Researchers discover a rare case of internal differences between the sexes of one fish genus

July 21, 2014 by Hayley Dunning, Natural History Museum
Size difference in the swimbladder (green) of the male (top) and female (bottom) of Psilorhynchus.

A team of scientists, including Museum fish researcher Dr Ralf Britz, have discovered that in one genus of fish the swimbladder, the organ used for buoyancy in water, is greatly enlarged in the males.

The larger swimbladder in males can be up to 98 times greater in volume, and is accompanied by a strange, unidentified organ that the females lack altogether.

Most differences between males and females of the same species of animal are external, such as body size or elaborate colouring. Apart from the gonads, there are few examples of organs inside the body being different.

Staying afloat

Members of Psilorhynchus, the genus of fish studied, are commonly known as South Asian torrent minnows, because they live in fast-flowing water.

The swimbladder is normally used to keep fish buoyant, but in fast-flowing water a buoyant could easily be swept away in the currents, so that fishes in this habitat usually have very small or absent swimbladders.

The male Psilorhynchus however doesn't have a problem in the water despite his larger swimbladder. Dr Britz and colleagues think this may be because the male has larger bones and muscles, adding weight to the body and offsetting the greater buoyancy.

Making noise

The different bone and muscle composition may also help explain what the mystery organ is for. Sitting on top of the swimbladder is a unique tuning-fork shaped organ made of dense connective tissue.

Changes in the muscles and skeleton surrounding the swimbladder in other similar fishes are often associated with sound production. Although the tuning-fork organ is unique among fishes, together with the modified skeleton and muscle, it may have a similar function.

'We think that the modified muscle is a drumming , using the enlarged bone to drum against the swimbladder like a drumstick,' said Dr Britz.

The fast-flowing that Psilorhynchus lives in is a very noisy environment, and this may explain why the males make such an effort to produce a sound that could be difficult to hear.

Surprising discovery

The genus Psilorhynchus has been known for 170 years, but it was only when lead author Dr Kevin Conway studied its internal anatomy that the enlarged swimbladder was discovered. It was analysed in detail in collaboration with Dr Britz and Dr Dustin Siegel. The enlarged swimbladder in occurs in all but one of the species in the genus.

'Our study nicely demonstrates that there still are scientific mysteries remaining to be discovered, one just needs to look,' said Dr Britz.

Explore further: New miniature fish discovered

Related Stories

New miniature fish discovered

July 3, 2014

A newly discovered fish species measuring up to 15.4mm long has added to the global diversity of 'miniature' fish.

Blue-bellied fish is a surprise catch

March 28, 2013

It is only 7mm longer than the world's smallest fish, and seems to only appear at night, but the bright blue belly of a tiny Amazonian fish caught the eye of a team of scientists who spotted it was a new species and genus.

Dracula minnow has teeth, almost

March 11, 2009

A new species of tiny fish with jaw structures that look like huge teeth has been identified, Natural History Museum scientists report in the Proceedings of the Royal Society journal today.

Sharksucker fish's strange disc explained

January 28, 2013

There's an old legend about a fish that attaches itself to ships and has powers to slow them down. The powers may be mythical but the fish is real. 

Recommended for you

Circadian regulation in the honey bee brain

January 17, 2018

Circadian clocks regulate the behaviour of all living things. Scientists from the University of Würzburg have now taken a closer look at the clock's anatomical structures and molecular processes in the honeybee.

Quick quick slow is no-go in crab courtship dance

January 16, 2018

Female fiddler crabs are sensitive to changes in the speed of a male's courtship display, significantly preferring displays that accelerate to those that are performed at a constant speed or slow down.


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.