Aliens in a Drop of Water

Unless you study plankton, many of the tiny creatures in saltwater will appear new and mysterious. Plankton includes any organism (plant or animal) that lives in water and drifts with the currents, rather than under its own steam. While some of these creatures stay small their entire lives, others grow up to be familiar ocean creatures. Marine organisms are different from many land creatures, in that they often change shape several times throughout their lives. Just as tadpoles transform into frogs and caterpillars change to butterflies, young sea creatures metamorphose many times as they grow.

But, while tadpoles are somewhat recognizable as the frogs they will someday become, many juvenile marine organisms are completely different than the adult forms we are familiar with. Moreover, rather than the single dramatic transformation of the butterfly, they may go through several different stage, none of which is looks exactly like the others. In fact, of the 40 animal phyla that are found in the ocean, only seven of them lack a distinctive-looking, free floating, planktonic stage in their lives.

Why do so many juvenile marine organisms look and behave differently than the adults they will someday become? Why don't these larvae hatch directly from the egg, as a smaller, cuter version of the adult, like chickens or turtles? And why do they float around in the open ocean, instead of staying home with mom and dad?

As it turns out, there are many advantages to looking and behaving differently from your parents (I mean, other than their terrible taste in music). First of all, free-floating larvae can travel far away from where they are born, which means they don't have to share food and space with their parents or siblings. This has the added advantage that they are less likely to mate with their relatives. If conditions get bad in one location and kill everyone there, the larvae are able to settle somewhere else.

These advantages are great, but there are also many disadvantages to being a planktonic larva. First of all, getting away from your family sounds great, but you might not find a decent place to live. Secondly, while you're tiny, lots of other large animals are trying to eat you. Very few larvae survive the planktonic stage to become adults, but research suggests that planktonic larvae have a better chance of survival than larvae that stay on the ocean floor. The strange, alien shapes of many larvae are defenses against this threat of predation, which leads me to my entire reason for writing this blog: crab zoeae.

If you look at the banner above this blog, you should see images around the words "Saltwater Science." When Sara, Jessica and I first saw the design for our blog, we loved the design, but weren't sure what those animals were. Was that round thing on the left an animal, or an egg? Maybe an Ostracod or a dinoflagellate?

By now, you may have figured out our mystery - the animals depicted in the Saltwater Science blog banner are different life stages of the same larval animal: a crab. The first image, to the left, depicts the crab curled up in an egg. The middle stage, right next to "Science" shows what's known as crab zoeae. The crazy-looking spikes on the crab's head are actually defensive spines. These spines make it difficult for fish to swallow the larval crabs, and can even lodge in the fish's throat and kill it. Fish quickly learn that it's no fun to eat spiny crab larvae. After 31 to 49 days, the zoeae enter the next stage of development, which is called the megalopa. This is shown in the image to the right of the "Saltwater Science" banner, and it looks more like a lobster than a crab. After another 6-20 days, the megalopa finally molts into the crab stage that we see at the beach and in the sea.

Sometimes, when you look into the microscope, you see aliens. But look closer; they may be more familiar than you think.

References

Morgan, S. G. (1989). Adaptive significance of spination in estuarine crab zoeae. Ecology 70 (2), 464-482.

Pechenik, J. A. (1999). On the advantages and disadvantages of larval stages in benthic marine invertebrate life cycles. Marine Ecology Progress Series 177, 269-297. (Open access)

Williams, A. B. (1965). Marine decapod crustaceans of the Carolinas. US Department of the Interior, Fish and Wildlife Service, Bureau of Commercial Fisheries.