Hammerjaw

Appearance

The large head is dominated by a massive, truncated lower jaw and large, high-set eyes. The lower jaw has a dark, almost black distal end, "chin". The lower jaw possesses at least one pair of oversized, transparent, and dagger-like teeth; the palatines possess 1–4 pairs of slightly smaller teeth. The body itself is scaleless and laterally compressed; it is covered in iridescent, silvery-gray guanine with the dark peritoneum peeking through in places. Dark smoky gray dorsally, the body tapers strongly towards a thin caudal peduncle and deeply forked caudal fin. The caudal peduncle is a smoky black color, darker than the body and tail. All fins are spineless; both the low-slung pectoral fins and abdominal pelvic fins are fairly small.

The single dorsal fin and anal fin are roughly equal in size; the anal fin's origin lies immediately posterior to the dorsal. The lateral line is uninterrupted and the gill rakers number 20–25. Like other members of their order, hammerjaws also possess a small adipose fin. The largest recorded hammerjaw measured 23 cm standard length.

Evolution

Very little is known of the hammerjaw's life history. It inhabits the mesopelagic and bathypelagic zones down to 4,000 m and is never caught in large numbers. It is inferred from their sporadic capture and sleek morphology that hammerjaws are swift swimmers—capable of avoiding sampling nets. Hammerjaws appear to spawn year-round; like many other deep-sea aulopiform fish, they are hermaphrodites. They are carnivorous and feed on squid and other pelagic fish; in turn, hammerjaws are known prey of lancetfish and tuna.