Latin name

Channichthys mithridatis

Other name

Channichthys mithridatis

Identification

The green icefish is distinguished from other species of the genus Channichthys by the following set of characters. In the dorsal (upper) lateral line 61-78 tubular bony segments (scales), in the posterior part of the medial (middle) lateral line 7-17 tubular bony segments; 11-16 gill stamens in the outer row of the lower part of the arch; vertebrae 55-58, including 23-25 trunk vertebrae and 31-33 caudal vertebrae.

Interorbital space narrow (10-13% of head length), markedly concave, with outer margins of frontal bones distinctly raised above orbit. The posterior edge of the jawbone extends well posteriorly to a vertical line through the centre of the orbit or beyond. 

Granulation (tuberculation) on the frontal bones and barbs of the first dorsal fin is absent or very weak, represented by smooth bony granules; on the maxilla and anterior half of the dentary, as well as on the rays of the gill membrane, granulation is completely absent. 

In the anterior part of the medial lateral line, rounded, punctate bone plates (bone scales) are usually absent or, if present, they are in the form of very thin, flexible, translucent plates that are barely visible. 

In the lower part of the first gill arch, stamens only form the outer row; on the inner side of the arch, stamens are usually absent or occasionally 1-2 stamens are found in the corner of the arch (observed in only 2 fish out of 29 examined).

Features of fish fins

The first dorsal fin has 6-9, usually 7-8 flexible barb rays, of which the 2nd and 3rd are the largest; the second dorsal fin has 32-34 rays; the anal fin has 30-32, usually 31-32 rays; the pectoral fin has 19-21, usually 20-21 rays. 

The first dorsal fin is very high, its height is 2.9-4 times the standard length of the fish, more or less triangular in shape (not trapezoidal), with a very low fin fold, reaching a height not higher than 3/4 of the length of the largest barb. The first and second dorsal fins are separated by a wide interdorsal gap. 

Fish colouring

The overall body colour of live fish varies from greenish to dark olive, with a darker top of the head. In mature post-spawning males, the colouration is generally much darker than in post-spawning females (sexual dimorphism and mating habits): the lower part of the body below the midline, the cheeks and gill covers are dark, and in some males almost black. On the sides of the body there are 3-4 broad vertical dark stripes, sometimes barely visible. The lower part of the body, including the narrow areas along the base of the anal fin and anal fin, is white, without any sign of pigmentation; on the bases of the pectoral fins and on the chest between the bases of the pelvic fins are silvery white spots. The spines of the first dorsal fin are dark green or grey; the colour of the dorsal fin fold varies from greyish black to black with silvery-grey areas along the spines. The rays of the second dorsal, pectoral and caudal fins are pale, grey-green; the fin pleats are greenish, transparent. In formalin-fixed fish, the overall green colouration of the body changes to grey-brownish or light brown.

Distribution

The known range of the species includes the coastal marine waters around the Kerguelen Islands (endemic). 

Habitat

Autochthonous subantarctic demersal or benthic pelagic fish. Relatively shallow, recorded in 2 demersal trawls at depths of 250-310 m.

Size

It belongs to the group of medium-sized species of the genus Channichthys. The largest specimen, the post-spawning female, reaches 437 mm total length and 387 mm standard length. The largest male (holotype) reaches 371 mm total length and 332 mm standard length.

Behavior

Judging by the rather runny "light" body shape, the weak bony tubercles of the skin and head cover bones, and the typical pelagic colouration, the lifestyle of the green icefish is more closely related to the benthic biotope and probably to the pelagic layer of the water than to a permanent presence on the surface, as observed in typical bottom-dwelling species of the genus.

Food and feeding habits

By mode and feeding strategy it is a predatory ichthyophagous fish hunter, apparently predominantly above ground in the pelagial zone. All known specimens of this species have been caught on dense, rocky bottoms devoid of any benthic (sedentary or wandering) fauna suitable for feeding. As in other predatory relatives, their gill stamens are relatively few and located only on the outside of the lower part of the first gill arch.

Reproduction

Females of C. mithridatis do not reach sexual maturity until they are about 30-32 cm long (26-28 cm standard length). Spawning occurs during winter in the southern hemisphere, probably in May-June. Most of the fish caught on 10 August 1990 with a total length of 33-34 cm were already spawned and had post-spawning gonads at maturity stages VI-II (males) and VI-III (females).

Fishing

Green icefish are caught with bottom trawls.

Relationship with a person

Harmless.