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Simon actually did some serious thinking the other day after my blog on the rarity of sexually selected characters in marine systems. He came up with a couple more examples of sexual selection, one of which was the difference between the sexes in mandarin fish. Mandarin fish males are much larger than females and have very elongated fin rays. Males behave differently from females, arriving at the mating grounds well before females do, securing and defending small territories in the broken Acropora rubble mandarin fish mate above.
Males compete fiercely over the ownership to especially attractive sites. Conflicts are most often resolved by a side by side measuring of each others strengths, where obviously weaker fish fold before fighting, as fighting would not do them any good anyway. However, at times the male pair will be more or less evenly matched and a vicious fight can be initiated, where males push, flick and bite each other.
The more equal the males are in strength, the longer this fight will last, and it can in certain circumstances last even longer than the whole mating session! This kind of mating behaviour, where males gather at certain spots for the sole purpose of mating is called lek, and is best known and studied in a number of birds, such as grouse and peacock.
When the mandarin fish females arrive to the lek, they check out the available group of suitors. Females prefer large males, so the largest males get the largest number of matings, with small males getting few or no matings at all. When a female has compared available males, the female approaches the chosen one, which then initiates a small “dance” around the female. If the female still wishes to proceed with the mating, the pair rises slowly to about half a meter above the rubble patch. The female can, and will often, break of the event at any stage of the rise. Sometimes the pair rises several times before the actual mating, while on other occasions the female regrets her choice, leaves the male and start searching for another male. If the female chooses to proceed, at the peak of the rise the pair will release a small squirt of eggs and sperm.
Preferred males can in lek systems attain a highly disproportionate number of matings. Successful males can mate with several females during one night, and be back in full force the next night. Despite successful males performing with several mates during a night and females generally only mating with one male at each mating event, males can mate every night while females have to refresh their energy reserves between matings. This is obviously because eggs are much more energy rich and costly to produce than the miniscule amounts of sperm needed to fertilize a batch of eggs from a female. Such a mating system, with males potentially fathering high amounts of offspring or very few to none at all, will be very conducive to high levels of sexual selection.
Mandarin fish are one of the most popular fish in the tropical fish tank hobby. Fish collectors target big males, as big males achieve higher prices. Sadly, mandarinfish have very specific dietary requirements, eating only small live crustaceans such as amphipods and copepods. A large proportion of the wild caught mandarinfish never adapt to aquarium life, and die after quite a short time. This fuels the demand for even more mandarinfish being caught, leading to either a decrease in male sizes at mating sites, or, in extreme cases, a serious deficit of males. It is without doubt a testament to our ability to penetrate into the most hidden corners of nature that certain populations of mandarin fish, where one male is enough to fertilize the eggs from quite a number of females, now have low reproductive success due to a lack of males! Mandarin fish do breed in captivity, a feature that with time might solve the problem, but for now better control over wild caught fish numbers and sex distribution would be needed.
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.
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.
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.
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.
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.
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.
Whenever you visit the same dive area several times, some dive sites seem to consistently deliver more than others, and, correspondingly, some less than you would wish. Here in Lembeh my favorites are the Aer Bajo sites and Hairball, where many uneventful starts of dives have been turned around to glorious experiences. The different TK sites on the other hand have for some reason not really caught my interest that much, despite that if I really think hard I actually have seen some really neat animals there. My expectations for yesterday´s morning dives were less than stellar, as the first dive was in Nudi retreat (coral dive, why??) and TK 3. Nudi retreat delivered a couple of cool Xeno crabs as well as a beautiful soft coral crab, which was fine, and perfectly OK. Happy with that. TK on the other hand, in the words of a well known dive resort owner, just kicked the balls out of any kind of negative feeling about that site.
Paulus, my dive guide for the day, first found a weird flat crab buried into the rope sponges that are so common in TK (pictures coming in a later blog). Just after that a great Janolus nudibranch posed nicely,
followed by a number of “commoner” nudis, and a beautifully colored devilfish, showing of its pectoral fins.
A couple of minutes later in a small patch of rope sponges and debris, two beautiful frogfish and one common sea horse were found.
That was just the start. After that, less than 5 meters apart, a flamboyant cuttlefish was hunting,
a hairy frogfish came walking by, and, finally,
a coconut octopus did his (or her) amazing show with a couple of beautiful shells, hiding, digging, watching me and walking around with the shells.
So, TK, I am officially sorry for my negative attitude. TK after this climbs significant steps on my favorite dive site list. Simultaneously, my wow for this trip of concentrating the photo shooting to a few of the best subjects on each dive, was blown to pieces. But what can you do, this was like letting kids loose in a candy store, Danes loose in a beer pub, Djengis Khan loose in a village of pacifists or Simon loose in a camera store!
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.
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?
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.
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!
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.
A number of marine animals do occur in families. Maybe not in the sense we usually use the word to describe human family relations, but at least in the sense of a mother being followed by one or some of her offspring. Dolphins and larger whales are examples that readily come to mind.
