Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world.

Someone’s got to do it (Image: Miguelverdu under a CC 3.0 share-alike licence)

Species: Lysmata wurdemanni

Habitat: Atlantic coasts of North and South America, and the Gulf of Mexico, having confusing sex

Changing sex is more common than you might think. Many animals start out as one sex, and then change into the other part-way through their lives. There are also plenty of animals that are both male and female at the same time.


But a few go one step further. They start out as one sex, and then transform into hermaphrodites.

The peppermint shrimp is one of these rare beasts. It gets its name from the red stripes that run along its translucent body, which make it look like a peppermint stick or candy cane.

It first matures as a male, and sometimes turns into a hermaphrodite with both male and female sexual organs. This lifestyle, named by an extreme-pronunciation enthusiast, is called protandric simultaneous hermaphroditism.

Pep up your sex life

But most peppermint shrimps prefer to stay male. All-male shrimps are more successful at finding mates than hermaphrodites acting as males – probably because they can put more effort into trying to find a mate – and they will delay changing sex if there are hermaphrodites present.

In fact, the decision whether or not to change is determined by the size of the social group. To maximise their chances of one day being able to mate, shrimp living on their own always turn into hermaphrodites, even though they end up growing more slowly because of the energy spent on making eggs.

It seems that the shrimp are playing an advanced version of “chicken” called the hermaphrodite’s dilemma, in which everyone tries to stay male as long as possible and hopes that someone else will jump first and turn hermaphrodite.

But perhaps making the switch isn’t such a tough decision as we thought. Junda Lin of the Florida Institute of Technology in Melbourne, Florida, has spent much of the past 10 years studying peppermint shrimp, and his latest study shows that being male is not as advantageous as it seemed. Although it costs males much less energy to make sperm rather than eggs, the business of mating itself is costly: males must find a hermaphrodite mate and then chase it for up to eight hours.

As a result, a single mating can cost the male 15 per cent of the physical growth he would otherwise have attained. This means that males that mate grow more slowly and end up smaller than hermaphrodites, which often outcompete them for food.

Choose to switch

Faced with such a complex lifestyle, the question immediately arises: what’s the point?

A straightforward switch from one sex to the other can make sense if the two sexes respond differently to size: if females do better when they’re large, but males do better when they’re small, as in the worm Ophryotrocha puerilis puerilis, whose growing males change sex once they reach a certain size.

Changing into a hermaphrodite might seem a natural extension of that strategy. After all, hermaphrodites can mate with whoever is available, and so should have the best chance of passing on their genes.

But in fact both maleness and femaleness have a lot of costs – females must produce food-rich eggs, males must engage in courtship – so doing both at once is a last resort. For this reason, throughout nature slow-moving animals that rarely meet a mate are the most likely to be hermaphrodites.

But if that’s your situation, why not do what many hermaphrodites do, and be both all your life long? It might be a sort of evolutionary laziness. Once an ancestral shrimp had evolved the ability to change sex – a major evolutionary change – it would be only a small step to keep its male sexual organs rather than dismantle them.

In other words, peppermint shrimp may have started out as conventional sex-changers, and then hedged their bets by keeping the old set of gonads.

Journal reference: Journal of Experimental Marine Biology and Ecology, DOI: 10.1016/j.jembe.2010.08.007

Read previous Zoologger columns: Death by world’s longest animal, Live birth, evolving before our eyes, Sympathy for the piranha, The world’s most fecund vertebrate, Whale-eater’s helpful sulphur-powered guests, Horror lizard squirts tears of blood, Secret to long life found… in a baby dragon, Eggs with an ‘eat me’ sign, How did the giraffe get its long neck?, The toughest fish on Earth… and in space.