Tanaidacea

(Tanaids)

Phylum Arthropoda Subphylum Crustacea Class Malacostraca Order Tanaidacea Number of families 20

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Small, mostly marine-dwelling crustaceans that are among the most diverse and abundant creatures in some marine environments

Illustration: Apseudes intermedius. (Illustration by Dan Erickson)

Evolution and systematics

The fossil record of tanaids is better known from the Ma-zon Creek faunas from the Middle and Upper Pennsylvanian (Carboniferous). These faunas are distinguished from others by the numbers and types of species and by their unusually fine preservation. The suborder Anthracocaridomorpha was noted in 1980 and is known from fossils only.

The systematics and phylogeny of the Tanaidacea are poorly understood. However, there is a consensus that suggests abandoning the ancient divisions Monokonophora and Dikonophora for three new suborders proposed in 1983: Apseudomorpha, Neotanaidomorpha, and Tanaidomorpha. Regarding phylogeny, Apseudomorpha shows the most ple-siomorphic features, Tanaidomorpha presents the most apo-morphic or derived features, and Neotanaidomorpha is placed in an evolutionary position between the Apseudomorpha and the Tanaidomorpha. Three suborders, three superfamilies, and 20 families comprise the Tanaidacea.

Physical characteristics

There is a carapace covering the cephalothorax, which is formed from the head fused with the first two thoracic segments; the cephalothorax bears one pair of antennules and one pair of antennae. Compound eyes can be absent or present. The two first pairs of thoracic appendages (thoracopods) are additional mouthparts called maxillipeds, the second pair (called gnathopods) is chelate, which is a distinctive characteristic of Tanaidacea. Thoracopods 3-8 are ambulatory pere-opods (articulate walking legs), which are usually similar in form. In some families of Tanaidacea, the distal articles of the first pereopod are flattened, possibly to use in digging or even in swimming. In Tanaidacea, the pleopods can be present or absent; when present, they are used in swimming and to produce a ventilatory current in tube-dwelling species. In the final body region, the telson and the last one or two pleonites are fused as a pleotelson, which bears a pair of terminal bira-mous or uniramous appendages called uropods. Ovigerous females possess a brood pouch (marsupium), formed from flattened plates arising from an inner proximal margin of coxa of certain pereopods (oostegites), within which the eggs are developed.

Most species of Tanaidacea are small, ranging from 0.039 to 0.78 in (1 mm-2 cm) in length. The tanaidaceans display some color patterns, mainly yellowish and blue or gray tonalities.

Distribution

The Tanaidacea are distributed in all oceans of the world, from tropical to temperate regions, and even in polar waters.

Habitat

Most species of Tanaidacea are benthic, although some species have been found in plankton. The tanaidaceans can be found at a wide variety of depths, from the littoral zone to deep waters, but the better known species are those that live in shallow waters. Although the tanaidaceans are almost exclusively marine dwellers, some species have penetrated freshwater habitats. This is the case of some euryhaline species. Some euryhaline species can also tolerate a wide range of temperatures, inhabiting from tropical to temperate regions, living in rivers, lakes, and estuaries. Most of the offshore species are found in sand, mud, and kelp holdfasts. Other species can be found in intertidal regions of sandy or muddy shores. There are some species with different habitats: Hexapleomera robusta inhabits minute tubes in fissures between the carapace scales of marine turtles; Pagurapseudes spp. is adapted to fit the pleon into small gastropod shells like the hermit crabs. Many tanaids can produce a cementlike material from tegumental glands in the pereonal cavity; this cement emerges from the tips of the dactyli of the anterior pereopods and is used to construct tubes with particles of sand and detritus. Generally, the tubes are open at both ends, and tanaids use the pleopods to create water currents. During the incubatory period, ovigerous females may close the ends of their tubes. The tubes built by tanaids stabilize the sediment, favoring the colonization of other sedentary species.

Behavior

Nothing is known of the behavior of tanaids.

Feeding ecology and diet

The Tanaidacea, like many others crustaceans, have exploited several feeding strategies. Most tanaids utilize feeding mechanisms that usually involve direct manipulation of food by the mouthparts and the pereopods, especially chelate anterior legs. They are raptorial feeders, eating detritus and its associated microorganisms. The larger food particles are manipulated by the chelipeds and maxillipeds and transferred toward the mouth. Some members of the Tanaidacea possess better-developed branchial chambers, maxillae, maxillipedal epipodites, and exopods on the chelipeds, and the first pair of pereopods that are used in filter or suspension feeding. In this feeding mechanism, the simultaneous action of the thoracic limbs creates a suspension-feeding current by alternate movements of adjacent limb pairs. Thus, surrounding water is drawn into the interlimb spaces, and particles are retained by setae on the inner face of these appendages. Then, the retained particles are carried to a midventral food channel and moved anteriorly toward the mouth. Some tanaids are predators, catching the prey with chelate pereopods or even directly with the mouth appendages. Then, with various mouthparts, mainly the mandibles, they shear, tear, or grind the prey.

Reproductive biology

As in all other peracarid crustaceans, tanaid offspring are held within a marsupium on the ventral face of an ovigerous female during their early development, and generally are released only when they have developed most of their appendages. The fertilized eggs develop into embryos, which gradually develop into the first larval stage (manca I), and then into a second, more mobile larval stage (manca II). In this second stage, the manca has partially formed the sixth pereopods; this is when the tanaid generally leaves the marsupium. In tube-dwelling tanaids, the larval stage is followed by a juvenile stage with completely formed appendages; these juveniles will develop either into preparatory males or preparatory females. However, hermaphroditism can occur in tanaids in many ways. For example, in Apseudes spectabilis, some specimens possess both mature eggs and male gonads filled with sperm. Hence, several researchers believe that self-fertilization is possible.

In the case of conventional reproduction of tube-dwelling species, a copulatory male enters the tube of a female, and the courtship takes place for a long period of time. Male and female lie with their ventral sides adjacent to one another, and sperm are deposited in the marsupium. After closing the mar-supium, the female releases the eggs. Then, the female expels the male from the tube, closes the ends of the tube, and incubates the brood.

Conservation status

Nothing is known of the conservation status of tanaids. No species are listed by the IUCN.

Significance to humans

No significance to humans is known.

1. Paraleiopus macrochelis; 2. Apseudes intermedius. (Illustration by Dan Erickson)
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