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Paternity testing shows: Size does matter for lobsters

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Researchers have taken DNA samples of 115 females, 1,150 fertilised eggs and 650 males from a reserve and from a control area.

Photo: Espen Bierud

If they have plenty of choice, female lobsters choose sex partners with a big body and a muscular crusher claw. However, new research shows that fishing pressure may affect their beauty ideal.

Marine scientist Tonje Knutsen Sørdalen has compared how lobsters choose partners in the Flødevigen lobster reserve with a control area outside it. There are clear differences.

“By DNA-testing males and females with fertilised eggs, we can discover which of the male lobsters have become fathers”, explains Sørdalen.

In the reserve, size matters

In the reserve, there is a ban on catching lobsters. As a result, it is home to more and bigger males than areas where fishing is allowed. The minimum size limit for catching lobsters is 25 centimetres, so bigger lobsters are more exposed to fishing pressure.

“In both areas, females choose a partner who is bigger than them. But the difference in size between the males and females is much greater in the reserve”, says Sørdalen.

“The relationship between the body size and claw size of males may also affect their sex lives. Big claws give an advantage when fighting, and they may be attractive to females”, she continues.

Stop caring when there are few males to choose from

In the lobster world, ladies do the chatting up, while the men fight and show off.

“In the area outside the reserve, it appears that the females don’t care about size. They don’t all go after the few, large males, as you might expect. They simply stop caring”, says Sørdalen.

She thinks a possible explanation is that since lobsters are so few and far between outside the reserve, the females find it difficult to judge what is attractive. The fact that they are less picky, may have long-term consequences.

Lobsters may become smaller in the long term

“If males no longer benefit from being big, it may result in lobsters becoming smaller in the long term. Our study is the first to demonstrate empirically that fishing activity can affect sexual selection”, she concludes.

Last fishing season, a maximum size limit of 32 centimetres was introduced for lobsters in Skagerrak, on the advice of the Institute of Marine Research.

The new study is published in the journal Evolutionary Applications: onlinelibrary.wiley.com/doi/10.1111/eva.12611/full