Cumacea (Cumaceans)

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Cumacea

(Cumaceans)

Phylum Arthropoda

Subphylum Crustacea

Class Malacostraca

Order Cumacea

Number of families 8

Thumbnail description
Cumaceans are quickly recognizable by their large carapace, which covers the first three thoracic somites and narrow abdomen. The first thoracic appendage is modified as a maxilliped, which has both feeding and respiratory functions.

Evolution and systematics

Cumaceans are the most modified of the orders usually grouped together in the superorder Peracarida. The fossil record is sparse, but a few specimens from the Mesozoic have been found. Eight families of cumaceans are recognized. As many as 26 families once were used to divide the cumacean genera, but many contained only one or two genera. Modern phylogenetic methods are being used to revise the current scheme. Molecular and morphological evidence suggests that reduction of the cumacean telson, which is a feature of three families, has occurred only once.

Physical characteristics

Cumaceans have a large carapace that extends backward and ventrally to cover the first three thoracic somites. The anterior aspect of the head usually bears a single middorsal eye, but some species have two eyes, each member of the pair being located anterolaterally. The mandibles are usually of the basic, generalized feeding type with strong molar and incisor, but the molar occasionally is modified into a long styliform process. Because the first three thoracic somites are fused to the head, the appendages on those somites are modified as maxillipeds. The first pair of maxillipeds has a complicated structure. The endopod functions as a feeding device, and the epipod elaborates into a large and sometimes convoluted gill. The second pair of maxillipeds also is involved in feeding, but in females the coxae of these appendages have small posteriorly directed brood plates. The third pair of maxillipeds usually is leg-like but can have other functions, such as forming an opercular covering over the more anterior mouth appendages. The other five pairs of thoracic appendages function as walking legs. These appendages may have exopods, or the exopods may be reduced or absent. The first five pairs of abdominal appendages are known as pleopods. With a single exception (a species from the deep sea), pleopods are not found on females. Among males, pleopods may be absent, may occur in reduced numbers, or may be fully formed. A freely articulated telson is present in five families but is fused to the last abdominal somite in three families. The last pair of abdominal appendages is the uropods. In most cumaceans the uropods are composed of a peduncle that leads to two rami, the endopod and the exopod. The structure of the uropod often has bearing on classification at the genus level.

Distribution

All seas, from shallow bays and estuaries to the deepest trenches. Caspian, Aral, and Black Seas. Some families, such as Diastylidae, Lampropidae, and Ceratocumatidae, are diverse in the deeper and colder waters of the ocean, whereas Nannastacidae and Bodotriidae are most diverse in shallow tropical waters. Pseudocumatidae is especially diverse in the Black and Caspian Seas.

Habitat

Most cumaceans are found in marine and brackish waters, but some range into water that is fresh for at least some of the time. Cumaceans tend to live in soft sediment where there is reasonable current motion but not heavy wave action. They bury themselves just below the sediment–water interface. Members of the family Gynodiastylidae are found in algal turf on the surfaces of stones or coral rubble.

Behavior

Cumaceans generally sit with their bodies just submerged in the sediment. They need to maintain contact with the overlying water to pump oxygenated water over their gills. The respiratory current generated by the movement of the first maxilliped epipod enters at the base of the walking legs, passes under the carapace, and exits at the front of the carapace, where part of the epipod forms a siphon. Some members of several shallow-water species after sunset undergo vertical migration from the sediment into the water column.

Feeding ecology and diet

Most cumaceans are fine-particle feeders. They obtain the particles by manipulating sediment grains with the mouth appendages or scraping the surfaces of larger particles. There has been some speculation that cumaceans are filter feeders, but there is no direct evidence to support that idea. Some members of the family Nannastacidae are considered, on the basis of the structure of the mandible, to be carnivores, feeding on small organisms such as foraminiferans or meiofaunal metazoans.

Reproductive biology

In most cumacean species males are semipelagic, swimming about until they find a female. In a few species within several families, males have lost their swimming ability and have developed grasping antennae, which are used to hang on to the abdomen of females until mating has occurred. Eggs are carried in a ventral brood pouch on the female, where they are incubated until hatching as juveniles. There is no pelagic larval stage. Both males and females of most species reach terminal molts at sexual maturity.

Conservation status

No species are listed by the IUCN. No cumaceans have been studied to the extent that it is known whether they are endangered. A few have been transported to foreign shores, where they have established themselves as invasive species.

Significance to humans

One cumacean species is known to be important as food for juvenile salmon along the northwest coast of North America. Others are commonly found in the stomachs of juvenile ground fish, such as cod. Most cumaceans are very small and insignificant from the perspective of marine food webs.

Species accounts

List of Species

Cyclaspis longicaudata
Diastylis sculpta

No common name

Cyclaspis longicaudata

family

Bodotriidae

taxonomy

Cyclaspis longicaudata Sars, 1865.

other common names

None known.

physical characteristics

The carapace is nearly globose and is completely smooth. The free thoracic somites are quite small, and the abdomen is very long and narrow. The male has five pairs of pleopods.

distribution

Cold waters of the North Atlantic, northern Norway to northeastern United States at depths from 395 to 16,400 ft (120–5,000 m).

habitat

Cyclaspis longicaudata is found in sandy mud. The sand-sized particles in deep sea sediment are derived from foraminiferan and pteropod shells.

behavior

Nothing is known. The morphological features suggest C. long-icaudata is not likely to spend much time swimming, except in the mature male stage.

feeding ecology and diet

Nothing is known.

reproductive biology

Juveniles and females with young in the brood pouch have been found at almost any time of the year, so it is likely that at least some populations are reproductive throughout the year.

conservation status

Not listed by the IUCN.

significance to humans

Cyclaspis longicaudata has been found in the stomachs of some deep-dwelling fishes.

No common name

Diastylis sculpta

family

Diastylidae

taxonomy

Diastylis sculpta, Sars 1871.

other common names

None known.

physical characteristics

Diastylis sculpta gets its name from the series of large ridges on the carapace that give it a highly sculptured appearance. The carapace of males is somewhat smoother, being streamlined for hydrodynamic purposes. The male has two pairs of pleopods.

distribution

Coast of eastern North America from the Gulf of Saint Lawrence to Long Island Sound.

habitat

Muddy sand bottoms in shallows of bays and estuaries.

behavior

Diastylis sculpta spends most of the day nestled into surface sediment. At night it makes short forays into the overlying water; the purpose of these forays is not known.

feeding ecology and diet

Although its specific food has not been investigated, D. sculpta spends most of its time in the sediment sifting through the particles, most likely looking for microalgae and other high-quality organic particles.

reproductive biology

In the Woods Hole, Massachusetts, region, D. sculpta is found from July to January with young in the brood pouch. More than one generation is represented during this period.