Thorntail Pipefish, Micrognathus pygmaeus (Fritzsche 1981)

In Australian waters, known from Cocos (Keeling) and Christmas islands, the Timor Sea, North West Cape, WA, and from the Great Barrier Reef, Cape York to Southport, Queensland. Elsewhere in the tropical western Pacific, from eastern Indonesia, Australia, Papua New Guinea, New Caledonia, Marshall Island, Bikini Atoll and the Ryukyu Islands, Japan. 

Inhabits sheltered coral reef lagoons, inner and seaward reefs, usually in crevices, small caves and gutters, mostly below 10m, but collected as deep as 20m.

Meristic features: Dorsal fin 18-21; Anal fin 3; Pectoral fin 10-13 (usually 11-12); Trunk rings 15-16; Tail rings 28-31; Subdorsal rings 1.5-0.25 + 2.75-4.25 = 3.75-5.0.

Body slender, elongate; head length 7.2-10.2 in SL, snout short, 2.6-3.3 in HL, snout depth 1.8-3.0 in snout length; median dorsal snout ridge low, entire, somewhat concave in lateral profile; longitudinal opercular ridge complete in young, incomplete to obsolete in subadults-adults.

Superior trunk and tail ridges discontinuous, inferior trunk ridge ends on anal ring, lateral trunk ridge continuous with inferior tail ridge; head and body rings without spines, denticules or serrations; scutella not keeled; principle ridges of posterior tail rings flared laterally with the posterior angles produced to hook-like points.

Dorsal fin origin between anterior margin of second last trunk ring and posterior margin of 1^st tail ring, the fin base not clearly elevated in subadults-adults.

Maximum length 65 mm.

Overall pale reddish to brown, often with irregular narrow bars on back and uppersides; lower sides and underside of trunk and tail often with dark bars; near-black blotches sometimes on lateral ridge of the 3^rd-4^th and the 7^th-8^th trunk rings. Specimens larger then 30 mm SL typically with a patch of 1-20 small dark spots on the opercle, spots most numerous in adult males. Fins usually flecked or shaded with brown.

Likely to feed on small crustaceans.

Males brood eggs in a brood pouch under the tail; pouch plates and pouch folds are present with an everted pouch closure; some males begin brooding at 23-25 mm SL, and most are brooding at 40 mm SL.

Brood pouch eggs are usually deposited in transverse rows in 1-2 layers, with up to 37 eggs.

Late yolk-sac larvae with well-developed dorsal and caudal fins, pectoral and anal fins are present. Planktonic young (13-15 mm SL) lack dermal flaps, have pointed posterior angles on most rings and the dorsal fin base is slightly elevated.

Of no interest to fisheries or aquaculture.

Australian Commonwealth legislation: Marine listed under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act).

The Thorntail Pipefish was previously called Micrognathus brevirostris, a species found only in the Western Indian Ocean.

M. pygmaeus differs from M. brevirostris in having more pectoral fin rays (modally 11-12 versus 10) and in having a lower average number of subdorsal tail rings (3.4 versus 4.2). Although both species have bars on the lower half of the trunk, M. brevirostris lacks the dark spots on the opercle that are usually present in M. pygmaeus.

Micrognathus from the Greek micro meaning small and the Greek gnathus meaning jaw.

Dianne J. Bray & Vanessa Thompson