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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.
When I arrived in Lembeh this time it was just after Christmas. However, I was up for another Christmas treat. Simon here from NAD had ordered a bunch of cool stuff from Nightsea, strobe filters, filters for the lens and a cool pair of yellow spectacles, which waited for me here. I have now tried this system during my stay and will in this blog give a short overview over what I learned from shooting it. But first of all I want to give a brief explanation over what fluorescence is, and why we can find it in nature. BTW, the complete system is available for rent here in NAD if you wish to try it out.
First of all, fluorescence is often confused with bio-luminescence. Bio-luminescence is found in more and more animals, and in a number of mushrooms as well. Well-known examples are those of plankton giving of light when disturbed, deep-water organisms with light organs, mushrooms glowing in the forests, fire flies and for northern areas glowworms. Bio-luminescence is the emitting of light involving a chemical reaction. Very generally, the light emitting substance is a protein called luciferin, which emits light through a chemical reaction catalyzed and oxidized by an enzyme, called luciferase. Thus, a chemical provides the energy fueling bio-luminescence, using oxygen in the process.
Fluorescence, in contrast, is the emission of light where energy from one (or on rare occasions two) photon excites an electron into a higher energy orbital. After a short time, the electron will return to its former level, emitting the excess energy as light of another wavelength. No oxygen will be used in this process, as the light emittance is fuelled by the energy in the photon. Many different subjects in nature fluoresce. Despite that, we can seldom see the fluorescence, as the emitted light level is low, or in wavelengths we cannot detect. One notable exception is the red or orange anemones that we sometimes can se in much deeper water than red and orange colors from sunlight penetrate water. Despite being fairly poorly understood on a biological level, fluorescence is used in many applications, from mineralogy, oil detection, microbiology and forensic work.
So why do things bio-luminesce or fluoresce? Bio-luminescence in marine systems is used for at least three widely different purposes. Most of the deep-sea bio-luminescence seems to be used in order to attract prey. Second, a number of fish living in the zone deep enough for a little light to get through, use bio-luminescence to counter shade the ventral side of them, so shading against the faint surface light can not be used by predators to find prey. Third, and maybe most speculative, it is thought that small crustaceans that bio-luminesce do so to deter small predators. Why would small predators be afraid of light? Well, if a small more or less translucent predator eats a bio-luminescing crustacean, the small predator will light up and attract the next step upwards in the food chain, increasing small predators risk risk of being killed them selves.
What is the point of fluorescence in marine systems? For a number of shallow water cnidarians, mainly the ones using zooxanthellae for their energy input, fluorescence has been suggested to be a way to control excess levels of sun exposure, limiting the damaging effects of uv-light.
Both proteins in the coral itself as well as chlorophyll in the zooxanthellae associated with the coral may fluoresce. The available evidence, however, does not really support this theory.
A number of other animals fluoresce. Some crustaceans, such as the anemone hermit crab fluoresce. Also some bristle-worms, fish and cephalopods fluoresce. There seems to be no reason for these animals to fluoresce, so much fluorescence simply seems to be a side effect of other processes in living creatures. Whatever the cause of fluorescence it really is quite magical to see the different sources of fluorescence light up leaving the rest of the surroundings pitch black when diving. Try it out, it is an experience I am sure you will not forget!
What did I then learn from shooting this system? A to me very surprising fact is that people are not wildly enthusiastic over the results! That might of course be my results that are lacking. Furthermore, the best results, bleak as that may be, are when there are more than one fluorescent color in the picture. Third, and maybe most important is that it is a lot of fun to try it! I look very much forward to try it on coral reef sites such as in the Red sea as well as on land in rain forests. In a month or so I might get back to you with results from that.
Meat, being high quality food, is a scarce resource in nature. The available amount of meat for any consumer is many orders of magnitude less than the amount of vegetation that is available. If you don´t believe me, just take a look out the nearest window and do a quick calculation over how much animal weight and how much tree or other plant weight you see out there. With notable exceptions such as my home area (where all you will see right now is a lot, and I mean a lot! of snow) and a camel market in the middle of a desert, meat will be extremely scarce and often not even around. Where I sit right now I see probably hundreds to thousands of tons of trees. I see one (very annoying) fly representing available meat.
Octopus are essentially soft globs of meat living in a world where meat in general is extraordinarily scarce and needed. Octopus will face a never-ending struggle to keep their meat to themselves, and not let anyone else use it for food. It is no wonder that octopus have a number of defenses to actually accomplish monopolizing their meat. Once being seen by a predator, octopus have several defenses. First of all they will try to escape, either by swimming quickly away from the predator or by getting into a small crevice or hole that the predator cannot penetrate. Second they can let go of an ink cloud in order to confound the predator, or, best-case scenario, have the predator attack the ink cloud as the octopus escapes. Third, some octopus can let go of limbs when threatened much like lizards loosing their tail, leaving the predator with a meal but keeping most of the octopus unharmed. Fourth, some octopus can use warning coloration advertising a highly venomous bite when threatened.
However, octopus go a long way to keep from having to use their active defenses. they do so by blending in in the environment, that is being camouflaged, in a way that is highly efficient and also very adaptive. One problem with using camouflage in general is that camouflage is site specific. Once out of the area the camouflage pattern is adapted for, the camouflage stops working. A Swedish soldier in his very efficient camouflage for Swedish winter conditions would, if transferred to tropical rainforest, stand out as a huge boil on the forehead of Naomi Cambell! As octopus are highly active, they have to be able to adapt their camouflage to changing environments, which they solve in a way very few other animals do.
