Friday, 14 October 2016

8) A variety of tangs

 Currently one of the most popular animated films is Finding Dory, and as might be expected the actual fish is invariably greeted by that name by children visiting the aquarium. As well as Dory (technically a Pacific Blue Tang Paracanthurus hepatus), there are four other tang species to be seen in the large marine tank.

Tangs are placed in the wholly marine family Acanthuridae, along with the surgeonfishes. The family name means “Spiny Tail”, and refers to the sharp spine they can erect at the base of the tail which they use in self defense. They may travel alone, but many species move in small schools or pairs as well. They are mostly small to medium sized fish, but the largest species can reach over 1m. They mostly have an oval to rectangular outline, and are compressed laterally, which enables them to wedge themselves in coral crevices at night, as they are diurnal. They have a very distinctive swimming style, seemimng to "fly" by flapping their pelvic fins.

Different species of tangs have slightly different diets, but they all have a strong preference for plant material, from larger microalgae down to plankton depending on species. The diets of some species make them distasteful or even toxic, but others are the target of commercial food fisheries.
As with many marine fish, they are pelagic spawners. The female, accompanied by one or more males, swims up towards the surface to release small, floating eggs to be fertilized. The larvae live in the plankton for several months before metamorphosing into miniatures of the adults to settle down on a reef.

Aside from some commercial fishing, tangs and many surgeonfishes are targets for the aquarium trade. They tend to be sensitive in captivity, being prone to parasites and also a degenerative disease known as “lateral line disease”, which affects several groups of both freshwater and marine fish in captivity and is often associated with dietary deficiencies. Unfortunately, as a result of the small size and long planktonic stage of the larvae there has been little success in captive breeding until recently, but in the past year two species have so far been raised and more should be produced in the near future. Before the commercial trade can be supplied however, it will be necessary to raise them at a lower price than wild individuals can be obtained for, which will be the next crucial challenge to overcome.

Currently Bristol Zoo aquarium has the following species on show:

Pacific Blue or Regal Tang Paracanthurus hepatus (see picture at top of this blog)
Currently being voiced by Ellen Degeneres in a multiplex near you, this is one of the larger species, growing to 30cm. Its range extends from East Africa across to the Philippines and north to Japan, and as it is not commercially fished except for the aquarium trade, aside from some use as bait fish, it is listed as Least Concern by the IUCN. This was successfully raised for the first time in 2016, but commercial production has not yet begun. It is primarily an open water feeder, targeting plankton but will also graze on algae.

Blue-Eyed Tang Ctenochaetus binotatus
Growing to a smaller size than the Pacific Blue tang, this species grows to around 20cm maximum, although at present the fish on show are juveniles. This genus is often referred to as bristletooth tangs, and there are numerous species. They are extremely keen grazers, continuously pecking at rocks, and as juveniles at least move around in a loose school. In the wild they are found as adults in rubble areas on the seaward side of reefs where they feed on algae films and diatoms. The diet accumulates ciguatera toxins and as a result they are not fished commercially.

Convict Tang or Surgeonfish Acanthurus triostegus
This is much smaller, growing to around 15cm, and more monochrome. It is more sociable than the previous species and is commercially fished for food. They are more adaptable to water conditions than pure reef fish, and are often found near stream outlets where they feed on algae films. They range across the Pacific as far east as the Galapagos.

Red Sea Sailfin Tang Zebrasoma desjardini

This is known from the Indian Ocean and Red Sea, where they are found either as lone individuals or in pairs rather than in schools. They are one of the larger Zebrasoma species, with a big male reaching 40cm, although they are usually much less in captivity. They are primarily grazers on filamentous algae, although they also feed on plankton and they have been observed feeding on jellyfish and ctenophores as well. They are widespread in the Indian Ocean, as far south as South Africa and as far east as Java. They tend to live in lagoons and inner reefs, and as adults they are usually found moving around as mated pairs.

Yellow Tang Zebrasoma flavescens
The other Zebrasoma species on show is much smaller, with a maximum length in the wild of around 20cm. As with the previous species they mainly obtain their food by grazing on the reef, although they have also been seen grazing on the shells of passing turtles. They move around either singly or in loose schools. This was the first species of tang to be successfully raised in captivity, with over 200 raised by the Oceanic Institute in Hawai’i in October 2015. They are potentially quite long lived, with an estimated maximum lifespan of 30 years and over 20 being recorded in captivity. That so many in practise live only a few years in captivity is a measure of how easy inexperienced hobbyists  can shorten lifespans if they do not know what they are doing.

Unicorn Tang Naso brevirostris
Finally, what will eventually be the largest tang in the tank is currently a juvenile, and is frankly rather a dull olive grey colour. Eventually it will reach as much as 60cm, but the most prominent feature has yet to develop. This is the massive forehead spike that gives this group of fish their name. The reason for this is obscure, but presumably has some species recognition function. As a result of their size, they may be a target for commercial or subsistence fishing’

Next week, another herbivorous fish from the reefs of the Indo Pacific.

Images from wikipedia

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