The Pacific leaping blenny, Alticus arnoldorum, has adopted a unique method of moving about on land. So successful is the adaptation that the blenny has moved beyond being amphibious and is now considered a terrestrial species.
A. arnoldorum feeds by scraping the algae from rocks that are above the water line, migrates with the tides, actively defends terrestrial territories and resides in moist burrows whilst the tide is out – A. arnoldorum even reproduces on land!
The key to this is the development of a tail-twisting ability (see picture above) when on land, thought to be the only known one of its kind in the fish world.
This particular characteristic permits the fish to move about on land with greater efficiency than amphibious blenny species, particularly when it comes to leaping and climbing, and so supports a terrestrial existence. You can see a video of their tail twisting ability online.
Tail-twisting
Research conducted by Tonia Hsieh of Temple University, Philadelphia, used high-speed video to reveal the nature in which A. arnoldorum moved about on land, comparing the results with that of aquatic and amphibious blenny species.
Only A. arnoldorum exhibited the tail-twisting ability in the laboratory. Species from the genus Andamia are also terrestrial blennies and share the tail-twisting, but were excluded from the laboratory tests as a result of welfare concerns.
In the research paper Hsieh says: "Tail movements in aquatic and amphibious blennies are limited to a side-to-side motion, like those of most other fishes.
"In contrast, terrestrial blennies twist their tail axially 90° before placement on the ground, using the lateral surface of their tail – rather than the ventral surface – to propel themselves forward.
"This unusual axial tail-twisting movement is unique to the two terrestrial genera, and thus represents a kinematic innovation."
Climbing blennies
The peculiar way in which the terrestrial blennies twist their tail also gives them an advantage when it comes to climbing, an important attribute for the particularly challenging environment they live in; the intertidal zone is prone to powerful wave action against exposed, algae-covered rock.
By twisting their tails so that the caudal fin lays laterally on a surface, the fish can increase traction and stay adhered to the slippery rock.
Terrestrial blennies only need periodical wetting by spray from the waves so that they can breathe through the skin, a process known as cutaneous respiration.
Hsieh believes the research is important in addressing "....evolutionary questions about water-land transitions and niche invasions."
You can also watch a video of a leaping blenny online.
For further information see the open access paper available from PLoS one: Hsieh S-TT, 2010 A Locomotor Innovation Enables Water-Land Transition in a Marine Fish. PLoS ONE 5(6): e11197. doi:10.1371/journal.pone.0011197