Texture and color are the two major components of an environment that a camouflaged animal must handle. Octopus handle texture in an impressive way. They can change from looking like a smooth coral to a piece of broken rubble in a matter of seconds. They do this by using muscles in their outer layer of skin, using their very acute eyesight to map an exact picture of the surrounding environment in order to match the texture of the environment with that of the outer surface of the octopus. Some octopus can go from smooth to looking like a hairy piece of decaying sea floor in mere seconds.
Octopus are even more impressive when handling colour matching of the environment. First of all, their sense of sight is excellent, enabling the octopus to know what they have to match. Second, the outer layer of octopus skin has a number of different specialized skin cells, which can change the colour, brightness and reflectiveness of the octopus. These cells are under control of the nervous system of the octopus, enabling extremely fast responses to outer stimuli.
In this way, octopus and many of their allies can live their life being highly successful despite inhabiting a very dangerous environment.
Few humans have gone through a sex change. Changing sex in humans is a major effort, involving long-term counselling, surgery and medical treatment. Very few other terrestrial vertebrates, if any, change sex during their life cycle. In contrast, in the underwater world, sex change is very common. And this sex change does not involve any surgery, any hospital stay and any medical treatment. How do marine animals change sex so easily, and why do they change sex in the first hand would be reasonable questions to ask.
First of all, in many animals sex is not determined by chromosomal factors. Thus in many reptiles such as sea turtles the temperature during egg development determines what sex the offspring will have.
In marine systems, non-chromosomal sex determination is mostly a consequence of social rank or, in some cases size, which in itself may be correlated to social rank. Thus most wrasses, basletts and groupers start out as females while large individuals turn into males, while anemone fish start out as males, and, typically, only the largest individual in an anemone changes to being female. In both sex-changing groups, all individuals have both testis and ovaries, but at the most one of those are active at a given time.
Why change sex then. The sex change from female to male is more or less exclusively seen in species where males keep harems or gather in communal mating areas and fight for dominance over females. In such species, it is utterly pointless (with one exception that I get back to in a later blog) to be a small male. As small males do not get any matings, and few individuals reach sized large enough to dominate other males, it makes a lot of sense to start out life as female to ensure participation in the reproduction. Only if an individual for some reason has a better than average life will it reach sizes large enough to change sexes. Such a sex change, when large, will give access to many females and will be very successful in terms of offspring production.
The sex change from male to female does, to my knowledge, only occur in species with strict monogamy. In fish, as in most animals, large individuals will have more excess energy to put into sex cells. As the miniscule sperm cells are cheap to produce, and eggs are many times larger and much more energetically expensive to make, it makes a lot of sense for a monogamous pair to let the larger individual be a female and the smaller a male. Obvious examples of this are the anemone fish, where typically only one individual is a female and at the same time the largest individual in the anemone, while the rest of the resident fish are males. Only when the female dies will the former second largest change sex and become a female.
Finally, some species have no sex determination, but lives life as both sexes. Such animals are called hermaphrodites. A well-known example from land is the common earthworm. The classic example from marine systems is the nudibranchs, where every individual is both male and female. When nudibranchs mate, both partners transfer sperm and both have their eggs fertilized.
When I visited Lembeh in September/October there were flamboyant cuttlefish around in numbers that I have never seen before. On more or less every dive, we saw one or more flamboyants. Blue ring octopus were not as common, but still there was hardly a day when we didn´t see them. This is very much in contrast to the situation now, at least considering adult octopus and cuttlefish. We have seen some, but while still here, they are not nearly close to the densities experienced in September/October. Where did they all go?
Actually, cephalopods, the group that octopus and cuttlefish belong to are quite short-lived. Not only that, they only reproduce once and then die! This pattern of reproduction, where an individual grows for almost all its life, mates and then die, are called semelparity.
Other animals continue living after reproduction and produce other litters some time after. Such a strategy, with multiple reproductive events after the onset of maturity, is called iteroparity.
The answer is that all life histories, that is the allocation of energy over time, consist of trade offs. Food is seldom unlimited, and if it where, time or developmental rate would limit the options available for an animal. The first ting that needs to be realized is that reproduction, having offspring, is expensive. Experiments have time after time verified that reproducing individuals of a lot of different species die earlier than non-reproducing individuals. That is the first trade off, choosing between a long life or offspring. And having no offspring is a fundamental screw up in nature, as your genes get lost for the future, so choosing long life rather than reproduction is not an option for animals looking for success in life.
The second trade off is that costs are related to the effort that is put into reproduction. Everything else equal, the more offspring that is produced, the less survival or future growth rate an individual will experience.
The third important trade off is that an individual need to keep significant amounts of energy reserves in order to survive over long time in the future. Up to around 90% of the energy budget might have to be allocated to future survival if that is the goal. Thus, an individual can produce a vastly higher amount of offspring in one reproductive event if no energy is saved for future survival. Thus, if an individual irrespective of reproduction faces quite high mortality rates, it could obviously be advantageous to “go all in”, reproduce with all the effort you can, and save nothing for a very uncertain future. And that is exactly what cephalopods do, invest all they got in one single “big bang” litter instead of saving themselves for another day.
The cool effect of that here in Lembeh is that there now are quite a number of minute, fully colored flamboyants around. The young flamboyants are really cool, brightly colored and have a lot of attitude.
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.