Phyllocarida (Leptostracans)

(Leptostracans)

Subphylum: Arthropoda

Class: Malacostraca

Subclass: Phyllocarida

Number of families: 3

Thumbnail description
Small crustaceans with a laterally flattened carapace enclosing the bases of the thoracopods (legs), a hinged rostrum covering stalked eyes, and a tapering abdomen ending in a forked tail

Evolution and systematics

Only one fossil leptostracan has been recognized in the literature, the Permian Rhabdouraea bentzi. It has a separate family status (Rhabdouraeidae; Schram and Malzahn, 1984). However, the fossil is only that of an abdomen and makes comparison to other leptostracans difficult. The regular segmentation of the body, the trunk musculature, nervous system, circulatory system, presence of a furca, and absence of uropods indicate that the Leptostraca are primitive malacostracans. Three extant families are recognized: Nebaliidae (with genera Nebalia, Nebaliella, Dahlella, and Speonebalia); Nebaliopsidae (with only one genus, Nebaliopsis, and only one species, N. typica); and Paranebaliidae, containing genera Paranebalia and Levinebalia. Almost 35 species are described. There is not a common name for the leptostracans, but names such as "leaflike-shrimps," "mud-shrimps," and "sea fleas," are used in the literature.

Physical characteristics

Leptostracans have a bivalved carapace (the two valves are fused dorsomedially but have no hinge) enclosing head and thorax, or enclosing head, thorax, and part of the abdomen, laterally compressed and smooth. The anterior end of the carapace dorsum continues as a movable articulated rostrum. The eyes are stalked, compound, with visual elements either present or absent (genus Dahlella). The antennules (antenna one) are usually biramous and the antennae (antenna two) are uniramous. The eight pairs of thoracic limbs (thoracopods) are leaflike turgor appendages, foliaceous (phyllopodous), and biramous. The abdomen has six pairs of appendages (pleopods): the first four pairs are birramous and large, and the last two are uniramous and reduced. The seventh abdominal segment continues as a telson having a long caudal furca.

Most leptostracans are 0.19–0.59 in (5–15 mm) long, but one species (Nebaliopsis typica) is a giant at nearly 1.5 in (4 cm) in length.

In life, specimens appeared mostly transparent (colorless), except for the bright red eyes. When viewed with the naked eye the thoracic region appears almost white. All leptostracans, except Nebaliopsis typica, show pronounced sexual dimorphism. Males also occur much more rarely than females. The carapace of the male is less deep than that of the female and the antennae are much longer.

Distribution

The genera of Leptostraca are distributed differently. Nebalia is cosmopolitan. Nebaliella is confined to cold waters, being found in Antarctica, southern Australia, and the high latitudes of the Northern Hemisphere. Nebaliopsis is a pelagic genus with a worldwide distribution. Paranebalia is found in central America, Bermuda, New Caledonia, and Australia. Levinebalia has been recorded only in Australia and New Zealand. Dahlella was collected from hydrothermal vents near the Galápagos, and Speonebalia has been recorded from only marine caves in the Turks and Caicos Islands.

Habitat

All leptostracans are marine, and have been recorded in waters from 3.2 ft (1 m) deep to more than 6,561 ft (2,000m); most species occur in <656 ft (<200 m). Many leptostracans, with the exception of the pelagic Nebaliopsis typica, seem to prefer mud bottoms that are low in oxygen content. However, leptostracans can be found in a variety of habitats ranging from the intertidal zone to the abyssal deeps.

Behavior

Young animals and females rest in the same place for hours, their thoracic limbs beating rhythmically, moving respiratory currents of water through the carapace. The mature males may swim long distances propelled by the four anterior pairs of pleopods. Each of these pleopods is hooked to its partner so that each pair functions as a unit when swimming.

When placed in laboratory tanks or small, concave "dishes" of glassware (watch glasses) with mud, leptostracans will swim right to the bottom and burrow in. Frequently, not one part of the body is moved at all, and even the heartbeat slows down; these are functional and physiological adaptations to living in low-oxygen environments.

Feeding ecology and diet

Living in muddy environments, often where organic matter is abundant, it is assumed that leptostracans are filter feeders, but they are often taken in baited traps, which indicates that they act as at least opportunistic scavengers.

Most of the benthic leptostracans suspension feed by stirring up bottom sediments. They are also capable of grasping relatively large bits of food (detritus particles, dead animals) directly with the mandibles.

Reproductive biology

The eggs of all leptostracans, except possibly in Nebaliopsis, are carried by the female in the thoracopod brood chamber beneath her carapace. Development is embryonic and the young hatch as post larvae. These post-larval, or mancoid, stages differ from the adults in having a rudimentary fourth pleopod. The free-swimming, pelagic larva of Nebaliopsis has been found in plankton collections. In this genus, the eggs may not be carried in a brood pouch but laid directly into the water.

Water temperatures influence the length of time taken to reach maturity, the size at maturity, and the incubation time of young.

Transformations in the shape of the carapace, antennae, pleopods, and furca are gradual from molt to molt in immature and subadult males, and deviate from the female morphology, which generally remains unchanged except when reproductive.

Conservation status

Nothing is known. No species are listed by the IUCN.

Significance to humans

Nebalia bipes is considered to be desirable as a living fish food and for larval rearing because of its suitable size and tolerance to the deterioration of bottom conditions.

Species accounts

Resources

Brusca, R. C., and G. J. Brusca. Invertebrates, 2nd ed. Sunderland, MA: Sinauer Associates, Inc., 2003.

Calman, W. T. "Crustacea." In A Treatise on Zoology, Part 7, third fascicle, edited by R. Lankester. London: Adam and Charles Black, 1909.

