Polymixiiformes

(Beardfishes)

Class Actinopterygii

Order Polymixiiformes

Number of families 1

Evolution and systematics

Beardfishes of the order Polymixiiformes comprise a single living family, Polymixiidae, containing one genus, Polymixia, and, according to the revisional study of Kotlyar (1993), ten species. These seemingly nondescript fishes have a remarkably checkered taxonomic history, and few fishes have been shifted back and forth in different phylogenetic schemes as have these poorly understood animals. Although most systematists agree that Polymixiiformes are basal acanthomorphs, their precise placement at the base of this huge radiation of spiny-rayed fishes remains a mystery. In view of the longstanding confusion regarding the phylogenetic position of beardfishes, it should be clear that the small size of the order is no indication of their evolutionary importance. Although the group is represented today by a single genus, the Polymixiiformes have a much more diverse fossil record, and at least two families, the Aipichthyidae and Polymixiidae, containing some six genera, are currently recognized.

Polymixiiforms appear first in the fossil record in the late Cretaceous period, some 95 million years ago, and have been viewed as being of considerable importance to our under-standing of the spiny-rayed fishes and their evolution. In 1964 the noted British paleoichthyologist Colin Patterson reviewed polymixiiform fossil diversity and discussed the striking similarities between them and certain families of living perciforms such as carangids (jacks) and monodactylids (moonfishes). Whether these similarities reflect the common phenomenon of convergent evolution or are the result of recent common ancestry remains an open question. Whatever the case, such an ongoing confusion serves to highlight the importance of Polymixiiformes to our understanding of the evolution of spiny-rayed fishes.

Physical characteristics

Polymixia are rather deep-bodied fishes, with prominent blunt snouts and large eyes. Their dorsal and anal fins bear well-developed spines, but a spine is lacking in the subabdominally positioned pelvic fins, which have a single, segmented, leading ray and six branched rays. All beardfishes are characterized by the possession of a pair of hyoid barbels. These "chin" barbels are internally supported by three bran-chiostegal rays of the hyoid arch, and their characteristic appearance lends their bearers the name "beardfishes." A 2001 study by Kim and his colleagues highlights the unique nature of the anatomy and muscular control of beardfish barbels.

Distribution

Beardfishes are known to occur in the Atlantic, Indian and Western Pacific oceans, as well as in tropical and subtropical waters. Worldwide collection data for Polymixia species are poor, and sampling in many regions is limited or nonexistent. When caught, most species are taken at depths ranging from 492–2,132.5 ft (150–650 m), but catches from as shallow as 164 ft (50 m) have been recorded.

Habitat

Very little is known of the habitat preferences of Polymixia species, but the presence of chin barbels suggests a bottom-dwelling mode of life, probably over sandy or muddy substrates.

Behavior

Nothing is known.

Feeding ecology and diet

The stomach of Polymixia is thick walled and muscular, often with over 100 pyloric ceca (pouches). Analyses of the gut contents indicate an opportunistic diet of crustaceans, squid, and small fishes. Interestingly, beardfishes are recorded among the stomach contents of the Indian Ocean coelacanths, as well as in those of a wide range of other fish predators.

Reproductive biology

Polymixia eggs are unknown, and the smallest individual so far identified is a 0.15 in (0.39 cm) postflexion larva, which was described in some detail by Konishi and Okiyama, who noted the presence of moderately developed head spination similar to that found in many beryciform fishes. Reproductive biology is virtually unknown, but sexual dimorphism in coloration has been noted in P. lowei, where an intense black marking on the anal fin and both lobes of the tail fin are present only in male fish.

Conservation status

None of the Polymixia species are included on the IUCN Red List. Although catches of some species are relatively high in some regions, there are no indications of overfishing.

Significance to humans

Beardfishes are marketed for human consumption in most regions; however, commercial catches are limited but growing. As catches of other species have declined, a number of Pacific beardfish species have been tagged as having unexploited fisheries potential.

Species accounts

List of Species

Stout beardfish

Polymixia nobilis

family

Polymixiidae

taxonomy

Polymixia nobilis Lowe, 1836, off Madeira.

other common names

Portuguese: Salmonete do Alto.

physical characteristics

Maximum length 19.6 in (50 cm). Deep-bodied, with a prominent blunt snout and large eyes. The ctenii on the scales are arranged in wedgelike rows, and the hyoid barbels are long and filamentous. Dark bronzy-grey dorsally and silvery ventrally.

distribution

Found only in Atlantic Ocean. In western Atlantic, they occur from Norfolk south to Lesser Antilles, Cuba, and Bahamas, where they are sympatric with Polymixia lowei. The stout beardfish is the only beardfish in the eastern Atlantic, where it occurs from the Azores and Canary Islands south to St. Helena. Records of Polymixia nobilis from the Pacific are now known to be misidentifications.

habitat

Semihard and soft bottoms on the continental shelf and slope.

behavior

Nothing is known.

feeding ecology and diet

Feeds on crustaceans, squid, and small fishes.

reproductive biology

Eggs are unknown and reproductive biology is virtually unknown.

conservation status

Not listed by the IUCN.

significance to humans

Stout beardfishes are of some commercial importance, particularly in Madeira, where they are marketed fresh and frozen.

Resources

Periodicals

Kim, B. J., M. Yabe, and K. Nakaya. "Barbels and Related Muscles in Mullidae (Perciformes) and Polymixiidae (Polymixiiformes)." Ichthyological Research 117 (2001): 409–413.

Konishi, Y., and M. Okiyama. "Morphological Development of Four Trachichthyoid Larvae (Pisces: Beryciformes), with Comments on Trachichthyoid Relationships." Bulletin of Marine Science 60 (1997): 66–88.

Kotlyar, A. N. "A New Species of the Genus Polymixia from the Kyushu-Palau Submarine Ridge and Notes on the Other Members of the Genus." Journal of Ichthyology 2 (1993): 30–49. Patterson, C. "A Review of Mesozoic Acanthopterygian Fishes, with Special Reference to Those of the English Chalk." Philosophical Transactions of the Royal Society of London 3 (1994): 213–482.

Other

"Coelacanths: Coelacanth Fact Sheet." South African Institute for Aquatic Biodiversity. Ichthos, 2001. (March 20, 2003). <http://www.saiab.ru.ac.za/educoel1.htm>

Melanie Stiassny, PhD