Mating system is defined as the general pattern by which males and females mate. In many malacostracans, mating systems involve dramatic premating behavior on the part of the male, such as courtship signaling, fighting among males, or defense of females before mating. In Crustacea, mating systems as well as factors that determine them are diverse. Life history patterns, ecological influence, functional morphology of copulation and insemination, and social determinants of both male and female behaviors are some key criteria employed in categorizing the mating systems found in Crustacea. Additional factors such as sperm competition and pre- and postcopulatory mate choice are also considered in shaping mating strategies. Ecological factors apparently contribute to the evolution of diversified mating systems as well as the attributed behavioral and morphological differences between the sexes. For example, crustaceans such as isopods, amphipods, shrimps, and crabs that have developed complex symbiotic relationships with other marine invertebrates like sponges, sea anemones, and corals have featured a wide array of social structures along with wide-ranging mating systems. Mating systems therefore refer to the procedure used in finding and securing a mate (usually a female), the number of mates an individual acquires, the type of pair bonds, and the nature of parental care (Subramoniam, 2013).
Sex attraction and sex recognition before mating also form an important component in the evolution of different mating systems in Crustacea. In addition, gamete morphology has a bearing on the establishment of different mating systems. For instance, the spermatozoa of malacostracan crustaceans are atypical and invariably nonmotile, and hence have to be delivered to the site of fertilization in the female body. This is in stark contrast to the condition found in many other marine invertebrates such as echinoderms, in which male and female gametes are simply released into the sea water to effect broadcast fertilization, without requiring any male-female contact. In addition, except the free-spawning dendrobranchiate penaeiodean shrimps, all other crustaceans incubate their yolky eggs in a brood chamber or attached to pleopodal hairs up until advanced larval development. Different crustaceans, therefore, resort to either internal or epizoic fertilization by delivering the sperm cells, packaged in special structures called spermatophores. The origin and occurrence of several mating systems correlate with environmental conditions as well as the
Sexual Biology and Reproduction in Crustaceans. http://dx.doi.org/10.1016/B978-0-12-809337-5.00004-6
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lifestyle, adopted to live and reproduce in the diversified environmental niches. Similarly, crustaceans exhibit a wide array of mating behaviors, which have evolved in response to special prerequisites for protecting the females, which in many decapods and peracarides, undergo an obligatory molting before mating and spawning. Thus, the precopulatory mate guarding by the males is prevalent in aquatic brachyuran decapods and marine isopods, and in many instances, the mate guarding has extended beyond copulation; with the result, the egg- brooding females continue to receive guarding from the males.
In decapod crustaceans, mating strategy of males also depends on the timing and duration of female receptivity. The latter will influence the amount of time a male invests in staying with the receptive female by defending or guarding her, or in searching for other receptive females so as to achieve maximum reproductive success (Parker, 1970). Furthermore, restricted female receptivity caused a male-biased operational sex ratio, increasing male-male competition and the potential for sexual selection. According to Emlen and Oring (1977), ecological and behavioral potential to monopolize mates is a major element of the mating strategy of a species. Thus, habitat difference as well as behaviors needed to survive in them has profound influence on shaping the different mating systems. For example, mate guarding is present in the intertidal grapsid crab species, Hemigrapsus sexdentatus, whereas in another species, Cyclograpsus lavauxi, it is absent. Although these two crabs have similar duration of female receptivity, habitat differences have caused differences in their mating strategies. Interestingly grapsid crabs mate during intermolt stage (Anilkumar et al., 1999).
The important factors or conditions responsible for shaping various mating systems are enumerated below.