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What is the world's smallest fish?

01 February 2006

Is the 7.9mm Paedocypris fish really the smallest in the world?

Ted Pietsch, of the University of Washington, disagrees and believes the title belongs to two males of the species called Photocorynus spiniceps , which are 6.2mm and 6.5mm long, the females being 46-50.5mm.

Hangers-on

Photocorynus spiniceps are deep-sea angler fishes found in Gulf of Panama and the Philippines. The males are a great deal smaller than the females and once they have found a female they attach themselves to the females back for the rest of their lives, a phenomenon known as ectoparasitism or sexual parasitism . The female can have many males attached to her and she might be considered a functional hermaphrodite.

Small and mature

The male Photocorynus could be described as just a sac with testes, as many of its organs have been reduced. Pietsch measured one male at 6.2mm and two others at 6.5mm and 7mm, noting the 'three newly reported parasitic males are also mature or at least close to maturity' and the testes of the smallest two 'are especially well developed, …each containing large amounts of developing spermatozoa, but without evidence of flagellated sperm.'

This is important because they need to be sexually mature adults before they can be counted as the world's smallest. Pietsch states that these males are 'among the world's smallest known sexually mature vertebrates as defined in terms of length, volume, and weight.'

Publishing research

Pietsch's work appeared in the September issue of Ichthyological Research, published by the Ichthyological Society of Japan.

Dr Ralf Britz and colleagues, who had their work on the Paedocypris fish published last week, said they had not seen the work before their own was published.

Smallest species rather than individuals

Britz points out that the two processes that lead to miniaturisation in Photocorynus and Paedocypris are very different and he raises the question whether the former sexual parasites can be comparable to a free-living fish. Photocorynus spiniceps  may have the smallest individuals but Paedocypris progenetica is the smallest species.

Britz adds that the most interesting and important aspect of this research is 'all these organisms, no matter which one is the smallest, are unusual in many aspects and deserve to be studied in greater detail.'

Pietsch said, 'There are always difficulties in talking about the smallest - would that be length, volume or weight - the debate goes round and round.'

Unexplored globe

Britz concludes, 'The whole exchange is quite amusing and in the end, what is really important is that we appreciate that there are still many areas on this globe that are unexplored, containing vast numbers of new species, among them very unusual ones that need to be discovered and their biology and anatomy explored.

'For some habitats we are facing a race against time, because they disappear faster than we can survey them.'

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