A Blog by Carl Zimmer

The Mystery of The Rare Male Sea Monkey

Here we see a happy, typical family of sea monkeys. Note the red bow and plump lips that indicate the female of the species, and the tall body and protective stance of the male. I assume that the father’s well-placed tail blocks some other clues to his identity. The parallels between the sea monkeys and the human family (see inset) are uncanny and surely nothing more than a coincidence.

Photo by justaghost, via Creative Commons. Image linked to source.
Photo by justaghost, via Creative Commons. Image linked to source.

The real life of sea monkeys (brine shrimp, or Artemia) is a pretty far cry from Ozzie and Harriet. Sea monkeys don’t live in families, for one thing. And in a lot of populations, the females have no need for males. Their eggs can develop into healthy embryos–and, eventually, adults–without the need of sperm. You can take that picture of sea monkeys and wipe Dad out.

From an evolutionary perspective, this father-free way of life has a lot going for it. Let’s say you’ve got a sexual pair of male and female shrimp in one tank, and two asexual females in the other. Let them breed for a while. Sexual species typically produce a roughly even ratio of sons and daughters. So only half of the sexual population can produce eggs, while every individual in the asexual one can. It won’t be long before the asexual population is far bigger than the sexual one. Out in the wild, this proliferation should mean that the genes for male-free reproduction should quickly dominate populations. Down with sex, in other words.

But this has only occurred in only about one in every ten thousand species of animals. Sex must have a powerful advantage that overcomes its disadvantage–what the late biologist John Maynard Smith dubbed the two-fold cost of sex.

Scientists have given this question a lot of thought, and they’ve come up with some possible answers that they’ve been testing in recent years. Maybe sex lets adaptations evolve faster, because mothers and fathers can combine genes into new combinations. Defenses against ever-evolving parasites might be especially important. There may be different explanations for different cases. Very often, when an asexual lineage emerges, it gains an extra set of chromosomes. That’s a lot of extra DNA to build when a cell divides–which requires a lot of phosphorus and other ingredients. Perhaps that’s a cost too great to balance the advantage of giving up fathers.

Or perhaps the rarity of asexual animals is the result of evolution playing out not in short-term competitions, but over vast stretches of time. Populations of sexual animals may be less prone to going extinct because they can adapt to more niches.

To better understand the evolution of sex, a number of biologists are looking to the exceptions to the rule. If the advantages of sex overwhelm its costs for 9,999 species out of every 10,000, then why is the opposite true in the remaining one? One lineage of microscopic animals called bdelloid rotifers has been asexual for 80 million years. Cornell scientists have suggested that they have remained asexual because they’ve found a way to resist parasites that’s as good as sex–by drying up and blowing away from their pathogens.

Brine shrimp. The orange masses are eggs. Photo by Paul Zahl, National Geographic
Brine shrimp. The orange masses are eggs. Photo by Paul Zahl, National Geographic

But there are other puzzles to the evolution of sex. And one involves sea monkeys. In a paper appearing in the Journal of Evolutionary Biology, Marta Maccari of the University of Hull and her colleagues describe a massive survey of brine shrimp from across Europe and Asia. They reared cysts from dozens of populations and closely examined the offspring over the course of two generations. The females in these populations can reproduce on their own. And yet in most of the populations they studied, they discovered a few males.

The males were exceedingly rare–around one in thousand in many cases, and around one in a hundred in a few. And yet they were healthy and fertile. The males couldn’t mate with females of their own population, but they readily had sex with other species. What’s more, their hybrid offspring were healthy and fertile, too.

If asexual animal species are rare, species with asexual females and rare males are even more rare. Only a few other examples have turned up, such as certain populations of snails in New Zealand. Maccari and her colleagues don’t think there’s a clear answer to why these rare males exist. But there are a few plausible possibilities.

Maybe it’s just a fluke. It’s possible, for example, that as eggs develop, a few accidentally lose a chromosome, altering their sex. Sons, in other words, are birth defects.

It’s also possible that some of the asexual brine shrimp have mutations that lead sometimes to males, and they pass their mutated genes down to their offspring. In her study on New Zealand snails last year, Maurine Neiman of the University of Iowa and her colleagues found, surprisingly, that producing a few sons that can’t mate with any females of your species doesn’t put asexual animals at a major disadvantage.

On the other hand, maybe rare males are a side-effect of brine shrimp biology. One way for females to reproduce is to combine two eggs, joining together their chromosomes into a full set. This process can produce lots of different combinations of the shrimp’s DNA, and that variation may help them adapt to the changing environment. Sometimes, though, those combinations may produce a fertile son.

The most interesting possibility Maccari and her colleagues raise is that the rare males are a way for the genes for asexuality to spread themselves. The males can’t mate with their own species, but they can interbreed with others. They may then introduce genes for asexual reproduction into the species, causing them to turn male-free. For brine shrimp, in other words, fathers may be a way of getting rid of fathers. I have no idea how you’d paint that on a box of sea monkeys, but I’d be curious to see someone try.

9 thoughts on “The Mystery of The Rare Male Sea Monkey

  1. Artemia salina, known as Sea-Monkeys respond very differently to their environment in a tank. The ratio of males and females is about 50/50. I have no idea about how their ratios are in their salt lake natural habitat. The study is probably correct there. I can send you a handbook and a sea-monkey set to conduct your own experiment if you’d like. :)

  2. I recently “hatched” a batch of sea monkeys for my kids. Only one was born out of the whole packet. Could this single sea monkey procreate and bring more life to the tank?

  3. Dr. Toyhunter,

    By chance would you be able to send me a handbook and sea monkey set so that I can conduct an experiment? This article has, t say the least, peaked my interest and curiosity. I am very intrigued and beyond fascinated by the endless capabilities and wonders that our world holds. Never did I begin to imagine, that what most (including myself) considered to be “just another novelty toy” is in fact so much more than what it appears.

    Please let me know if you are willing to do so and how I can reach you or what I can provide to you, if so.

    Thank you,

  4. Absolutely, but also keep in mind that the eggs can continue to hatch based on thermocline as of their egg and the temperature of their environment. I have had entire families die and two days later the tank is swarming with new ones.

  5. Do the males have horns? In my two batches of sea monkeys there is an aggressive one (only one) that has horns and seems to dominate the others. I noticed that it attached itself to the other by cradling the egg, wrapping its horns around the egg from underneath and then swims around dragging the other around the tank abusively.

  6. Are these still for sale using the order form at the top? I have a group home for developmental disabled women and they would love to order some but have very little money and this is a great price.

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