‘Men are from Mars, Women are from Venus’. It’s a well-known saying that highlights just how different men and women really are. And it’s the same throughout the natural world: males often look and behave very differently to females because of the separate roles they play. It’s a topic we’ve covered before on this blog – consider the tiny, parasitic male anglerfish, for example – but no animal exhibits the striking differences between the sexes more astonishingly than the green spoon worm.
The picture above depicts an adult female green spoon worm, which can reach 12cm long. The male isn’t pictured, and for good reason – he’s a mere 3mm long, thousands of times smaller than the female. The female spoon worm produces a vivid green pigment in her skin called bonellin, and although this chemical is highly toxic to most other organisms, it plays a significant role in the worm’s sexual differentiation. In fact, the planktonic, free-swimming larvae of this species are initially sexually indifferent – their sex is only chosen when they fall to the bottom of the ocean.
If a larva lands on the sea floor itself, it develops into a female. But if a larva comes into contact with the bonellin on the skin of an adult female, it is masculinised by the process (although perhaps ‘masculinised’ is not the right word, considering he’s the smallest male in nature in comparison to his mate). Now a tiny male, he is sucked into the giant female’s body via her feeding tubes. It is there that the male spends the remainder of his life, living as a parasite within her genital sac, producing sperm to fertilise her eggs. Since the majority of a male’s tiny body is taken up by his reproductive organs and is devoid of any other structures, he must absorb any nutrients he needs directly from the female herself.
Choosing your gender in this way is certainly unorthodox. Why should the green spoon worm have evolved in this way? Well, the chance of a male spoon worm finding a mate are pretty slim, and so too are the chances of a female worm finding a suitable burrow. So if a genderless larva lands on the sea floor, it’s better to become a female so it can find a home; if it lands on an already-mature female then it’s better to become a male so he can fertilise her. The ability to decide on a gender at the last possible moment gives the species great flexibility and enables them to make the most of a bad situation.
As far as bad situations go, though, having a body that consists of little more than a pair of gonads and being forced to live inside your gigantic mate certainly takes some beating. Not even the male anglerfish is jealous of the green spoon worm, and that’s saying something.