Slimehead

Taxobox
name = Slimeheads or roughies


image_width = 200px
image_caption = Orange roughy, "Hoplostethus atlanticus"
regnum = Animalia
phylum = Chordata
classis = Actinopterygii
ordo = Beryciformes
familia = Trachichthyidae
subdivision_ranks = Genera
subdivision = "Aulotrachichthys"
"Gephyroberyx"
"Hoplostethus"
"Optivus"
"Paratrachichthys"
"Parinoberyx"
"Sorosichthys"
"Trachichthys"

Slimeheads, also known as roughies and redfish, are mostly small, exceptionally long-lived, deep-sea beryciform fish constituting the family Trachichthyidae (derived from the Greek "trachys" ["rough"] and "ichthys" ["fish"] ). Found in temperate to tropical waters of the Atlantic, Indian, and Pacific Ocean, the family comprises approximately 45 species in eight genera. Slimeheads are named for the network of muciferous canals riddling their heads.

The larger species — namely the orange roughy ("Hoplostethus atlanticus") and Darwin's slimehead ("Gephyroberyx darwinii") — are the target of extensive commercial fisheries off Australia and New Zealand. Many populations have already crashed, while others are showing signs of severe overfishing; due to slimeheads' slow rate of reproduction, the future viability of these fisheries has been put into question. Orange roughies are food fish and are marketed fresh and frozen, whereas Darwin's slimeheads are utilised for their oil and made into fishmeal.

Physical description

With a typically deep-bodied, laterally compressed form, slimeheads are conspicuous for their large, titular heads, large eyes, and (in some species) bright colours. The head is especially notable for its network of mucus-filled canals, which constitute the cranial portion of the lateral line system. Similar cranial networks are found in the beryciform fangtooths (Anoplogastridae) and the stephanoberyciform ridgeheads (Melamphaidae). The trachichthyid head is typically blunt with a large and oblique mouth; the snout may project slightly in front of the upper jaw. A short, sharp spine is present on the preoperculum and/or operculum and posttemporal bone, the latter spine directed posteriorly. Species of the genera "Optivus", "Paratrachichthys", and "Sorosichthys" differ in form from other members of the family; their bodies are more elongate.All fins are spinous (excluding the low-slung pectoral fins) and rounded: there is a single dorsal fin with 3–8 spines and 10–19 soft rays; the pelvic fins are thoracic with one spine and 6–7 soft rays; the anal fin has 2–3 spines and 8–12 soft rays; and even the forked caudal fin possesses 4–7 procurrent spines on each lobe. The scales of slimeheads are ctenoid but vary interspecifically; they range from deciduous to adherent. In most species the ventral scales between the pelvic fin and anus have been modified into a median ridge of large, bony scutes. The lateral line is uninterrupted and fairly obvious; its pores are largely obscured by the scales' well-developed spinules or "ctenii".

Slimeheads range from a bright brick red with identically shaded fins, to dusky grey or silver, to black with dusky grey to transparent fins. The reds quickly fade to orange following death. Some species (e.g., "Aulotrachichthys latus") are reported to be bioluminescent, probably via symbiotic bacteria as is found in other beryciform fish. The largest species is the orange roughy at a maximum standard length (SL; a measurement excluding the caudal fin) of 75 cm and a weight of 7 kg; however, most slimeheads are well under 30 cm SL.

Life history

Most slimeheads are sluggish and demersal, spending most of their time near the bottom of continental slopes. Cold, moderate benthopelagic depths (ca. 100 – 1,500 m) with usually hard, rocky substrates are frequented. The most elongate species are typically the most active and frequent the shallowest depths; for example, the slender roughy ("Optivus elongatus") is found in photic coastal waters and is associated with rocky reefs. This species is nocturnal and hides in crevices during the day. "Trachichthys australis" is of the same habitus, but is rather deep-bodied and resembles a soldierfish. Both young and adult slimeheads feed primarily upon zooplankton such as mysid shrimp, amphipods, euphausiids, prawns and other crustaceans, as well as larval fish. Slimeheads store energy as extracellular wax esters, which aid the fish in maintaining neutral buoyancy.

Slimehead behaviour is not well studied, but some species sporadically form dense aggregations. In the case of the orange roughy, these aggregations (possibly segregated according to sex) may reach a population density of 2.5/m²;. The aggregations form in and around geologic structures, such as undersea canyons and seamounts, likely where water movement and mixing is high, ensuring dense concentrations of prey items. The aggregations do not necessarily form for the purpose of spawning; it is thought that the fish cycle through metabolic phases (feeding and resting) and seek areas with ideal hydrologic conditions to congregate during their inactive and active phases. Observations of orange roughy aggregations during submersible dives have also shown that the fish lose almost all pigmentation while inactive, during which time they are very approachable. The orange roughy's metabolic phases are thought to be related to seasonal variations in the fishes' prey concentrations, with the inactive phase being a means to conserve energy during lean periods.

