufficient to prevent mating limitation from constraining population propagation during the productive season.…”

“…The consistent difference in swimming patterns between males and females in both pheromone-producing species is unlikely to be related to sexual differences in feeding behavior, since in P. elongatus the males do not feed at all, while males of C. typicus do feed (Ohtsuka and Huys 2001). There are also several other reports of differing swimming patterns in male and female copepods, typically with males swimming faster than females (Uchima and Hirano 1988;Doall et al 1998;Tsuda and Miller 1998).…”

Macroevolutionary patterns of sexual size dimorphism in copepods

“…As we will show, copepods provide an excellent test case and allow quantitative examination of major SSDbased theory. The subclass Copepoda, which are members of the class Maxillopoda, are crustaceans and possibly the most abundant animal group on the Earth [14]. The striking variability in mate seeking behaviours (including sedentary and roving types in some parasitic families) and sex ratios allows for quantitative and qualitative tests of model predictions [1].…”

“…Some copepod families (within the Diaptomoidea) require repeat mating because they are unable to store sperm and often have near equitable sex ratios [17,18]. Others (many non-Diaptomoidea families) are able to store sperm [19] and can produce multiple batches of eggs from a single copulation event [14,20]. Higher rewards from single mating events may lead males to high mortality risk when mate searching.…”

Rare sexual anomalies in Temora stylifera (Dana, 1849) (Copepoda: Calanoida)

“…In most calanoid species the geniculated A1 is used for mate recognition and initial capture of the female, by grasping its caudal rami or setae (Ohtsuka and Huys, 2001). Generally, the right side is the geniculate and prehensile appendage, presenting one or two hinged joints, which enables the antennule to fold back upon itself (Blades-Eckelbarger, 1991), a common pattern in numerous families including the Temoridae.…”