Cumacea Krøyer, 1846
* Krøyer, H.N. (1846). On Cumaceerne Familie. Naturh. Tidsskr. 2(2): 123-211 pls 1-2
Cumacea is an order of small marine crustaceans, occasionally called hooded shrimp. Their unique appearance and uniform body plan makes them easy to distinguish from other crustaceans.
Cumaceans have a strongly enlarged carapace (head shield) and pereon (breast shield), a slim abdomen and a forked tail. The length of most species varies from 1 to 10 millimetres (0.039 to 0.39 in).
The carapace of a typical cumacean is composed of several fused dorsal head parts and the first three somites of the thorax. This carapace encloses the appendages that serve for respiration and feeding. In most species, there are two eyes at the front side of the head shield, often merged into a single eye lobe. The five posterior somites of the thorax form the pereon. The pleon (abdomen) consists of six cylindrical somites.
The first antenna (antennula) has two flagella, the outer flagellum usually being longer than the inner one. The second antenna is strongly reduced in females, and consists of numerous segments in males.
Cumaceans have six pairs of mouthparts: one pair of mandibles, one pair of maxillules, one pair of maxillae and three pairs of maxillipeds (Jones, 1976 and Brusca & Brusca, 2003).
Cumaceans are mainly marine crustaceans. However, some species can survive in water with a lower salinity, like in brackish water (e.g. estuaries). In the Caspian Sea they even reach some rivers that flow into it. Few species live in the intertidal zone.
Most species live only one year or less, and reproduce twice in their lifetime. Deepsea species have a slower metabolism and presumably live much longer.
Cumaceans feed mainly on microorganisms and organic material from the sediment. Species that live in the mud filter their food, while species that live in sand browse individual grains of sand. In the genus Campylaspis and a few related genera, the mandibles are transformed into piercing organs, which can be used for predation on forams and small crustaceans (Bacescu M. & Petrescu I., 1999).
Many shallow water species show a diurnal cycle, with males emerging from the sediment at night and swarming to the surface (Akiyama et al., 2004).
Like Amphipoda, cumaceans are an important food source for many fishes. Therefore, they are an important part of the marine food chain. They can be found on all continents.
Reproduction and development
Cumaceans are a clear example of sexual dimorphism: males and females differ significantly in their appearance. Both sexes have different ornaments (setation, knobs, and ridges) on their carapace. Other differences are the length of the second antenna, the existence of pleopods in males, and the development of a marsupium in females. There are generally more females than males, and females are also larger than their male counterparts.
Cumaceans are epimorphic, which means that the number of body segments doesn't change during the different developmental stages. This is a form of incomplete metamorphosis. Females carry the embryos in their marsupium for some time. The larvae leave the marsupium during the so-called manca-stadium, in which they are almost fully grown and only miss their last pair of pereopods.
History of research
The order of Cumacea was already known since 1780, when Ivan Ivanovich Lepekhin described the species Oniscus scorpioides (later renamed to Diastylis scorpioides). During that time, many scientists thought that the cumaceans were some kind of larval stadium of decapods. In 1846, they were recognized as a separate order by Henrik Nikolaj Krøyer. Twenty-five years later about fifty different species had been described, and currently there are more that 1,400 described species. German zoologist Carl Wilhelm Erich Zimmer studied the order Cumacea very intensively.
The fossil record of cumaceans is very sparse, but extends back into the Mississippian age (Schram et al., 2003).
umaceans belong to the superorder of Peracarida, within the class of Malacostraca. The order of Cumacea is subdivided into 8 to 11 families, about 139 genera, and 1593 species .
* Paulson, Otton Mikhailovich, Studies on Crustacea of the Red Sea, with notes regarding other seas. Part I. Podophthalmata and Edriophthalmata (Cumacea).
1. ^ Krøyer, H.N. (1846). On Cumaceerne Familie. Naturh. Tidsskr. 2(2): 123-211 pls 1-2
Source: Wikispecies, Wikipedia: All text is available under the terms of the GNU Free Documentation License