By Matt Walker
Editor, Earth News
Even the tiniest creatures have a discerning sex life, scientists have discovered.
They have found the first evidence that zooplankton, tiny creatures that inhabit the ocean in their billions, actively choose their sexual partners.
Despite being blind, the plankton try out and then reject the sexual advances of others, reserving their affections for bigger mates.
That suggests sexual selection plays a key role in plankton evolution.
Details are published in the journal Oecologia.
Most larger animals are known or assumed to choose their partners.
This process is known as sexual selection, and is one of the major processes, along with natural selection, by which evolution takes place.
It can lead to a host of sexual behaviours, where the two sexes joust and compete with one another to choose the best mate.
It results in males chasing females, males competing with one another, and females being just as choosy about their partners, often using surreptitious means.
Plankton diversity, drawn in 1904 by natural historian Ernst Haeckel
For example, due to sexual selection, some females, such as brown trout, will even fake orgasms to deter unsuitable partners.
Marine zooplankton are the most abundant multicellular animals on earth, comprising a collection of different tiny creatures such as copepods, krill and invertebrate larvae.
Yet until now, it was not known whether these tiny creatures are also subject to sexual selection.
Many, such as copepods, are small, blind and are carried along by currents, making it difficult for them to find one another in a vast ocean.
To overcome this, female copepods release pheromones to attract males, which try to actively swim to seek out mates.
Considering their vast numbers, encounters between copepods may be relatively rare, and they have to get within a few millimetres to evaluate each other.
Because they are blind and can only recognise each other by smell and perhpas movement from extremely close distances, a copepod can only perceive of a single other copepod at one time, meaning they cannot directly compare two partners.
Despite these relatively rare mating opportunities, the creatures remain choosy about which other copepods they reproduce with, researchers Dr Sara Ceballos and Professor Thomas Kiørboe of the National Institute of Aquatic Resources in Charlottenlund, Denmark have discovered.
They studied mate preferences in the marine copepod Acartia tonsa and found that both males and females pass up mating opportunities, holding out till they come into contact with larger, and hence more attractive potential partners.
Larger males are more attractive to females, as size in copepods is passed on to the next generation.
Larger females lay more eggs, so it is also good for males to mate with females that can produce more offspring.
"This is the first demonstration of sexual selection by mate choice in a planktonic organism," write the researchers.
Hypothetically, that could mean that sexual selection will drive the evolution of different sized or shaped plankton, as over many generations each sex selects mates with particularly attractive traits.