You wouldn’t think that the sex life of the horseshoe crab would be that complicated, but you’d be wrong. Like every red blooded guy out there, horseshoe crabs have to think long and hard about the best way to attract a mate (even though horseshoe crab blood is actually blue). While human males have many options to choose, from say writing a romantic poem, or buying a lovely bunch of flowers for their sweetheart, crabs are generally limited to two different options.
In animal behaviour circles, these choices are called alternative reprodcutive tactics (ARTs). For horseshoe crab males, the first option is to attract a female out in the sea, attach together and make the long journey to the spawning beaches, where they can get freaky. This option is dependant on the male being attractive enough in the eyes of the female to be worth attaching to. The second option decidely third-wheel-ish. Instead of putting the effort into attracting a female oneself, the male could wait alone at the beach for an attractive male to come along with a female in tow. He can then, rather rudely, join in on the action. In this case, he is bargaining on his own sperm to be able to compete with that of the other male’s to fertilize the female’s eggs first. This doesn’t always work, however, or no male would ever chose the first option.
Why would a horseshoe crab chose to risk not being able to breed then? The answer is, that both options have their benefits, but they also have their costs. Finding your own female in the ocean obviously has its benefits – you’re definitely going to get some action and a chance of reproducing when you make it to the spawning beaches. But what is the cost?
Scientists from the University of Florida wanted to delve into this unexplored crevice of crab sexuality, and so begun examining the two types of males – those they describe as attached (the sexy ones) and satellite males (the uninvited threesome-enjoying ones).
What they found was the attached males who were sexy enough to find a female produced, on average, 57% less poo, than satellite males. Their digestive tracts contained 2.5 times less food in it.
They also measured the levels of the rare isotope of nitrogen – δ15N. Attached males had much higher levels of δ15. Higher levels of this isotope usually mean that the animal is nutritionally stressed, because of a recent period of fasting. However, it can also mean that the crab has been eating foods naturally high in δ15. To test which was causing the effect, they analysed the contents of the crabs gut. They found the attached males had actually been eating mainly sea grass – which itself contains only very low levels of δ15.
It seems that attaching to a female prevented the crabs from eating as much as they normally would (completely different from the human phenomenon of males putting on weight after they get married.) The scientists believe this is the root cause of male horseshoe crabs performing different reproductive tactics. Although attaching to a female early on can help your chances later on, it means forgoing eating. This self-induced starvation may harm the males later on, as they might not have the energy to move or to grow or to invest in future sperm. It’s obviously up to the crabs to decide, at any one time, what the best strategy for them is. Good luck to them.
- Horseshoe crab blood is highly sought after by pharmaceutical companies. Apparently it’s really good for soaking up contaminants.
- Seabirds love to eat the eggs. So much so that the female crabs have to lay over 88,000 eggs just to make sure some survive.
- In the 1900s, horseshoe crabs were dried for use as fertilizer and poultry food supplements before the advent of artificial fertilizers.
Smith, M., Schrank, H., & Brockmann, H. (2012). Measuring the costs of alternative reproductive tactics in horseshoe crabs, Limulus polyphemus Animal Behaviour DOI: 10.1016/j.anbehav.2012.10.021