Lophogastrida

(Lophogastrids)

Phylum Arthropoda

Subphylum Crustacea

Class Malacostraca

Order Lophogastrida

Number of families 2

Thumbnail description
Small shrimplike marine crustaceans with elongate carapaces covering, but not fused to, all thoracic somites (segments); stalked eyes; and strongly developed swimming legs on the first five abdominal somites

Evolution and systematics

Lophogastrids appear to be most closely allied to the strictly fossil pygocephalomorphs; indeed, species recognizable as lophogastrid relatives are known from the Pennsylvanian Period, about 325–286 million years ago. In these groups the sternites (ventral shields) are wide, with the thoracic legs being located on the outer edges of the somites.

The order Lophogastrida contains two extant families, the Lophogastridae with six genera, and the Eucopiidae with the single genus Eucopia. Many scientists now regard lophogastrids as a suborder of the order Mysidacea. They differ from the other mysid suborder in having gills on most thoracic appendages; no statocyst (organ governing the sense of balance) in the uropod; and no modifications of the anterior pleopods in the males. Some authors have suggested that any resemblance to other mysids is purely superficial and that the lophogastrids should have their own order. The latter classification is followed here.

Physical characteristics

The body of a lophogastrid is long and shrimplike, with the head and thorax covered by a loosely fitting carapace. The carapace is extended in front as a rostrum, which is an anatomical structure resembling a bird's beak. The rostrum can be very large in one genus of lophogastrids. The rear portion of the carapace may cover from five to seven thoracic somites. It extends over the sides of the animal to the bases of the thoracic legs. Both pairs of lophogastrid antennae are biramous; that is, they have two branches. All lophogastrids have eyes on moveable stalks. The antenna (=second antenna) exopod (outer branch) is shaped like a large scale. The mandible, or lower jaw, is of the rolling crushing type, with a toothed incisor and large crushing molar. The mandible palp, which is a jointed sensory appendage, can be rather large and extends upwards in front of the head to the area between the antennular peduncles (small stemlike process at the base of an antenna). In some genera, at least, the maxillules, or first pair of maxillae, possess a backwardly-directed endopod (posterior process) in the form of a palp. On the maxilla (upper jaw), the exopod forms a large setose (bristly) lobe that resembles the scaphognathite (lateral flap on the second maxilla) of decapod crustaceans.

The thoracic limbs of lophogastrids are modified in various ways; however, all have well-developed endopods and exopods. The first pair is always modified as a maxilliped because the first thoracic somite is incorporated into the head. In essence, such modifications usually involve shortening of the endopod as well as the development of specialized lobes and setae for handling food. The remaining thoracic limbs, now called pereopods, are numbered from one through seven. The pereopods are all usually similar to one another in the structure of the endopod as well as in the strength of the swimming setae (bristles) on the exopods. In the genus Eucopia, however, the first three pereopods are short and stout with subchelate (pincerlike) tips; the fourth through the sixth pereopods are highly modified as elongate grasping appendages; and the seventh pereopod is reduced to a limb with bristles. Most lophogastrid pereopods bear gills at their base. Oostegites, or brood plates, are present on all seven pairs of pereopods.

All lophogastrids, both males and females, have well-developed biramous propulsive pleopods on the first five abdominal somites, and broad flattened uropods on the sixth abdominal somite. The uropods do not possess statocysts. A flattened and somewhat pointed telson terminates the body. In males of the genus Gnathophausia, the endopod of the second pair of pleopods is slightly modified for sperm transfer.

Distribution

Lophogastrids occur in all oceans except the Arctic. They are generally bathypelagic (found below 3,280 ft [1,000 m]), but some species may be found in waters as shallow as 164 ft (50 m) and others as deep as 13,120 ft (4,000 m). Most of the known species are found primarily in the Pacific and Indian Oceans, but a few are common in the Atlantic.

Habitat

Members of this group occur in the pelagic oceanic zone (the open sea beyond the continental shelf).

Behavior

Few observations have been published on the behavior of living lophogastrids. They are obviously difficult to observe but have been reared in captivity recently at the Monterey Bay Aquarium in California. Because lophogastrids are bathypelagic they spend almost all their time swimming, using the pleopods for propulsion and the exopods to maintain a flow of water around the gills for respiration.

Feeding ecology and diet

The lophogastrids that have been investigated to date are almost all predators. Others whose feeding habits are not known seem to lack filter setae on their mouth appendages, suggesting that they too are predators. Only Gnathophausia seems to use its mouthparts to filter large particles from the seawater.

Reproductive biology

Lophogastrid mating has not been observed. Eggs are extruded into the female's ventral brood pouch where they are kept until hatching. When the young hatch, they possess a full set of appendages on the thorax.

Conservation status

No species are listed by the IUCN.

Significance to humans

Lophogastrids are undoubtedly eaten by various pelagic fishes, some of which are commercially important.

Species accounts

List of Species

Giant red mysid

Gnathophausia ingens

family

Lophogastridae

taxonomy

Gnathophausia ingens Dohrn, 1870.

other common names

English: Deep water giant mysid; German: Rote Riesengamele.

physical characteristics

The carapace possesses a very long rostrum, which extends almost to the end of the antennule, and a long posterior spine that reaches to the end of the second abdominal somite. The carapace is also folded inward along the ventral margin, creating a semi-enclosed gill chamber. The antennal scale is also very long, unsegmented, and sharply pointed at its apex. Gnathophausia ingens is the largest pelagic crustacean known, reaching a length of 12.2 in (31 cm). (Illustration shown in chapter introduction.)

distribution

Giant red mysids are commonly found in deep waters below the tropical and subtropical waters of the Atlantic, Pacific, and Indian Oceans. Although they have been sampled between 656–13,123 ft (200–4,000 m), they are most common at depths of 1,965–4,920 ft (600–1500 m).

habitat

Bathypelagic in the oceanic zone (open sea).

behavior

Little is known about the behavior of this species.

feeding ecology and diet

Gnathophausia ingens apparently feeds on large particles filtered from the seawater. It also feeds on small dead aquatic organisms.

reproductive biology

Gnathophausia ingens has a very long period of larval development, estimated to be about 530 days. Adult females probably have more than one brood, and live for almost 3000 days, based on growth estimate data. From hatching to adult, an individual giant red mysid passes through 13 instars (stages between molts).

conservation status

Not listed by the IUCN.

significance to humans

Probably serves as food for larger fishes that are harvested commercially.

Resources

Books

Nouvel, H., J.-P. Casanova, and J. P. Lagardère. Ordre des Mysidacés. Mémoires de l'institut océanographique. Monaco:[n.p.], 1999.

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

Periodicals

Childress, J. J., and M. H. Price. "Growth Rate of the Bathypelagic Crustacean Gnathophausia ingens (Mysidacea: Lophogastridae). I. Dimensional Growth and Population Structure." Marine Biology 50 (1978): 47–62.

Tattersal, O. S. "Mysidacea." Discovery Reports 28 (1955): 1–190.

Les Watling, PhD