Saturday, 27 August 2011

Aquarium tour: Arrowhead Pufferfish

Next to the brackish water display is a tank that most of the public walk past, as it appears at first glance to be empty. This is their loss, as it actually contains two of the oddest fish we have, the Arrowhead Puffer fish Tetraodon suvattii.

Pufferfish belong to the order Tetraodontiformes, a group which contains some of the oddest fish on earth today. They are characterised by odd body shapes, slow and precise movement, and often some form of bony beak. The order includes (besides the puffer fish) the trigger fish, box fish, and the giant ocean sunfishes. Aside from some puffer fish, almost all are marine. Most are reasonably sized to large fish, but some are quite small and one or two (such as the sunfish) are gigantic.

Puffer fish get their name of course from their habit of swallowing water (or air if they are fished out with a net) to make themselves look bigger. This may be to make them appear too large for a predator to swallow, or it may be used as a display in threat or courtship. Their other claim to fame is their sequestering of toxins in their liver. This is tetrodotoxin, and despite its lethality it is eaten as the famous Japanese dish fugu, which is prepared from some marine species found near Japan. Apparently the minute amounts in a properly prepared dish impart a pleasant tingling sensation on the lips as it is eaten. If it improperly prepared the only hope is to support the patient on a heart-lung machine until the patient recovers. There is usually at least one death every year or so apparently. My favourite description of this meal comes from one of Sir Terry Pratchett’s books – “fish and chips – for men!”

From their odd appearance one might think that all puffer fish are much the same. In fact, they have radiated into a variety of ecological niches, varying from coral reefs to acidic freshwaters all over the tropics, although they shun colder waters. Most species are marine to brackish water specialists, and they tend to avoid open water, where they are exposed to predators. Many species are found in mangrove swamps where the salinity varies. The Mekong river, where our Arrowhead puffers originate, has at least five endemic species, with habitats ranging from muddy shallows to weedy river margins to rapids.

Almost all are specialist predators of molluscs and crustaceans, and to help them in that they have a continuously growing bony beak. This may cause problems if they are fed softer food in an aquarium than they would obtain in the wild, as the beaks can overgrow, requiring specialist dental care to grind down the beak.

Our Arrowhead puffers exploit a slightly different niche. Instead of searching plants or rocks for aquatic snails, they are sit-and-wait predators of small fish. In the mud of the Mekong where they originate, they half bury themselves and wait, sometimes for days, until a smaller fish swims overhead, Then they suddenly dart up and either swallow the fish whole or bite it in two . The whole strategy is rather reminiscent of the marine angler fishes, without the lure.
T.suvattii in lurking mode
In our aquarium, the tank is furnished with a deep layer of fine sand and some river cobbles about the size of the puffers (ours are about adult size at 15cm). They can be quite aggressive with each other – they are in a large tank but are always at opposite ends. This territoriality is one of the reasons most puffers in the aquarium trade are wild caught – tank breeding of some species has occurred, but the low demand for such fish and the difficulty of breeding them means that there are no commercial fish farms producing them.

Having said that, T.suvattii is one of the few species that has been bred. What the exact trigger for breeding is not clear – lowering barometric pressure in summer may simulate the start of the monsoon, when many South East Asian species breed. Most of the reports of breeding seem to occur in August or late summer in any event. The eggs are laid (after a leisurely courtship) on a smooth hard surface and are guarded by the female. The newly hatched fry feed on microorganisms and small crustaceans. Once adult, they only feed about once per week – this is important in an aquarium as overfeeding is easy but can result in water pollution, which is rapidly fatal to all puffer fish.

With their predatory lifestyle, one can see that this is definitely not a fish for the standard mixed home aquarium. It demands (and deserves) a tank by itself and a fair amount of attention. As a group, all the Tetraodontiformes including puffer fish, trigger fish, and box fish seem to have an intelligent and curious personality, and definitely get to know their keepers. This raises the question of aspects of aquarium fish care that are often overlooked – the psychology of the fish themselves and how a captive environment may affect it. That discussion I will take forward next week. In the meantime, here are a couple of links you might like:

Video of T.suvattii spawning:
An account of a T.suvattii spawning:

(Images from

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