S.E.X. (Sexual Evolution Xenoforms)

Hermaphroditism: the phenomenon whereby an individual of a certain species can produce both male and female gametes simultaneously or subsequently.

This scientific term comes from an ancient Greek legend of Publio Ovidio Nasone, who tells of Hermaphrodite, a beautiful boy, son of Hermes and Aphrodite with which the nymph Salmace fell madly in love. Being constantly rejected by the boy, she asked the gods to join him forever in one body. Her wish was accepted and the two became one and their bodies were mixed into a creature of both sexes, half male and half female.

How gentle is Greek literature.

But returning to our marine organisms, many fish for example have this condition. Or rather, at this point we must distinguish hermaphroditism:

  • sequential, in which there is a change in sex during the life of the animal;
  • simultaneous, in which the organism presents both sexes at the same time. But be careful, there is never self-fertilization because what we are talking about is called insufficient hermaphroditism.

Furthermore, when we talk about sequential hermaphroditism we must also distinguish:

  • proterandria: individuals born male who become female;
  • proteroginia: individuals born female who in the course of life become males.

This sexual flexibility among fish is quite widespread and at least fourteen families of fish contain species that show proteroginia (e.g. Labridae, Scaridae, Serranidae), while proterandia is present in eight families of fish (e.g. Sparidae, Pomacentridae, Muraenidae). Simultaneous hermaphroditism, on the other hand, is very rare among reef fish but much more common in depth fish.

The theoretical explanations between simultaneous and sequential hermaphroditism are completely different and, therefore, we will deal with the two phenomena separately.

Ghiselin proposed a model to explain sequential hermaphroditism, the one called “dimension-advantage model”. According to this theory, which would explain the proteroginia, the fertility of a female is limited by the number of eggs that she can contain or do, which in turn is controlled by its size, its energy reserve or both. So it makes little difference if it mates with one male or many. Male fertility is not limited by the number of spermatozoa that an individual can produce, but rather by the number of females it mates with and their fertility. In other words, male fertility is potentially much more variable than female fertility.

To understand better the concept of Ghiselin: in monogamous species where both members of a pair normally have the same size, the fertility of males and females is roughly equal. Instead, coral reef fish have mating systems in which larger males monopolize the deposition of females. In this situation, the size is therefore important and small males may not generate at all due to the low fertility that concerns them. Since the distribution of fertility differs between the sexes with the size, we would expect the change of sex to be adaptive: an individual is born female and remains so until the dimensions are still too small and subsequently becomes male upon reaching the right size. In this way he will be able to generate as many children as possible throughout his life. So proteroginia will be favored by natural selection (e.g. cleaner fish (Labroides dimidiatus)).

Other coupling systems, on the other hand, provide proterandia. Males usually produce millions of sperm cells and small individuals are physically capable of fertilizing females of almost any size. In mating systems where monopolization does not occur and where mating is often random, it is advantageous to be a small male (likely that any mating will occur with larger individuals) and a large female (taking advantage of the ability to produce more eggs) (e.g. clown fish (Anemonfish)).

In both cases, the removal of the large/dominant individual from a population involves a change of sex in the next larger individual, but no change in the rest of the local population. Within a few hours the first behavioral changes begin to manifest and in a few days the sexual organs change with the appearance of the male or female ones.

Simultaneous hermaphroditism, instead, has as adaptive meaning the ability, in this condition, to be able to produce as many offspring as possible during one’s life. This condition is typical of those species that show a low population density such as at depth. But it has also been found in small sea bass common on Caribbean coral reefs (of the genus Hypoplectrus).

Various methods can be used to adopt this reproductive strategy:

  • “Egg trade” in which a couple alternates sexual roles during mating. Whenever an individual functions as a female, she expels some, but not all, of her eggs; while as a male, he fertilizes his partner’s eggs. In this way both individuals are forced to invest the same reproductive effort.
  • Another method of preventing desertion from this reproductive strategy, by avoiding non-monogamous mating (on a given day), is to reduce the partner’s opportunities to find another partner. In this case the onset of darkness and, therefore, the possibility of reproducing only at sunset, indicates a rather limited time (before the search for the refuge for the night), and therefore the impossibility of further coupling.

These concepts are very far from human reproductive abilities, which seem almost only belonging to mythology. But it is not so, as if to tell us once again that life has shown itself in its most diverse forms, which may even be strange to our eyes, but which have allowed it to perpetuate itself.

The passage of geological eras and the various conditions of habitats during these allowed the emergence of mythical individuals, who have extraordinary abilities, always and only aimed at a single purpose. I will never get tired of saying it: life.

And fortunately, that this still persists, always and forever.

Maria Bruno

Credit: Florent’s Guide to The Tropical Reef

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