Slimehead

Slimeheads—also called roughies or redfish—are mostly small, deep-sea fish that live a very long time. They belong to the family Trachichthyidae (from Greek words meaning "rough fish") and are found in temperate and tropical waters across the Atlantic, Indian, and Pacific Oceans. The family includes about 50 species in eight genera. Their name comes from the network of mucus-filled canals that riddle their heads. The larger species, especially the orange roughy and Darwin's slimehead, are heavily fished commercially off Australia and New Zealand. Many of their populations have already collapsed, and others show severe overfishing. Because slimeheads reproduce so slowly, the future of these fisheries is uncertain. Orange roughies are sold fresh or frozen as food fish, while Darwin's slimeheads are used for oil and fishmeal.

Slimeheads typically have deep, flattened bodies, large heads, big eyes, and (in some species) bright colors. The head is notable for its mucus-filled canals, which are part of the lateral line system. Similar head canals appear in fangtooths and ridgeheads. The head is usually blunt, with a large, slanted mouth; the snout may stick out slightly past the upper jaw. Short, sharp spines sit on the preoperculum, operculum, and post-temporal bone, with the last spine pointing backward. Species in the genera *Optivus*, *Paratrachichthys*, and *Sorosichthys* have more elongated bodies than other slimeheads.

All fins are spinous (except the low pectoral fins) and rounded. The single dorsal fin has three to eight spines and 10–19 soft rays; the pelvic fins are placed on the thorax, with one spine and six or seven soft rays; the anal fin has two or three spines and eight to 12 soft rays; even the forked tail fin has four to seven procurrent spines on each lobe. Their scales are ctenoid but vary between species—some fall off easily, others stay attached. In most species, the scales on the belly between the pelvic fin and anus form a ridge of large, bony scutes. The lateral line is clear and unbroken, though its pores are mostly hidden by the scales' spiny projections.

Slimeheads range in color from bright brick red (with fins the same shade) to dusky gray or silver, to black with dusky gray or clear fins. The red fades quickly to orange after death. Some species, like *Aulotrachichthys latus*, are thought to be bioluminescent, likely from symbiotic bacteria, as seen in other beryciform fish. The largest species is the orange roughy, reaching a maximum standard length of 75 cm and weight of 7 kg; most slimeheads are well under 30 cm.

Most slimeheads are sluggish and live near the bottom of continental slopes, at cold, moderate depths of about 100 to 1,500 meters, usually over hard, rocky ground. The more elongated species tend to be more active and live in shallower water. For example, the slender roughy (*Optivus elongatus*) is found in sunlit coastal waters near rocky reefs; it is nocturnal and hides in crevices during the day. *Trachichthys australis* has a similar shape but is deeper-bodied, resembling a soldierfish. Both young and adult slimeheads eat zooplankton—mysid shrimp, amphipods, euphausiids, prawns, other crustaceans, and larval fish. They store energy as wax esters, which helps them stay neutrally buoyant.

Slimehead behavior is not well studied, but some species form dense groups from time to time. For orange roughy, these groups (possibly separated by sex) can reach a density of 2.5 fish per square meter. They gather around undersea canyons and seamounts, likely where water movement and mixing are high, bringing lots of prey. These groups do not necessarily form for spawning; instead, the fish may cycle through feeding and resting phases, seeking areas with good water conditions for each phase. Observations from submersible dives show that inactive orange roughy lose almost all their pigment and become easy to approach. Their metabolic phases may be tied to seasonal changes in prey, with the inactive phase helping them save energy when food is scarce.

Slimeheads spawn in open water: they form spawning groups and release eggs and sperm into the water. Some species show evidence of seasonal migration. The fertilized eggs and larvae drift with currents until the larvae are strong enough to swim on their own. Only the commercially important species have been studied in detail. Darwin's slimehead larvae and juveniles live in shallow coastal waters, while orange roughy early life stages stay in deeper water (around 200 m). Slimeheads grow very slowly and live a long time; the orange roughy is among the longest-lived animals known, with a maximum reported age of 149 years (though this is disputed). Their predators are not well documented.