This fish prefers to be out of water! The Pacific leaping blenny is back in the news again with the publication of further research on how they avoid being eaten by birds and lizards.
Whoa, back up a bit you say. A fish living out of water! Yep, and this isn’t just a fish that can drag itself across land to get to another pool of water or scoot around on the mud to find food like mudskippers. Sure, those fish are pretty cool but they are just amphibious fish – fish able to tolerate spending time out of the water but spend more time in the water than out. The Pacific leaping blenny has outdone them all (well except for all those tetrapods out there:-).
There are many species of blenny, all of which are very small and most of which live in tidal pools and along rocky coasts in tropical regions. Many species of blenny are amphibious with the ability to live out of water for a few hours at time and scavenge algae growing on the rocks. But one particular species (Alticus arnoldorum), the Pacific jumping blenny, has decided that living on land is to be preferred.
OK, “on land” might give the wrong impression. These fish still need to stay moist and so they live near the water’s edge. However, they really prefer to be out of the water and so they spend nearly their entire lives on land including reproduction. Unlike other species of blenny, members of this species can breathe through their skin as well as through gills allowing them to do enough gas exchange that as long as their skin is moist they can live out of the water indefinitely. In fact, they will avoid being in the water!
This species has adapted its behavior to living out of the water to the extent that males displays courtship behaviors to defend their territory which consist of rocks with holes where they can hide and seduce mates.
Several populations of blennies from locations around the islands of Guam have different coloration patterns. These populations have been studied and it has been observed that the fish from each populations had coloration patterns that matched the rocks they lived on. Further research revealed evidence that these coloration patterns provide protection against being seen by predators.
This species of blenny can crawl around, pulling itself along with its front fins but its most impressive skill is its ability to curl its tail up and then flick it back out straight causing the fish to leap. Thus the amazing leaping blenny!
This amazing adaptation is found in one other closely related species of blenny but other blenny species that are less inclined to live out of the water don’t seem to be able to bend their tails in this fashion. Thus, this appears to be a new adaption allowing these fish to propel themselves up on rocks and from rock to rock. This helps them to avoid being dragged back into the water in the tides and helps them to get to new food sources and to avoid predators.
What can the blenny teach us about how organisms change?
As you read this story about the Pacific leaping blenny it may raise a number of questions in your mind. Let me guess what some of those may be and provide some hypothetical answers. ‘
In the blenny are we seeing the evolution of an aquatic animal to a land-dwelling animal? There is no simple answer to this question. The best I can do is say, maybe yes and maybe no. Clearly this species of blenny is far more capable of living on land compared to its cousins which are unable to live in the same environment. The Pacific leaping blenny has evolved new characteristics that provide it with a greater capacity to survive in aerobic environment including being able to breath more efficiently through its skin and be able to maneuver (eg. jump) from one rock to another more effectively than any other fish could. In these ways the blenny is in the beginning stages of becoming a land-dwelling animal.
Will it become another branch of land-living animals in the future? It isn’t impossible but it is very unlikely. This animal has found a very good niche to live in and is well adapted for that environment. For these “fish” to move further onto land would require that they continue to adapt in many ways such as their respiratory functions and their leg structure. To do so there would need to be a reason for them to do so? Why live on land when their current environment suits them quite well? One possibility is that if there were a food source further from the water then it may be advantageous for them to venture further from the water. However in today’s environment such a trek would be far too dangerous given that there are birds, reptile and mammals that would be very interested in eating the blenny. In this case we see that Darwin’s mechanism of natural selection acts against any further attempts for these fish to colonize the land. Any fish that strays further from their protective rocks no matter how great the food supply may seem on land will undergo severe negative selection due to predation. As a result natural selection selects for blenny’s that are best adapted to their present environment. This is called stabilizing selection and it provides a negative pressure on a species to explore new habitats.
However, consider if there were no land-animals on the islands in which the blenny lives. If predation pressure were completely removed there very well may be a benefit for the blenny being able to move to higher ground. If this happened, then rather than stabilizing selection, the blenny would now undergo directional selection whereby any variations among individuals that allows for prolonged air exposure, greater mobility, and increased flexibility in diet would be selected for thus driving the blenny to become more and more adapted to that land habitat. Under these conditions one might expect that the blenny may very well become a full-fledged land-dwelling animal albeit it would take a long time.
Does this mean that a blenny could become an amphibian someday? No, while it may be possible for the blenny to adapt to life on land, because amphibians come from a very different lineage of fish called the lobed-finned fish, the evolution of the blenny would result in a land animal that would likely have its own unique adaptions to land resulting from the fact it started with different building blocks. For example, the lobed-finned fish have different fin structures than the blenny and therefore if the blenny were to eventually adapt its fins to life on land their feet or method of locomotion on land might be quite different than the amphibians. So they would not become amphibians though they may have many similar features to today’s amphibians.
What is the creationist’ take on the jumping blenny?
Dr. Elizabeth Mitchell of AiG is the only young-life creationists to have addressed the blenny that I have found. In a short note she makes these comments about the leaping blenny:
By evolutionary standards, the blenny’s move toward land should have provided a survival advantage if those adaptations were to be preserved. Instead, the “evolutionary” changes evidently restricted this fish to a very limited survivable environment. …
The blenny is not making any evolutionary progress toward becoming a terrestrial creature. It does not have lungs. It does not have legs. Nor does it have the beginnings of them. It is breathing using the same anatomical equipment God equipped aquatic creatures with on the fifth day of Creation. It is actually fairly restricted in its habitat, perhaps due to a loss of genetic variability within its created kind. But it certainly has not acquired the information to become a land animal. It’s a fish.
As I said above, the question of what the blenny is doing with respect to its future is tricky one. Since the land is already colonized, there really isn’t enough of a “survival advantage”on land to cause any further adaption of the blenny to land. As a pointed out, Darwinian natural selection doesn’t say that species have to change but rather can also explain why species are restricted to particular habitats. In this case there is no reason to believe the blenny is moving toward land. They are only “moving toward” adapting to conditions that make it best fit for then under the current environmental constraints in which they live which may very well mean it is not changing much at all. Were that environment to change then it is possibly the blenny may change along with it.
Mitchell uses the familiar refrain they species today may be due to a loss of genetic variability within a created kind but what does this mean? Is she saying that the leaping blenny is somehow reduced in its capacities? Far from that. Leaping blenny has abilities that none of its relatives have nor do they need. The unique muscle structure is an adaption of the blenny tail and the front fins have grasping qualities that are adaptions to the rocks. I have no reason to believe that the leaping blenny has any less genetic variability or number of genes than any other blenny.
We can’t know what would have become of the blenny had there been no selection against moving onto land. To say it doesn’t have legs doesn’t mean that it doesn’t have structures that have the capacity to become legs. But, maybe the blenny would become a new form of land animal that moves by jumping with one large rear muscle and thus legs are not needed. Dr. Mitchell seems to think that evolution has some direction it must follow and the blenny must make a lung and legs just like other land animals in order to live on land. I would not limit God’s creative capacities in this way.
More of the story of the blenny can be found here including a great video of the blenny “leaping” around from rock to rock. http://www.sci-news.com/biology/science-pacific-leaping-blenny-01584.htm
Terry J. Ord and S. Tonia Hsieh. (2011) A Highly Social, Land-Dwelling Fish Defends Territories in a Constantly Fluctuating Environment. Ethology, 26 DOI:10.1111/j.1439-0310.2011.01949.x