Cannon, H. G. "Leptostraca." In Klassen und Ordnungen des Tierreichs, Vol. 5, Section 1, Book 4, no. 1, edited by H. G. Bronn. Leipzig, Germany: Akademische Verlagsgesellschaft, 1960.

Dahl, E., and J.-W. Wägele. "Sous-classe des Phyllocarides (Phyllocarida Packard, 1879)." In Traité de Zoologie. Anatomie, Systématique, Biologie. Crustacés, Tome VII, Fascicule II, edited by J. Forest. Paris: Masson et Cie, 1996.

Fretter, V., and A. Graham. A Functional Anatomy of Invertebrates. London: Academic Press, 1976.

Haney, T. A., and J. W. Martin. "Leptostraca." In Common and Scientific Names of Aquatic Invertebrates of the United States and Canada, edited by D. Turgeon. Bethesda, MD: American Fisheries Society Special Publication, forthcoming.

Hessler, R. R., and F. R. Schram. "Leptostraca as Living Fossils." In Living Fossils, edited by N. Eldredge and M. Stanley. Berlin: Springer-Verlag, 1984.

Kaestner, A. Invertebrate Zoology, Vol. 3, Crustacea. New York: Wiley-Interscience, 1970.

McLaughlin, P. A. Comparative Morphology of Recent Crustacea. San Francisco, CA: W. H. Freeman and Company, 1980.

Rolfe, W. D. I. "Phyllocarida." In Treatise on Invertebrate Paleontology, Part R, Arthropoda 4, Vol. 1, edited by R. C. Moore. Lawrence: Geological Society of America and University of Kansas Press, 1969.

Ruppert, E. E., and R. D. Barnes. Invertebrate Zoology, 6th ed. Fort Worth, Texas: Saunders College Publishing, 1994.

Schmitt, W. L. Crustaceans. Ann Arbor: University of Michigan Press, 1965.

Schram, F. R. Crustacea. New York: Oxford University Press, 1986.

Brahm, C., and S. R. Geiger. "On the Biology of the Pelagic Crustacean Nebaliopsis typica G. O. Sars." Bulletin of the Southern California Academy of Sciences 65 (1966): 41–46.

Dahl, E. "Crustacea Leptostraca, Principles of Taxonomy and A Revision of European Shelf Species." Sarsia 70 (1985): 135–165.

Hessler, R. R. "Dahlella caldariensis, New Genus, New Species: A Leptostracan (Crustacea, Malacostraca) from Deep-sea Hydrothermal Vents." Journal of Crustacean Biology 4, no. 4 (1984): 655–664.

Martin, J. W., and G. E. Davis. "An Updated Classification of the Recent Crustacea." Natural History Museum of Los Angeles County Science Series 39 (2001): 1–124.

Martin, J. W., E. W. Vetter, and C. E. Cash-Clark. "Description, External Morphology, and Natural History Observations of Nebalia hessleri, New Species (Phyllocarida: Leptostraca), from Southern California, with a Key to the Extant Families and Genera of the Leptostraca." Journal of Crustacean Biology 16, no. 2 (1996): 347–372.

Olesen, J. "A New Species of Nebalia (Crustacea, Leptostraca) from Unguja Island (Zanzibar), Tanzania, East Africa, with a Phylogenetic Analysis of Leptostracan Genera." Journal of Natural History 33, no. 12 (1999): 1789–1809.

Schram, F. R., and E. Malzahn. "The Fossil Leptostracan Rhabdouraea bentzi." Transactions of the San Diego Society of Natural History 20 (1984): 95–98.

Spears, T., and L. G. Abele. "Phylogenetic Relationships of Crustaceans with Foliaceous Limbs: An 18S rDNA Study of Branchiopoda, Cephalocarida, and Phyllocarida." Journal of Crustacean Biology 19 (1999): 825–843.

Walker-Smith, G. K. "Levinebalia maria, a New Genus and New Species of Leptostraca (Crustacea) from Australia." Memoirs of Museum Victoria 58, no. 1 (2000): 137–148.

Walker-Smith, G. K., and G. C. B. Poore. "A Phylogeny of the Leptostraca (Crustacea) with Keys to the Families and Genera." Memoirs of Museum Victoria 58, no. 2 (2001): 383–410.

"Anchialine Fauna of the Bahamas (Speonebalia cannoni)." April 16, 2003 [July 30, 2003.] . "The Biology of Sea Fleas." [July 30, 2003.] . "Leptostraca." [July 30, 2003.] . "Leptostraca—Sea Fleas (Tree of Life Web Project)." 2002 [July 30, 2003.] . "Leptostracan Morphology…A Look with the Scanning Electron Microscope." [July 30, 2003.] .

Lowry, J. K. "Crustacea, the Higher Taxa—Leptostraca (Malacostraca)." October 2, 1999 [July 30, 2003.] .

[Article by: Estela C. Lopretto, PhD]

A subclass of the crustacean class Malacostraca containing the extant order Leptostraca and the fossil order Archiostraca. The Phyllocarida has a long fossil record, and many early fossil taxa were referred to this subclass. However, studies of presumed phyllocarids from the Burgess Shale have shown that only the archiostracans agree with the definition of the Phyllocarida.

Phyllocarids are distinct from other malacostracan crustaceans because of two other characteristics considered to reflect the primitive condition, which strengthen the hypothesis of early separation from the main evolutionary line. The first is the presence of a bivalve carapace. The second is an abdomen consisting of seven fully formed somites and terminating in a telson that bears caudal rami. See also Crustacea; Leptostraca; Malacostraca.