P.B. Marko/Ecology Letters
Solenosteira macrospira, left, carries snail eggs on its shell. But not all of the eggs were fertilized by him. Females, like the one on the right, deposit the eggs into papery capsules and attach them to the males' shells.
A male Solenosteira macrospira, left, carries snail eggs on its shell. But not all of the eggs were fertilized by him. Females, like the one on the right, deposit the eggs into papery capsules and attach them to the males' shells. P.B. Marko/Ecology Letters
A man is not a mollusk, and many men probably think that's a good thing. And it's not just because a mollusk is a squishy invertebrate with a shell. It's also because for at least one species of mollusk, the males do all the heavy lifting when it comes to childcare.
The species of mollusk we're talking about is Solenosteira macrospira, a marine snail about 2 inches long. These snails live off the coast of Baja California, and during the mating season, the beach is awash with male and female snails in connubial bliss.
But after the deed is done, something very strange happens. In most snail species, the female deposits the eggs her partner has fertilized in the sand or attaches them to a rock. But when it comes to Solenosteira macrospira, the female deposits the eggs into papery capsules and attaches them to the male's shell. And it's the male who has to lug around these developing snails for months.
It's not unheard of in the animal kingdom for males to bear the brunt of early child rearing.
Take sea horses, for example, says Richard Grosberg, an evolutionary biologist at the University of California, Davis. "Male sea horses take care of the offspring. Male water bugs also carry eggs and babies on their back. There are toads that do the same thing as well."
But Grosberg says in the case of these sea horses and water bugs and toads, the offspring males are caring for are their own. Grosberg began to wonder if the same was true about these snails. It's not an unreasonable question; it's quite a Bacchanalian scene on the beach at mating time.
"There are thousands and thousands of males and female snails, all mixing and changing partners presumably, so we wondered whether or not the males that were caring for the babies were actually the fathers of the babies that they were caring for," he says.
So Grosberg and his colleague Stephanie Kamel brought the males back to the lab and did paternity tests on those itty bitty snails the males were schlepping around. And lo and behold: "We found that in fact, very few of the offspring that the male was carrying around were his," Kamel says.
In other words, says Kamel, not only was the male snail doing all the heavy lifting — and becoming an easy target for birds with a bunch of bright white eggs on his shell — "he was taking care of babies from, basically, 25 other guys."
Kamel and Grosberg report their findings in the journal Ecology Letters.
From a strictly male-chauvinist-pig point of view, this behavior is incomprehensible. But Grosberg says it's also odd biologically speaking because normally parents don't put themselves out for the offspring of others.
"In purely selfish terms, the smart thing to do, if snails were smart, would be to only care for their own offspring," he says.
But biologist Suzanne Alonzo of Yale University sees this snail sex saga somewhat differently.
"I find the results striking, but I actually don't find them surprising," she says. "I think they actually make a lot of sense." By unselfishly carrying around the eggs of others, a male snail is showing that he's a caring father and that makes females willing to mate with him.
"So as long as he mates with her and sires a bunch of offspring, it doesn't really matter if they're on his back or someone else's back," she says.
So yes, they're carrying some other jerk's kids, but then some other poor slob is carrying around theirs — a kind of molluscan version of all for one and one for all.