We often encounter animals that seem to be living in families. Examples are of course the anemone fish, where two adult fish live as a dominant couple defends an anemone, and smaller conspecifics also live in the anemone seemingly without doing much (all to similar in behaviour to my kids!). A number of shrimp also seem to live in families. Tiger shrimp,
bumble bee shrimp
and sometimes harlequin shrimp can occasionally be seen in groups of two large individuals and one small, leading many observers to draw the conclusion that the small individual is the offspring of the larger ones, and that the larger ones happily tends for their “baby”.
What is that then with the “seem to be living in families” I include above? Well, the reality of the anemone fish and shrimp family life is that the small individuals are not related to the large ones. For the shrimps, the interaction between small and large individuals have not been studied a lot, while anemone fish relationships have received quite some attention from researchers. Suffice to say in this blog entry that the relationship between large anemone fish and small conspecific inhabitants of an anemone is far from that we would expect from parents and their beloved offspring.
Why am I so sure that these shrimp and anemone fish are not related? Well, the shrimp and the anemone fish all adhere to the standard way that marine fish and crustaceans, and almost everything else marine, use to multiply. Eggs in marine environments generally hatch into larvae that spend some time as plankton. Plankton are animals that are moved around by currents and have little option to decide wherever they are going. Most larvae are plankton for quite some time, which means that currents sweep them far from their mother and father. Whenever the time (weeks to months) as plankton is over and the larvae metamorphose into the adult form, the shrimp and the anemone fish settle. The likelihood that larvae should happen to reach metamorphosis size at the same time it is located close to its parents is simply extremely small. Thus, the small anemone fish and different shrimp mentioned above that live together with large, adult conspecifics are extraordinarily unlikely to be related to those adult individuals. This means that relatedness, and thus family ties, can not be used to explain why the small individuals live with the larger ones.
I will in my next blog entry shortly discuss the relationship within an anemone fish “colony”. Be prepared for a grizzly world, combining slavery, forced starvation, castration due to stress, a just-to-happy willingness to let conspecifics meet a certain death outside the anemone and cannibalism, all taking place in one and the same anemone. Quite a far way from the cutie, cuddly feeling that Nemo left most of us in!
Well, as Lembeh must be the frogfish capital of the world, and frogfish are some of my favourite critters, I think that frogfish actually merits another blog entry. So here it is.
Frogfish are probably most known for their built in bait, an illicium or “fishing rod” that is topped with the esca, the worm- or shrimplike lure it self. The illicium and esca are formed from the foremost rays of the anterior dorsal fin, and can be moved in different directions in front of the mouth when prey comes close enough.
Frogfish belong to the family Antennariidae in the order Lophiiformes, which among others also include the supercool deep water anglerfish with lures that are shining with light. With a few exceptions, the most notable being the Sargasso frogfish, frogfish are mostly shallow water fish, living among rubble, sea weed, coral heads and other underwater structures that can aid the frogfish in hiding their outline from prey and predators.
In Lembeh strait a number of species are found. I have seen eight or nine species in Lembeh, but there are probably one or two more around.
Frogfish are notoriously hard to determine to species. Many species come in different colors, so color is at best a weak clue to what they are. Colors seem to change with background, making camouflage excellent, but on the other hand not helping very much in species determination.
A combination of habitat, body shape, and, maybe most important, characteristics of the lure can most often be used to at last narrow it down to a couple of likely names. A few, however, such as the hairy frogfish with it´s wormlike lure and the warty frogfish with it´s warty exterior are very characteristic and often easy to name. Others, such as the hispid, the painted and the giant (also called Commersons) frogfish are not to hard to determine if the lure is visible.
Then there are a number of frogfish with lures that are hard to see and quite similar markings that I find pretty hard to determine to species underwater.
And finally, Lembeh is home to at least one undescribed species, which has not been named by scientists yet.
The lure of the frogfish indicates that frogfish are specializing on fish as prey. With a few exceptions, fish is exactly what frogfish eat. When a small fish comes close to the frogfish, the lure is waved in an enticing way in front of the mouth of the frogfish. The prey fish will often be conned into believing that it is the luckiest day in its life, which most often is the last feeling the prey fish will have. When the prey comes close enough, the frogfish extends its enormous mouth, inhaling a lot of water and a small, and highly likely very surprised, fish. The mouth movement of the frogfish is so quick that scientists have problems explaining how that speed is even possible.
The teeth of frogfish are very pointed and directed backwards towards the interior of the mouth. Thus, anything caught by a frogfish will have a really hard time escaping, while the direction of the teeth will allow the frogfish to move the prey inside the mouth. Frogfish are extremely elastic, being able to swallow prey as large as themselves.
Frogfish have three ways of moving. First, like most fish, they can swim through the water column using their caudal fin. They can also use a water jet by expelling water through the small gill openings behind the pelvic fins. Finally, frogfish can more or less gallop over sandy bottoms, in a rather ungraceful running motion resembling what a cross between a sea lion, a football and a horse would look like in full speed.
So, now I am of running an ecology/photo course in the Red sea. Those of you that follow this series will probably have the next entry available in a couple of weeks.