Marine research on cumaceans from around the world.

Life History of the Cumacea

Cumaceans have several distinct life history stages. Females lay eggs into a brood pouch, and the embryos develop through several molts within the brood pouch. When released from the brood pouch, the young are known as mancae. The first manca stage is entirely without the fifth pair of walking legs, and the second manca stage has the last pair of legs represented by a pair of limb buds. It is assumed that there is a single molt between the first and second manca stages, and a single molt between the second manca stage and the juvenile form, with a complete fifth pair of walking legs.

The manca stages are not usually assignable to a species with confidence, as the carapace and uropods typically have not developed the ornamentation and proportions of the juveniles and adults. There are from 1 - several juvenile molts, with the individual increasing in size with each molt. Females in some species are reported to be capable of up to three broods, with the release of a brood followed by molting into a pre-brooding state with small brood plates, then a molt into the ovigerous state with large brood plates. Males are generally interpreted to be terminal, meaning that the molt into the adult male form is the last molt, and the males die thereafter.

There are a few papers (e.g. Bishop 1982, Bishop & Shalla 1994, Corey 1969, Corey 1976, Cartes & Sorbe 1996, Akiyama & Yamamoto 2004a, Akiyama & Yamamoto 2004b) detailing life histories for a few species, but it is clear even from these few studies that there is wide variety in life history strategies in the Cumacea, even within a single species (Akiyama & Yamamoto 2004b). Strategies range from continuous, twice or thrice yearly semelparous reproduction in intertidal and shallow subtidal species (Corey 1969, Corey 1976, Bishop & Shalla 1994, Cartes & Sorbe 1996, Corbera et al. 2000) to iteroparous deep sea species with discrete seasonal reproduction timed to coincide with the fall of the spring bloom, with life spans in excess of three years (Bishop 1982, Cartes & Sorbe 1996), and periods of brooding in excess of one year (Bishop 1982, Bishop & Shalla 1994, Cartes & Sorbe 1996).

The recent studies by Akiyama & Yamamoto (2004a, 2004b) describe the life history of the shallow subtidal species Nippoleucon hinumensis, reporting a 6.5 month diapause in the life history associated with high water temperatures, although there is a minority subpopulation (0.7%) that is non-diapausing.

Sexual dimorphism may be pronounced in the Cumacea, although it is not always present to the same degree. The picture shows the range of sexual dimorphism in the different families. The other figure gives two examples of sexual dimorphism within the family Gynodiastylidae, Allodiastylis johnstoni in which sexual dimorphism is maximal, and Gynodiastylis ampla, in which sexual dimorphism is minimal. If the Allodiastylis male had not been collected with the female, it is very unlikely that the two forms would have been associated with each other. On the other hand, in G. ampla the male and female forms are very similar, and one could easily identify the male on the basis of the female description.

Females and juveniles are similar in carapace morphology, appendage setation, proportions of the telson and uropods, and are easily recognizable as belonging to the same species. The males, on the other hand, may be nearly indistinguishable from the females and juveniles, or modified to a greater or lesser extent with loss or reduction of carapace ornamentation, increase in size and lens number in the ocular lobe, increased setation, changes in proportion and shape of the telson and uropods, elaboration of one or both pairs of antennae, increased numbers of exopods on the pereopods, and the presence of pleopods.

The changes in adult male cumaceans are modifications to decrease drag while swimming, increase the capacity to swim, to identify and locate females in a pre-mating condition, and in some cases there are informative character states present in adult males that are not present in juveniles or females, such as specialized types of setae associated with the modified antennae. The variations in males also tend to be somewhat predictable, such as a ventral expansion of the carapace to accommodate the large second antenna.

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