Slimeheads are non-guarding pelagic spawners; that is, spawning aggregations are formed and the fish release eggs and sperm en masse directly into the water. There is evidence of oceanodromy (seasonal migration) in some species. The fertilized eggs (and later the larvae) are planktonic, floating with the currents until the larvae develop the strength to determine their own way. Only the economically important species have had their reproduction studied in any detail: the larvae and juveniles of Darwin's slimehead are pelagic and frequent rather shallow waters near the coast, whereas in orange roughy the early life stages are apparently confined to deeper water (ca. 200 metres). Slimeheads are very slow-growing and long-lived fish; the orange roughy ranks among the longest-lived animals known, with a maximum reported age of 149 years (however, this age is disputed). Predators of slimeheads are not well known, but include large deep-roving sharks; cutthroat eels; merluccid hakes, and snake mackerels.

Species

There are 45 species in eight genera:
* Genus "Aulotrachichthys"
** "Aulotrachichthys latus" (Fowler, 1938).
* Genus "Gephyroberyx"
** Darwin's slimehead, "Gephyroberyx darwinii" (Johnson, 1866).
** "Gephyroberyx japonicus" (Hilgendorf, 1879).
** "Gephyroberyx philippinus" Fowler, 1938.
* Genus "Hoplostethus"
** "Hoplostethus abramovi" Kotlyar, 1986.
** Orange roughy, "Hoplostethus atlanticus" Collett, 1889.
** Black slimehead, "Hoplostethus cadenati" Quéro, 1974.
** "Hoplostethus confinis" Kotlyar, 1980.
** "Hoplostethus crassispinus" Kotlyar, 1980.
** "Hoplostethus druzhinini" Kotlyar, 1986.
** "Hoplostethus fedorovi" Kotlyar, 1986.
** "Hoplostethus fragilis" (de Buen, 1959).
** "Hoplostethus gigas" McCulloch, 1914.
** "Hoplostethus intermedius" (Hector, 1875).
** Flintperch, "Hoplostethus japonicus" Hilgendorf, 1879.
** Giant sawbelly, "Hoplostethus latus" (Fowler, 1938).
** "Hoplostethus marisrubri" Kotlyar, 1986.
** Mediterranean slimehead, "Hoplostethus mediterraneus mediterraneus" Cuvier, 1829.
** "Hoplostethus mediterraneus sonodae" Kotylar, 1986.
** Silver roughy, "Hoplostethus mediterraneus trunovi" Kotylar, 1986.
** "Hoplostethus melanopterus" Fowler, 1938.
** Smallscale slimehead, "Hoplostethus melanopus" (Weber, 1913).
** "Hoplostethus mento" (Garman, 1899).
** "Hoplostethus metallicus" Fowler, 1938.
** "Hoplostethus mikhailini" Kotlyar, 1986.
** "Hoplostethus occidentalis" Woods, 1973.
** "Hoplostethus pacificus" Garman, 1899.
** "Hoplostethus rifti" Kotlyar, 1986.
** "Hoplostethus rubellopterus" Kotlyar, 1980.
** "Hoplostethus shubnikovi" Kotlyar, 1980.
** "Hoplostethus tenebricus" Kotlyar, 1980.
** "Hoplostethus vniro" Kotlyar, 1995.
* Genus "Optivus"
** Slender roughy, "Optivus elongatus" (Günther, 1859).
* Genus "Paratrachichthys"
** "Paratrachichthys argyrophanus" Woods, 1961.
** "Paratrachichthys atlanticus" (Collett, 1889).
** "Paratrachichthys fernandezianus" (Günther, 1887).
** "Paratrachichthys heptalepis" Gon, 1984.
** "Paratrachichthys novaezelandicus" Kotlyar, 1980.
** "Paratrachichthys prosthemius" Jordan & Fowler, 1902.
** "Paratrachichthys pulsator" (Gomon & Kuiter, 1987).
** "Paratrachichthys sajademalensis" Kotlyar, 1979.
** Sandpaper fish or common roughy, "Paratrachichthys trailli" (Hutton, 1875).
* Genus "Parinoberyx"
** "Parinoberyx horridus" Kotlyar, 1984.
* Genus "Sorosichthys"
** Little pineapple fish, "Sorosichthys ananassa" Whitley, 1945.
* Genus "Trachichthys"
** "Trachichthys australis" Shaw, 1799.

References

*
* [http://journals.cambridge.org/bin/bladerunner?30REQEVENT=&REQAUTH=0&500000REQSUB=&REQSTR1=S0025315402005519 "Habitat, behaviour and colour patterns of orange roughy "Hoplostethus atlanticus" (Pisces: Trachichthyidae) in the Bay of Biscay"] "Journal of the Marine Biological Association of the UK" (2002), 82:321-331. Pascal Lorance, Franz Uiblein, and Daniel Latrouite. Retrieved March 2, 2005.
* "Fishes: An introduction to ichthyology". Peter B. Moyle and Joseph J. Cech, Jr; p. 365. Printed in 2004. Prentice-Hall, Inc; Upper Saddle River, NJ. ISBN 0-13-100847-1