Finding Nemo’s wrong biology

Finding Nemo is an amazing movie. In the movie, a fish called Marlin loses his wife and eggs in an attack by the fearsome Barracuda fish. Only one egg survives. When it hatches, Marlin names the baby fish as Nemo. Due to the painful memories of the loss, Marlin becomes overprotective of his son. But in spite of the protection of his father, Nemo ends up in a fish tank, and Marlin undertakes a heroic journey to bring him back.

Though a heart warming and eminently watchable movie, Finding Nemo has a biological inaccuracy. But before we can understand that, we turn our attention to an ancient divide – the male-female difference.

The fact that even the fish have fathers and mothers seems unremarkable to us. We take the division of any population into males and females for granted. The males and females of human beings have strong differences of body and mind, so much so that the two genders are supposed to be natives of different planets. The animals we see and recognize (mostly mammals) like dogs and cattle seem to have similar differences of gender, though we don’t actually know whether oxen don’t like to ask directions and cows are terrible drivers.

In human beings, gender is more of a social concept, defined by behaviour, role and responsibility. Most societies now recognize biological sex as different from gender. While the concept of gender is meaningful only in human beings, the biological differences in the male and female sexes are applicable to all living organisms.

But what exactly is a male or female? Biology’s definition of sex is very simple and it has to do with what kind of reproductive material the individual produces.

Sexual reproduction is basically the fusing together of two cells, each carrying only half the genetic material required. The fusing creates a cell with full genetic material, which then grows into a complete organism. This mixing of genetic properties gives sexually reproducing living beings an advantage in survival. This is the reason behind the popularity of sexual reproduction among species.

The two cells that fuse are not alike. One is very small but can move fast, the other is big, but stationary. These two cells are called gametes, and it is this asymmetry in the gametes that gives rise to the male-female difference. The smaller gamete is called sperm and the individual that produces it is called male. The larger gamete is called ovum (or egg cell) and the individual which produces it is called female.

(To set the record straight, there are species in which both the cells are alike, but we won’t talk about them here, to keep matters simple.)

Looking at the familiar world of living beings around us, we can easily make two rules:
- An individual is either a male or female
- Once an individual is a male (or female), it remains so for the whole life

So obvious these rules look, it hardly seems necessary to write them. But nature is far more innovative than that. In fact, none of the above rules is true.

Well, firstly, there are the hermaphrodites. There are many species in which an individual is both male and female. To begin with, most plants are hermaphrodites. But even if you take just the animals, over 5% of the species are hermaphrodites. The most familiar examples are the earthworms and garden snails.

Hermaphrodites don’t actually need another individual for reproduction. They can fertilize themselves, but usually they don’t, for the main reason that underlies sexual reproduction – mixing of genetic material is good for survival. Many species, especially plants, however fertilize themselves when it becomes difficult to find gametes from another individual.

Usually the biological sex is determined by the genetic material. In mammals the default sex of an individual is female. But if the genetic material contains a certain piece called the Y-chromosome, the individual becomes male. In birds, the default is male, and presence of a W-chromosome makes the bird female. The sex assigned by genetic material remains fixed for life.

Some species however use more colourful methods of sex determination. In many reptiles, the sex is determined by the temperature at which the egg is hatched. In turtles, for example, a lower hatching temperature produces a male.

But the weirdest strategy is used by the species that change sex during their life. And here’s where we meet Marlin the fish once again. The fish species used in Finding Nemo is Ocelleris clownfish, or simply clownfish. In clownfish, the smaller fish are male and the largest fish becomes female. If the largest fish is to die, the next largest fish will become female!

So in a real life plot of Finding Nemo, Marlin the father fish will become Coral the mother after the Barracuda attack, while Nemo will continue to remain male. According to published stories, this was actually pointed out to the makers of the movie, but they chose to ignore this scientific detail for the purpose of simplicity, a position I wholeheartedly support.