Acentronura

Acentronura
	Shortpouch pygmy pipehorse (A. tentaculata)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Order:	Syngnathiformes
Family:	Syngnathidae
Subfamily:	Syngnathinae
Genus:	Acentronura; Kaup, 1853
Type species
	Hippocampus gracilissimus; Temminck & Schlegel 1850
Synonyms
	Atelurus Duméril, 1870;

Acentronura is a genus of pygmy pipehorse native to the Indian and Pacific oceans. The name is derived from the Greek ακεντρονουρα,^{[dubious – discuss]} or a-kentron-oura, and refers to the lack of a sting on the tail.^[1]

Species

There are currently two recognized species in this genus, several previous members having been moved to the genera

Amphelikturus and Idiotropiscis:^[2]

Acentronura breviperula Fraser-Brunner & Whitley, 1949 (Dwarf pipehorse)
Acentronura gracilissima (Temminck & Schlegel, 1850) (Bastard seahorse)
Acentronura tentaculata (Günther, 1870) (Shortpouch pygmy pipehorse)

Distribution

Acentronura species are mainly found in shallow tropical and subtropical waters of the Indo-west Pacific and the

pelagic and therefore may be found deeper than this.^[1]^[5]^[6]

Biology

Adults may be found in or just above sandy or muddy substrates, around the base of macro

prehensile and used for anchorage, winding itself around pieces of algae or seagrass.^[1]^[6]^[7] However, the front part of the body is typical pipefish, with the head and body held in line rather than bent through and angle like seahorses. There is sexual dimorphism and the males are somewhat larger and more robustly built than the females. Because they are so small, the brood pouch is also large in proportion to the body, giving the males a somewhat more seahorse-like appearance than the females which have the typical slim linear form of pipefishes.^[6]

Acentronura fish also have characteristically fused jaws like other members of the Syngnathidae. For those species where the feeding habits are known, the diet consists of small

crustacea which are snapped up by the small mouth at the tip of the snout, as they float past in the current.^[1] With a slim snout, relatively large eyes and raised rear part of the skull, the head is somewhat reminiscent of that of a seahorse, in contrast to the short, slim body. There are often projections sticking out from the body and head at intervals along the length of the fish.^[5]^[6]

Reproduction

Acentronura species are

ovoviviparous so they give birth to live young. Like seahorses (Hippocampus spp.) and many other members of the Syngnathidae, the eggs are transferred at mating into a brood pouch on the ventral surface of the male. The brood pouch extends from just behind the anus to about halfway along the fairly short tail and is relatively large compared to the small size of the fish. It is formed by elongated folds of the skin surface which are less well protected by bony plates than the rest of the body. The eggs are incubated within individual skin cells in the brood pouch, hatch, and are released as their yolk sac is exhausted. .^[1]^[7]

Acentronura tentacula is the best known of these species, but it is likely that other species have similar habits. These fish are most commonly seen in pairs, and this has led to the supposition that they are

monogamous. However it is not known whether they do form long-lived pairs bonds or if these are quite changeable (as has recently been confirmed for some seahorse species, which were also assumed to be monogamous). In A. tentacula, the only species about which much is known, these new-born fish become free-swimming pelagic members of the plankton until they are part-grown, when they settle into their preferred adult habitat.^[1]^[8]