In many malacostracan crustaceans, molting continues into the reproductive adulthood, playing a pivotal role in timing the transfer of sperm/spermatophore via mating to the females. Hence, in females of most crustacean species, mating and fertilization are closely associated with female molting. The females molt first and mating occurs thereafter, followed by the release of egg, which will be fertilized by the sperm contained in the spermatophore. Importantly, females are receptive for several minutes to a few days after molting and hence mating must take place during this time period, and the rest of the eggs will be resorbed in the ovary. However, other species maintain the ability to mate throughout most of their molt cycle.

Hartnoll (1969, 2000) distinguished two types of mating in decapod crustaceans: “soft-female mating” and “hard-female mating.” Soft-female mating was defined as copulation occurring immediately after molting of the females, usually preceded by a lengthy premolt courtship, including precopulatory guarding by the male. In the portunid crab, Callinectes sapidus, after a prolonged mate guarding of the premolt female, copulation occurs immediately following her pubertal molt (Gleeson, 1991). With this last molt, the female enters terminal anecdysis after reaching maturity for the first time. The female blue crab incidentally mates only once during her lifetime, as the one-time mating provisions adequate sperm, stored in its seminal receptacle, for fertilizing eggs from subsequent spawning.

Hard-female mating was defined as mating, in which the female copulates during intermolt stage after a relatively brief courtship. In species inhabiting terrestrial and semiterrestrial habitats, mature females copulate in the hard-shelled condition. Examples are the land hermit crabs of the genus Coenobita and the brachyuran crabs of the Grapsoidea and Gecarcinidae. Briefly, the observations of Hartnoll on brachyuran crabs led to the general preposition that a lengthy premolt courtship behavior is associated with soft-female mating, whereas a relatively brief courtship behavior happens with hard-female mating.

The situation in penaeid shrimps is somewhat different, the molting condition of mating female is determined according to the type of thelycum it possesses. In the open thelycum type, the females copulate in the hard-shell condition, whereas in the closed thelycum type, the female molts just before mating. In spite of this difference in the molting condition of these two types of penaeid shrimps, there is no difference in the mating behavior between the open- and closed-thelycum species, the copulation occurring after a brief interaction between a male and a female (Asakura, 2009). Nevertheless, several other malacostracans, including the peracarid species, such as isopods and amphi- pods follow molt-mate-spawning sequence during the reproductive cycle. It is tempting to suggest that the evolutionary origin of crustacean mating systems is interrelated to the occurrence of molting in the reproductive adults, almost alternating with the female reproductive cycle. In the continuously reproducing intertidal mole crab, Emerita asiatica, molting cycle precisely alternates with the reproductive cycle, yielding high fecundity and faster body growth (Gunamalai and Subramoniam, 2002).

Furthermore, continued molting in the adult female is also an intervening factor for reproductive activities, resulting in the relegation of both mating and egg-laying to postmolt and intermolt stages. Understandably, mating in the postmolt condition is a physiological necessity, as the female gonopore will be accessible to intromittent organs only in the soft-shelled condition. Since molting females are vulnerable to predation, they need physical protection, which is offered by the males that seek fresh-molt females for mating. As a result, a new behavioral trait, namely precopulatory mate guarding by the males has evolved in many crustacean taxa, in which molting precedes mating. Since the spawned eggs are attached to the pleopodal hairs in the ventral sternum to allow brood development, in many cases, precopulatory mate guarding extended to postcopulatory guarding of the brooding females. Thus mate guarding has emerged as the central behavioral component in crustacean reproduction.

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