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The big-belly seahorse or pot bellied seahorse, Hippocampus abdominalis, is one of the largest seahorse species in the world with a length of up to 35 cm. Seahorses are members of the Syngnathidae family, and are in fact teleost fish.
The big-belly seahorse is found among algae, seagrasses, and rocky reefs in shallow water, and attached to sponges and colonial hydroids in deeper areas. They also attach to jetty piles and other man-made objects, and can be found in estuaries. They usually inhabit waters less than 50 metres deep, but have been found as deep as 104 metres. Juveniles are pelagic or attached to drifting seaweed.
The big-belly seahorse has a forward-tilted, long-snouted head, distended but narrow pot belly, and a long coiled tail. It swims using its dorsal fin with a vertical stance - when not swimming it coils its prehensile tail around any suitable growth, such as seaweed, waiting for planktonic animals to drift by when they are sucked up by the small mouth set at the tip of the snout much like a vacuum cleaner. Seahorses are voracious feeders, eating mainly crustaceans, such as shrimps, and other small animals living among the seaweed such as copepods and amphipods. They do not masticate so they can eat to excess because of their small gut tract. Each eye moves separately making it easier for them to see food and predators.
It is quite easy to distinguish males from females. The male have a smooth soft pouch-like area at the base of its abdomen between where the stomach meets the tail on the front side. Males also have a fin here but it is less obvious. The female will have more of a pointed stomach with a very obvious fin at the base of the stomach.
In the wild, the breeding can commence when the seahorses are about one year old and experiments show that this can be reduced to about eight months when in captivity. Breeding in big-belly seahorses is usually observed during spring/summer.
Courtship initiation involves a series of colour changes and postural displays. Dilating the opening of their brood pouch slightly, males inflate the pouch to balloon-like proportions with water by swimming forwards, or by pushing their body forwards in a pumping action, then closing the pouch opening. At the same time they lighten their pouch in colour to white or light yellow. Males also brighten their overall body coloration, typically intensifying the colour yellow. Males repeatedly approach their selected female with their head tucked down, and dorsal and pectoral fins rapidly fluttering.
If the female is not receptive she ignores the male, who then looks for another potential mate. If no females are receptive the male stops displaying and deflates the pouch by dilating the pouch opening and bending forwards, expelling the water inside. If a female is receptive to a courting male, she reciprocates with her own colour changes and head tucking, typically intensifying the lighter colours such as yellow and white, highlighting the contrast between these colours and their overall darker blotching and banding patterning. A series of short bursts of swimming together in tandem then ensues, sometimes with tails entwined, or with the female tightly rolling her tail up. This has often been described as ‘dancing’. After coming to rest, the male attempts to get the female to swim towards the water surface with him by repeatedly pointing his snout upwards.
If the female responds by also pointing her snout upwards then the final stage of courtship follows. This involves both males and females swimming directly upwards towards the water surface with both their heads pointing upwards and tails pointing straight down. If they reach the water surface, one, or both seahorses, can often be seen and heard to snap their heads. To transfer her eggs to the male, the female faces the male, slightly above him. Pressing the base of her abdomen against the male's pouch she then squirts her eggs through the opening in the front of his dilated pouch.
The male seahorse can give birth to up to 490 babies at a time. Their colouring is a variable shade of brown, mottled with yellow-brown and with darker splotches. The tail is often circled with yellow bands. In deeper water where the tail is anchored to other colourful forms of life, such as sponges and hydroids, they often take on these colours.
- A. B. Wilson and K. M. Martin-Smith (2007) Genetic monogamy despite social promiscuity in the pot-bellied seahorse (Hippocampus abdominalis), Molecular Ecology, 16, 2345–2352.
- Froese, Rainer and Pauly, Daniel, eds. (2006). "Hippocampus abdominalis" in FishBase. May 2006 version.
- Tony Ayling & Geoffrey Cox, Collins Guide to the Sea Fishes of New Zealand, (William Collins Publishers Ltd, Auckland, New Zealand 1982) ISBN 0-00-216987-8
- Chris M. C. Woods, National Institute of Water and Atmospheric Research, Received 16 July 2002; received in revised form 16 October 2002; accepted 14 November 2002 pp. 538. Effects of varying Artemia enrichment on growth and survival of juvenile seahorses, Hippocampus abdominalis. (Aquaculture 220 (2003)).
- Chris M. C. WOODS, New Zealand Journal of Marine and Freshwater Research, 2000, Vol. 34 pp. 475-485. Preliminary observations on breeding and rearing the seahorse Hippocampus abdominalis (Teleostei: Syngnathidae) in captivity. (The Royal Society of New Zealand 2000).
- Chris M. C. Woods, New Zealand Journal of Marine and Freshwater Research, 2002, Vol. 36: 655–660. Natural diet of the seahorse Hippocampus abdominalis. The Royal Society of New Zealand 2002
- Chris M. C. Woods, New Zealand Journal of Marine and Freshwater Research, 2005, Vol. 39: 881–888 Reproductive output of male seahorses, Hippocampus abdominalis, from Wellington Harbour, New Zealand: implications for conservation. (The Royal Society of New Zealand 2005).
- Chris M. C. Woods, National Institute of Water and Atmospheric Research, Received 17 December 1999; received in revised form 2 May 2000; accepted 9 May 2000. pp 377-388. Improving initial survival in cultured seahorses, Hippocampus abdominalis Leeson, 1827 (Teleostei: Syngnathidae) (Aquaculture 190, 2000).
- Chris M. C. Woods & FiammaValentino, National Institute of Water and Atmospheric Research,Wellington, New Zealand. Naples Zoological Station‘A. Dohrn’,Villa Comunale1, Naples, Italy. Frozen mysids as an alternative to live Artemia in culturing seahorses Hippocampus abdominalis (Aquaculture Research, 2003 34, 757-763).
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