ARTICLE 135    2015


by Jenny Edwards


The stars of the show at both the events held at Narooma wharf were the Big-bellied Seahorses (Hippocampus abdominalis).  Divers were able to borrow one or two from the nearby inlet to allow people to see them close up in tanks on the walkway.
It is easy to see from the photo why they are called big-bellied.  Both males and females have this profile but the males have an added pale-coloured pouch extending from the lower belly to the tail. 
The Big-bellied Seahorse is a slow but strong swimmer, using a pair of pectoral fins near its head for balance and moving about vertically using the dorsal fin on its lower back.  They are more active at night and at dusk than during the day.
These seahorses occur all along our south-east coast but are very hard to see.  They are usually found in relatively shallow water among seagrasses and sea weeds, hanging on with their long, curled tails, and very well camouflaged by their yellow-brown colouring.  Where the water is too deep for plants they can sometimes be seen clinging to sponges. Here the seahorses are typically brighter coloured similar to the sponges on which they live. Nets and pilings are also convenient places for the seahorses to attach and wait for their prey to come close.
Big-bellied Seahorses eat a variety of crustaceans from planktonic larval stages of crabs to tiny shrimps.  The seahorses suck in their prey when it drifts within range.  The food has to be small enough to be swallowed whole as the seahorse does not have any teeth.
In mating season mature Big-bellied Seahorses, a year or more old, pair off at dusk and dawn.  If not interrupted by competing males, the pair will hold each other by the tail then circle each other, sometimes for up to a few hours.  When the male finally inflates his pouch this seems to be irresistible to the female.  She deposits her eggs via a tube into his pouch where they are fertilised, protected, have the salt and water content regulated and are aerated.  After about 30 days the young are born at night to minimise the risk of their being eaten, including by other seahorses.
The new babies are about 16mm in length from snout to tail-tip and for the first one or two days are relatively straight.  They rise to the surface and grasp debris with their tails and start eating voraciously.  When they have reached a size of 20-30mm they settle to the bottom.
In captivity the species can live for at least seven years but it has been estimated from observations of their growth rate that they would take around 10 years to reach a length of 25cm and some specimens have been measured at 35cm.
As with most seahorses, they are threatened with over exploitation and loss of habitat.  Over 20 million animals were traded globally in 1995.  Most were dried for traditional medicine but hundreds of thousands were sold in the aquarium trade and a similar number dried to make souvenirs.  The farming of seahorses is now becoming more successful which we hope will give the wild seahorses a sporting chance.


Big-bellied Seahorse at Narooma


Photo - Rob Richardson