Landlubber Fish Suggests That Tides Influenced The Invasion Of The Land By Animal

A leaping blenny attached to a rock at mid tide.

It is well known that, around 390 million years ago, fish began to evolve bodies that could cope with the strain of living on land. Eusthenopteron sported fins whose bones were shaped and arranged in a leg and foot like structure. Its ancestor Panderichthys had a skull which was akin to that of an amphibian. This evolutionary chain continued until true amphibian such as Acanthostega and Ichthyostega colonised the dry and air filled world of the arthropods.

New research into the lifestyle of a small fish called a leaping blenny suggests that the tides would have played a major part in this transition. This remarkable little fish is of course aquatic. However most of its day to day activities take place on rocks that are above water level at mid. They use a combination of a tail twisting movement and enlarged fins to grip onto any available surface. They can also flick their tails and twist their bodies to jump many times their own body length to quickly reach higher ground.

While it still needs to stay moist to survive, it does not need to be completely submerged in water to breathe. They shelter in crevices in the rocks at high and low tides, emerging to feed, mate and socialise at mid tide, when the water level is not submerging the rocks that they make their home, but close enough for them to keep their bodies moist enough to survive. Today most fish are aquatic. However a few fish families such as the lungfish or the leaping blennies give scientists a unique laboratory to gain insight into amphibian evolution.

The remains of a Devonian tidal pool with a metamorphic bedrock.
Amphibians could have first evolved in such a place

It possible that early amphibian ancestors would have rode the tides up to high rocks, carried out their daily activities on moist mudflats and reed beds and then ride the tide back down into the sea. It is likely that such creatures would have lived in tidal freshwater pools connected to rivers that fed into the sea. By utilising the mudflats, swamps and reed beds that would have been regularly flooded by the pools around which they form, fish could have slowly evolved into true amphibians, setting the stage for the second invasion of the land.