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Posts Tagged ‘evolution’

Sexual selection – a driving force in marine systems?

Prey are well adapted to evade predators, and predators are correspondingly well adapted to catch prey. For most of us it is pretty reasonable to accept that such adaptation happens by natural selection, leading to long-term evolution of animals, making them better to either catch prey or evade predators, whatever end of the food chain you happen to be on. Thus natural selection affects traits such as foraging efficiency or anti-predator behaviours that lead to longer lives, quicker growth rates and, both directly and indirectly, higher reproduction rates. Most of my earlier blogs have more or less built on the assumption of natural selection affecting adaptations of animals.

Bright warning colour on nudibranch. Probably a result of natural selection favouring nudis clearly advertising their poisonous properties.

Bright warning colours on nudibranch. Probably a result of natural selection favouring nudis clearly advertising their poisonous properties.

There is  another kind of selection, sexual selection, that is a little bit harder to understand. Sexual selection is the process where traits that directly affect the likelihood of securing a mate is changed over time, leading to the evolution of traits that sometimes seem to act contrary to natural selection in that sexually selected traits rather decrease life expectancy and growth rates. There are many examples of traits governed by sexual selection on land. Bird song, brightly colored males in many birds and lizards, antlers on deer and males adapted for fighting other males for access to females are examples that we all can relate to. It is thought that sexual selection in terrestrial systems are well as important as natural selection in shaping many aspects of populations and also a major force in driving speciation.

Male macaque with teeth adapted to fight other males over harem rights

Male macaque with teeth adapted to fight other males over harem rights

Are there examples of sexual selection in marine animals? Well, such examples are without doubt not as obvious as on land. The most obvious is dimorphism between sexes, that is that the two sexes differ in size.  Many fishes, such as many species of wrasses and groupers, have males that are much larger than females. Males of such species secure their mating by either fighting with other males for mating rights or showing of to females in order to make the female choose the performer.  This is certainly a sexually selected character. Some crabs seem to have very large males as compared to females, and that could be related to mating coinciding with female molting. Males can detect this molting some days before it actually happens, and try to protect “their” female from other males. Thus large males will be able to fend of smaller males, thus monopolizing pre-molting females.

Large male crab guarding female crab against other males

Large male crab guarding female crab against other males

When females are lager than males, it is very seldom a sexually selected character. In most marine monogamous species with a size difference between the sexes, the female will be the larger. This is not due to the female competing for mates, as the pair is monogamous, but rather that size affects egg production positively much more than size affects sperm production. Thus, in many cases, it makes sense for a monogamous pair to consist of a large female and a small male.

Large female false clown anemone fish with small male

Large female false clown anemone fish with small male

Another possible example of a sexually selected trait could be the extraordinarily long “nose” that some xeno crabs have. I have no idea if this is correct, or even if “nose” length of xeno crabs are related to sex, but is could be.

Xeno crab with an extremely long "nose"

Xeno crab with an extremely long “nose”

Otherwise, I find it surprisingly rare with clear sexually selected traits in marine animals. It could be related to the mating methods many marine animals use, where sex cells are released into the water and is more or less anonymously left by themselves to find a suitable cell to fuse with. This method of mating somewhat precludes mate choice or mate competition, thus making the force of sexual selection very weak compared to that of natural selection.  I will get back to mating methods on reefs and reef-near areas in a later blog.

Why good things can be bad.

In every documentary I have seen and book I have read about Lembeh, it is stated that the waters of Lembeh are exceptionally productive partly due to the currents that bring nutritious water through Lembeh strait regularly, partly due the black lava sand more or less defining Lembeh that leaks nutrients into the water. It is easy to envision that such an environment with loads of biological production would be very nice to live in for the creatures inhabiting the strait.

Wire coral gobies, feeding on plantkon that is swept through the strait by currents

Wire coral gobies, feeding on plantkon that is swept through the strait by currents

Surprisingly enough, evolution seems to have been working overtime  in Lembeh, partly shaping the foraging skills of the animals here, but even more obviously perfecting their skills of evading predators. Why is the pressure on prey animals tougher here in the seemingly benign waters of the strait than in other less productive environments?

Lizard fish with the elongated fin rays of a former sand diver in Lembeh

Lizard fish with the elongated fin rays of a former sand diver

Mathematical models of predator and prey populations give us the answer.  For every population there will be a maximum number of individuals that the environment can provide for. This number is called the carrying capacity of a population. If one increases the carrying capacity for a prey population, the prey population will increase the number of offspring that is produced. However, if there are predators around, predators will take advantage of the increase in the reproduction of the prey population, and the predators will increase their population size, leaving the prey at a low but very productive density. Thus every prey individual alive will be faced with a much higher risk of being killed by a predator than in a less productive environment, setting the scene for evolution to try out more and more bizarre and elaborate ways for the prey to survive the onslaught of the predators. Obviously there will be competition between prey on being safer than anyone else which will feed evolution with a drive to use whatever genetic variance giving anyone an advantage over conspecifics or individuals of other species.

Soft coral crab with an almost unbelievable camouflage on its soft coral host in Lembeh

Soft coral crab with an almost unbelievable camouflage on its soft coral host

Why stay here then? Is there really anything good at all living in productive environments then given that you as prey face a never-ending threat from predators? Well, it turns out that the alternative is just as bad. In prey populations that are controlled by food availability rather than predators, prey will reach densities where the food resources are heavily used and most everyone is on the verge of starving to death. The sad truth about being an animal in nature is that, with very few exceptions, you either live a life where every day is a constant struggle to make ends meet, feeding on the very scarce resources not already being utilized by someone else, or you live in constant fear of being torn in pieces by something bigger and fiercer than you are! Sucks, doesn´t it!

Shrimp seeking refuge in anemone

Shrimp seeking refuge in anemone

Luckily for me, as I really enjoy exquisite examples of prey adaptations to evade predators, many prey animals in Lembeh will be on the “lots of food around, but holy smoke it is scary here”  end of the scale. Every dive here will give even a first time diver in the area many examples of what living in such an environment does to prey animals. Also, as a nice side effect, the strait is littered with predators, which we will get back to in a later blog.