If you have seen a sea robin on a dive, you’ll know that this ‘walking' fish has a set of six legs that move rapidly together to propel it across the seabed.
Now researchers at Harvard University have indicated that in some sea robins these legs, which are in fact pectoral-fin extensions, have evolved to do far more than scuttling – including ‘tasting’ food.
The work to reveal sea robins' walking aids as sensory organs was carried out by the university’s Department of Molecular & Cellular Biology in collaboration with Stanford University.
When walking over a soft seabed, sea robins had been seen to stop and scratch at the surface to dig out buried prey such as shellfish, and their legs were found to be sensitive to both mechanical and chemical stimuli. When the scientists buried capsules containing single chemicals, the fish were easily able to find them.
By chance a fresh consignment of sea robins arrived in the laboratory during the study, and the scientists were surprised to find that they all lacked this skill – until they realised that the new arrivals were a different species of sea robin.
Those they had been studying, Prionotus carolinus or the northern sea-robin, had shovel-shaped legs covered in protrusions called papillae, similar to human tastebuds, while the non-digging fish, P evolans (striped sea robin) had rod-shaped legs with no papillae. They used their legs only for locomotion and mechanical probing.
Papillae are relatively recent evolutionary developments, say the scientists. Northern sea robins are found only in the western Atlantic, and all sea robin species are closely related to gurnards.
Sea robins could serve as an evolutionary model for trait development, because genetic transcription factors that control development of their legs are also found in the limbs of other animals – including humans.
A second study looked in detail at the gene-transcription factors involved in development of the sea robins’ unusual trait, and the researchers succeeded in generating hybrids of the two species. The studies have been published in Current Biology here and here.
Blind cavefish
Meanwhile, a species of blind cavefish has been found to make up for its lack of vision by increasing the number and location of the tastebuds from inside its mouth to its head and chin.
The pale pink, almost translucent Mexican tetra (Astyanax mexicanus) evolved to be sightless in the Pachon and Tinaja caves of north-eastern Mexico, where vision would be wasted in the dark environment. Only the faint outline of eye sockets remains in the species, though it is otherwise similar to a silvery, sighted fish found above ground.
Scientists had already established in the 1960s that certain populations of blind cavefish had extra taste buds on their head and chin but the genetic processes that caused this had never been studied before.
Now biologists at the University of Cincinnati have established that at birth the number of tastebuds is similar to that of the surface fish, but that additional tastebuds appear from five months and were still doing so at 18 months, the maximum age of the captive fish.
As these tastebuds appear, the fish acquire a keener sense of taste – meaning that they show less interest in eating live food and more in ostensibly unappealing sources of nutrition such as bat guano.
The team are now studying which other flavours most appeal to these blind fish. Their study is published in the journal Communications Biology.
Also on Divernet: Coming to coral reefs: Bluebot swarms, Robot snailfish – and Emperor dumbo, Mmm, tastes good – why corals like plastics
