Hippocampus abdominalis Lesson 1827

Perhaps the most unique and unusual feature of seahorse biology is the fact that it is the male and not the female who becomes pregnant. When mature, males develop a pouch on the belly, known as the brood pouch. In this species the pouch is formed after 6 months of life, but males don't breed until they are about one year of age (3). Breeding takes place in spring and summer; the female inserts her ovipositor into the male's pouch and lays her eggs. The male then fertilises them and they become embedded into the wall of the pouch. The pouch is very similar to the womb found in female mammals; a placental fluid removes waste products and supplies the eggs with oxygen and nutrients. As pregnancy progresses, this fluid gradually becomes similar to the surrounding seawater, so that when the young seahorses are 'born' the change in salinity will not be such a shock (5). After around 30 days of pregnancy the male goes into labour, typically at night when there is a full moon (2) (3). After hours of thrusting, the miniature seahorses, which look exactly like the adults, are released from the pouch (3). The most numerous brood reported numbered over 1100 offspring (10). The offspring are fully independent after birth and must fend for themselves (3) (5). They are pelagic in the first stage of life, or hold onto floating debris at the surface with their tail (2). They settle on the bottom after they reach a length of 30 mm (3). The big-belly seahorse is more active at dusk and night than in the day (3). They feed on crustaceans, such as amphipods and shrimp, which are sucked into their tube-like snouts and ingested whole (6).

A pressing requirement to assist in the conservation of this species is the need for further research on the big-belly seahorse. In order to effectively conserve a species, its biology, ecology, range and abundance must be fully understood and the threats facing it must be known (8). In addition, Marine Protected Areas need to be established to provide areas in which the species is protected (3). In November 2002 all seahorses were listed on Appendix II of the Convention on International Trade in Endangered Species (CITES); this means that the massive trade in seahorses must be regulated to ensure that the survival of wild populations is not threatened. However, Indonesia, Japan, Norway and South Korea have opted out of the listing for seahorses (7).

The big-belly seahorse is one of the largest of all seahorses, growing up to 35 cm in length (2) (4). Like other seahorses, the head is held at right angles to the body, the eyes can move independently of each other, and the tail is prehensile. Instead of having scales, as most other fish, seahorses have a layer of skin stretched over bony plates that are visible as rings passing around the trunk. Swimming is powered by the rapidly oscillating dorsal fin, and they steer using the fins on either side of the body (the pectoral fins) (9). As the common name suggests, this species has a large swollen belly (4). In common with most other species of seahorse, the big-belly seahorse is well camouflaged; individuals may be brown, yellow, grey, white, orange or mottled, with dark spots and blotches on the head and trunk, and the tail often has alternating pale and dark bands (2). Males differ from females in that they have longer tails, a shorter, more robust snout and more dark markings (4), they also typically have a yellow mark close to the top of the brood pouch (2).

Inhabits harbours and sheltered coastal bays (3). They can be found amongst algae, seagrasses and around rocky reefs in fairly shallow water. In deeper water they typically attach to sponges (2). Unlike most seahorse species, the big-belly seahorse is a relatively strong swimmer and has been known to swim over hundreds of meters in the course of a day (11). Adults are also known to occur in open water and to raft on algal rafts and seagrass. Artificial structures appear to be important habitats for this species - in particular jetties, nets and salmon cages (10).

This seahorse is found in the south-west Pacific around Australia and New Zealand (2). It is known in Australia from Newcastle, New South Wales southwards, throughout Victoria, Tasmania and westwards as far as the northern Great Australian Bight in South Australia (9). In New Zealand, it is widespread around both North and South Islands (3). It has been suggested that the populations that make up this species are actually two separate species. However, this taxonomic splitting is still quite contentious and there is little evidence for the existence of two species (10).

Classified as Data Deficient (DD) on the IUCN Red List 2006 (1). In Australia, all seahorses and pipefish are subject to export controls under the Environment Protection and Biodiversity Conservation Act 1999 (3). All seahorses are listed on Appendix II of CITES (7).

The main global threats to the big-belly seahorse are habitat loss, incidental by-catch in commercial fisheries and over-exploitation (10). Although this species is sold locally and internationally for the aquarium trade and collected and dried for use in the oriental medicine trade, typically as a tonic and as an aphrodisiac, such exploitation is strictly controlled in Australia (2). Similarly, while it has been recorded as by-catch, numbers are generally low and many core habitats are not fished with trawl gear (10).