Thermosbaenacea

(Thermosbaenaceans)

Phylum Arthropoda

Subphylum Crustacea

Class Malacostraca

Order Thermosbaenacea

Number of families 4

Thumbnail description
Small crustaceans with a short carapace (shell) and seven free thoracic somites (segments)

Evolution and systematics

Thermosbaenacea is an order comprising seven genera arranged in four families. A total of 34 species are known as of 2003. Thermosbaenaceans were first classified within their own superorder but are now generally regarded as members of the superorder Peracarida. They and other peracarid orders that possess a short branchial carapace have sometimes been grouped into a superorder known as Brachycarida. A recent study of thermosbaenacean evolution called attention to the close relationship of thermosbaenaceans to mictaceans, an order known only since 1985, and spelaeogriphaceans, very rare crustaceans that live in fresh groundwaters.

Physical characteristics

Thermosbaenaceans have a small, elongate body. The head is short and possesses a short carapace that extends backwards over the first few thoracic somites. The first thoracic somite is fused to the head; the appendage of this somite is modified as a maxilliped, or feeding appendage, located behind the jaw. There are seven free thoracic somites, each with a biramous walking leg. Each walking leg consists of a coxa (basal segment attached to the body) and basis (segment attached to the coxa); an exopod with one or two segments; and an endopod with five segments. There are six abdominal somites known as pleonites; only the first two have short appendages called pleopods. A broad and flattened telson and a pair of flattened uropods form the tail fan, located posterior to the last pleonite. The head appendages, especially the maxillae and maxillipeds, are usually broad and carry a variety of setae (bristles). The maxilliped often has a branchial epipod that extends backwards under the short carapace. The carapace of mature females develops a large inflated area that shelters the eggs.

Distribution

The distributional pattern of thermosbaenaceans lies within the limits of the ancient Tethys Sea, an ancient body of water that existed during the Mesozoic Era, 248–65 million years ago. It extended from what is now Mexico across the Atlantic Ocean and Mediterranean Sea into central Asia. All Holocene species are found in the zone once covered by the Tethys Sea or along its former coastlines. All extant species in this order are known from ground waters.

Habitat

The first thermosbaenacean discovered, Thermosbaena mirabilis, was found living in hot springs near El Hamma, Tunisia, where the Romans built bathing houses. The name of the animal derives from the fact that these are thermal springs with water temperatures of 111–118°F (44–48°C). Other thermosbaenaceans were subsequently found in underground springs; most species, however, do not live in thermal springs. In the Caribbean, several species were found in wells drilled for drinking water. Others have been found in caves with water running through them. The water in some of these caves is oligohaline (low salinity) rather than fresh. In addition, a few species in the genus Halosbaena are known from interstitial marine waters or marine caves.

Behavior

Thermosbaenaceans move primarily by walking, but can also swim by using their thoracic limbs for propulsion.

Feeding ecology and diet

Thermosbaenaceans living in hot springs feed on blue-green algae, diatoms, and other microalgae lining the rocks. Little is known about the diet of species living in other habitats. The mouth appendages of these organisms, however, are well festooned with setae located on the distal edges of the appendage segments, suggesting they might be used to sweep small particles from the crustacean's substrate. One species is known to feed on plant detritus.

Reproductive biology

Mating has not been observed. The eggs are incubated in a dorsal brood pouch formed by the swollen carapace of the mature female, and are bathed in water passing past the respiratory epipod. The young resemble miniature adults when they hatch. In one species, the young hatch before the sixth and seventh legs are fully formed, and so must undergo some development outside the brood pouch.

Conservation status

The type species of the order seems to have disappeared from the type locality. Most thermosbaenaceans are protected because their habitats are protected. None are listed by the IUCN.

Significance to humans

Thermosbaenaceans have no known significance to humans.

Species accounts

List of Species

No common name

Thermosbaena mirabilis

family

Thermosbaenidae

taxonomy

Thermosbaena mirabilis Monod, 1924, El Hamma, Tunisia.

other common names

None known.

physical characteristics

The body is more or less cylindrical in shape, with the thorax somewhat shorter than the abdomen. The thorax bears only five pairs of legs, each with well-formed exopods. The maxilliped is modified and lacks an endopod. The abdomen has two small pairs of pleopods on its first and second segments. The telson is quite large, as long as the last three abdominal somites. Eyes are not present. (Illustration shown in chapter introduction.)

distribution

Known only from a limited number of thermal springs in Tunisia.

habitat

Thermal springs with temperatures above 111°F (44°C), generally with highly mineralized water.

behavior

Crawls on the surfaces of rocks in search of food.

feeding ecology and diet

Thermosbaena mirabilis has been found to feed on several species of blue-green algae in the thermal springs.

reproductive biology

The testes are located in posterior part of the head and the first somite of the thorax. Long vasa deferentia lead from the thorax to the end of the abdomen and then back to the eighth thoracic somite where the male gonopore is located. Ovaries occupy the entire thorax in females, with the gonopore

on the sixth thoracic somite. When mature the ovary also extends into the abdomen. Eggs are carried in a brood pouch formed by a lobe of the female carapace, although deposition of the eggs in this location has never been observed. When shed from the brood pouch, the young have five pairs of thoracic limbs beyond the maxilliped and look like miniature adults.

conservation status

The animal disappears when the baths are cleaned, but apparently repopulates afterward, perhaps from a larger population living deep underground. Not listed by IUCN.

significance to humans

None known.

Resources

Books

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

Periodicals

Wagner, H. P. "A Monographic Review of the Thermosbaenacea (Crustacea: Peracarida)." Zoologische Verhandelingen 291 (1994): 1–338.

Les Watling, PhD