Anaspidacea

(Anaspidaceans)

Phylum Arthropoda

Subphylum Crustacea

Class Malacostraca

Order Anaspidacea

Number of families 4

Thumbnail description
These crustaceans do not have a carapace, one thoracic somite is incorporated into the head, with its appendages modified as maxillipeds, and the remaining thoracic legs have both epipods and exopods

Evolution and systematics

The order Anaspidacea contains four families that have sometimes been divided into two suborders, the Anaspidinea and Stygocaridinea. The family Anaspididae has been known since the Triassic and exhibits strong relationships to the extinct order Palaeocaridacea. Anaspidaceans are generally thought to be among the most primitive of eumalacostracan crustaceans (that is, members of the subclass Eumalacostraca), especially because their thoracic limbs possess both epipods and exopods. However, details of their circulatory system, foregut morphology, and other aspects of their internal anatomy suggest that anaspidaceans are derived from ancestors leading to the eucaridan decapods. The lack of a carapace has caused debate about their exact placement among eumalacostracan crustaceans, but anaspidaceans are generally thought to be ancestral to at least the line leading to decapods, if not some of the other eumalacostracan groups as well.

Physical characteristics

The anaspidacean head does not bear a carapace. Eyes may be on stalks, sessile on the head, or absent altogether. The first thoracic somite is fused to the head while the remaining seven thoracic somites are free. The first thoracic appendage is modified as a maxilliped, that is, it has a different morphology from the remaining seven pairs of thoracic limbs, and is modified as a feeding structure. Thoracic limbs two through eight have the endopod developed into full walking legs. On these appendages, the epipods, which emanate from the outside margin of the coxa, function as flattened gills, and the exopods, which come from the basis, function for water movement. Sometimes the exopods are not present. The pleopods may be strongly developed, reduced, or absent. In the Anaspididae, the telson and uropods form a tail fan, but in other families, the uropods are elongate and the telson short. Anaspidaceans can be up to 1.9 in (5 cm) in length, but some species are less than 0.39 in (1 cm). Color is usually a dull brown.

Distribution

Anaspidaceans show a classic Gondwana relict distribution pattern, being found only in Tasmania, southeastern Australia, New Zealand, and southern South America. The family Anaspididae are known only from a small number of localities in Tasmania and Victoria, Australia, while the Stygocarididae are more widely dispersed with species known from Victoria, Australia, New Zealand, Chile, and Argentina.

Habitat

The larger anaspidaceans are generally found in cool mountain streams, lakes, and swamps, while the smaller stygocaridineans are dwellers of the groundwater, living among the sand grains. The swamp-dwelling anaspidaceans live in the burrows of freshwater crayfish, while those found in lakes tend to live in the algal macrophyte mats on the lake bottom, and the stream-dwellers patrol over and among larger rocks of the streambed.

Behavior

Anaspidaceans are not good swimmers. Instead, they spend most of their time walking over the substrate. The exopods of the thoracic legs are in nearly constant motion, most likely circulating fresh oxygen-bearing water past the flap-like epipods. When walking, the legs move in a metachronal pattern, which continues to the pleopods. In fact, in the larger species, the pleopods have the same motion as the walking legs, so that at first glance the animal looks to have a continuous set of legs all the way to the posterior end of the body. When startled, anaspidaceans are capable of an upward jump in which the body is flexed about midway along the back. On relaxation, the animal settles to the bottom and walks about as if nothing had happened. There appears to be no territoriality in anaspidaceans. When two individuals meet, they may touch antennae, but as often as not, one merely walks over the body of the other.

Feeding ecology and diet

Anaspidaceans are generalist feeders, eating organic detritus obtained from the substratum. The larger species may also be scavengers and appear capable of scraping organic films from the surfaces of small pebbles.

Reproductive biology

Mating has so far not been observed in this group. Eggs are laid freely on vegetation or stones and are not guarded or cared for. Hatching occurs 30–60 weeks after the eggs are laid. The young emerge as juveniles. The longer developmental periods are associated with an over-wintering dormancy period.

Conservation status

Anaspidaceans are restricted in their distribution, but for the most part occur in areas where landscape development is minimal. As with many Gondwanan freshwater crustaceans, however, the introduction of trout into the rivers and streams by European colonizers has meant that some species survive only in the small tributaries where fishes cannot go. These crustaceans evolved in the absence of freshwater fishes, so they have no natural defenses against those introduced predators. At present, none of the anaspidaceans are considered to be threatened, and none are listed by the IUCN.

Significance to humans

Anaspidaceans represent an interesting evolutionary branch of crustaceans, and as such, are important in telling the history of life on Earth.

Species accounts

List of Species

No common name

Anaspides tasmaniae

family

Anaspididae

taxonomy

Anaspides tasmaniae Thomson, 1892.

other common names

None known.

physical characteristics

Eyes stalked; body elongate, with thoracic and abdominal somites of nearly similar size; pleopod of outer ramus long and capable of touching the substrate; telson and uropods forming a tail fan. (Illustration shown in chapter introduction.)

distribution

Tasmania.

habitat

Freshwater streams and shallow pools.

behavior

Walking involves the use of both thoracic and abdominal appendages, moving in a metachronal beat from anterior to posterior. It employs a vertical escape jump when startled, but otherwise walks constantly about in search of food.

feeding ecology and diet

Has been observed to gnaw on large plant fragments, scrape the surfaces of small pebbles with its mouth appendages, and will scavenge carcasses of small dead organisms.

reproductive biology

Mating behavior is unobserved at present. Eggs are laid on plants or bark. Young hatch as juveniles.

conservation status

Not listed by the IUCN.

significance to humans

An intellectual curiosity.

Resources

Books

Schram, F. Crustacea. Oxford, U.K.: Oxford University Press, 1986.

Periodicals

Swain, R., and C. I. Reid. "Observations on the Life History and Ecology of Anaspides tasmaniae." Journal of Crustacean Biology 3 (1983): 163–172.

Les Watling, PhD