Monday, February 6

Are sharks the best lovers?

Scientific popularization and “best lover” seem to be concepts that are not very compatible. The first thing, even though a language accessible to a wide and diverse public is used, it is a rigorous task based on scientific data and, therefore, objective. In contrast, the consideration of “good lover” must be one of the most subjective concepts that may exist.

In fact, it is fascinating to verify the variability of practices, requirements and procedures considered by our peers as stimulants for obtaining optimal performance in mating rituals. The brilliant maxim of “there are people pa’ tó” is absolutely applicable to love strategies.

That is why I will focus on clearly objective and testable arguments. However, the first thing to be clear about is how sharks do it.

Very rare for a fish

In most aquatic animals, fertilization is external. This is easily understandable if we remember the fact that spermatozoa are mobile cells that swim. The possession of a terminal scourge, which they whip like Indiana Jones his whip, gives them their propulsion in a fluid and dense medium such as water.

With this idea in mind, we can imagine that the seas, oceans, rivers and lakes are, basically, a huge soup where the chickpeas are the ovules released by the females of many species and the noodles (micronoodles, since they are much smaller ), the spermatozoa of their corresponding males.

The specific chemotaxis and affinities of the membrane proteins of both types of sex cells cause each sheep to go with its mate. That is, that the spermatozoa of a certain species fertilize the ovum that corresponds to it without experiencing any type of molecular attraction towards the ovules of the multiple and varied other species.

Most of the fish taxa present this modality of fertilization. The exception is the chondrichthyans, a group of cartilaginous fish that includes, among others, sharks.

Sharks are cooler than anyone. Sharks have internal fertilization.

What does it mean to have internal fertilization?

Internal fertilization is a transcendental evolutionary invention. And no, I’m not referring to how funny or interesting the physical encounter necessary for it to take place might be, as readers may be thinking. It is a first line evolutionary conquest because it is very profitable, energetically very profitable.

The free discharge of gametes into the environment is an unsafe process where the chances of reaching the final objective and fertilization occurring are very low. Let us not forget that gametes, like any biological material, are fundamentally food for animals.

Furthermore, assuming that the improbable process of karyogamy is completed and the zygote is formed, that embryonic development progresses correctly without circumstances that abort it is a really improbable fact.

So how do you circumvent this cruel fate?

Well, playing with the quantities. Males and females produce vast numbers of gametes “hoping” that even a tiny proportion of them will succeed. The rest will be wasted reproductive material (although used as part of the menu of anyone who passed by hungry).

It seems clear that reproductive performance would improve substantially if gametic losses were minimized. Or, if you prefer a more direct language, if the sperm could access the ovules in a protective, calm and safe little love nest.

Well, oh, wonder! The copulatory appendages appeared. Physical and exclusive channels of contact between gametes. Direct meeting bridges without predators, without waves, without thermal oscillations… without surprises.

The consequent increase in the efficiency of the reproductive process led to natural selection favoring everything that involved an improvement in the copulatory organs. Sharks entered this evolutionary line and… they succeeded!

The miracle of the pterygopods

The copulatory appendages of male elasmobranchs (basically sharks and rays) are called pterygopods or claspers. And they are amazing.

The use of this qualifier is not capricious. These organs include such remarkable characteristics as the following:

1.- First of all, we will talk about size (an objective, measurable character that tends to obsess the staff quite a bit). When they reach sexual maturity, their wingspan can make up a very large proportion of their total body length. In fact, in Leucoraja erinacea we are talking about a third of the length of its body (and almost half if we eliminate the segment corresponding to the tail). Here, of course, size matters.

2.- Secondly, they do not need any special stimulation to reach a rigidity that allows efficient copulation because they have a skeletal system. Yes, how do you hear it? Pterygopods do not need to stand at attention: rigidity is guaranteed from the factory. The basipterygium, a terminal cartilage of the pelvic fins, is extraordinarily elongated, entering the end of the clasper and giving it rigidity. In addition, this rigidity is maintained throughout life, as it is caused by an internal stem of a skeletal nature.

3.- In many species, this cartilage gives rise, in its terminal part, to a skeletal piece (the rhipidion) that flattens and widens to ensure maximum dispersion of the sperm inside the female’s genital ducts.

4.- I reserve the last piece of information for last. Sharks do not have a copulatory organ… they have two! Remember that I have previously commented that the claspers are modifications of the terminal end of the pelvic fins. As these are paired, arranged one on each side of the anus, the result is that they have two pterygopods.

In short, the pterygopods of sharks are huge, they are always in perfect state of review (stiffness), they have a built-in sprayer and they do not need Viagra when the years go by mercilessly. In addition, and in case something fails in one of them, there is another spare. Let’s see what male human improves that!

This article has been published in ‘The Conversation‘.

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