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Rolie Polie Cousins

Making a family tree of tiny crustaceans

These isopods found at Cabrillo Beach, Los Angeles, California show spectacular color variation. Photo: Leslie Harris, NHMLA Polychaete Collections Manager.


Imagine stumbling across a huge family reunion with hundreds of people, and you don’t want to leave the celebration until you’ve figured out where each individual fits on a big family tree. That’s kind of what it’s like to be a phylogeneticist.

Phylogeny is the branch of biology that deals with evolutionary relationships. It’s all about figuring out common ancestors, when different species emerged, and how they’re all related to each other. These relationships are key for understanding the lives of living things — from how they behave to where they live and how they function.

It’s a lot of work.

a photo of two isopods, marine pillbug. one is wrapped around the other.
A scanning electron microscope shows an up-close view of two of these small isopods — a male wrapped around a female. This particular species doesn’t exhibit much sexual dimorphism (that is, the males and females look very similar), and the male guards a single female until they mate. Other species are very sexually dimorphic, with males being larger or having ornamentation (much like how male deer have large antlers), allowing them to fend off competing males. In those species, one male can guard a whole group of females.

Take these little isopods for example, members of a family called Sphaeromatidae (pronounced “sphere-oh-matt-it-dee”). These little marine rolie polies (or pillbugs, if you prefer) are common in shallow waters, especially in the tropics to temperate regions, and they’re teeny tiny, most just 3-5 mm long. They were first described 150 years ago, but we still barely understand them because they’re such a complicated bunch. There are over 1,000 different kinds, and their family tree is hard to figure out because you can’t assume that just because they look alike, they are closely related. And to further complicate things, in many species the males look nothing like the females, so it’s impossible to match them up. And good luck if you find juvenile individuals; they can look very different from the adults, so we can’t reconcile those either.

“I've been fascinated by this family of isopods since I was introduced to them when I first came to the Natural History Museum,” said Regina Wetzer, Associate Curator of the Marine Biodiversity Center.

an up-close photo of an isopod, marine pillbug, that is bright orange and has large protrusions on its tail that look like antlers.
This brightly-colored isopod was found in Redondo Beach, California. Its head is at the top of the photo, and its impressive tail is pointing down. In these particular species (Paracerceis sculpta), males guard whole groups of females, fending off competing males with their impressive tail fans, which they can use as pinchers. Photo by Leslie Harris.

Wetzer and her collaborators collected specimens of these marine rolie polies from all over the world: the Cook Islands, Fiji, Zanzibar, Mombasa, Singapore, Tonga, Palau, Great Barrier Reef, New Zealand, Gulf of California, and Chile — not to mention the ones she found right here off the Southern California coast. “They are most diverse in shallow water tropics, common in coral rubble, sponges, holdfasts, and on docks pilings too. Who knows how many tons of coral rubble my colleagues and I have hauled to the surface,” said Wetzer.

From these samples, she and her collaborators pieced together the very first family tree for this incredibly diverse and complicated group. It’s just the beginning, though; we still have a lot to learn about these little crustaceans. Wetzer is particularly interested to discover if their sexual dimorphism — when males and females look totally different — is something that has evolved over and over again with the group, or if this is something that was present in their common ancestor, and some species have since lost that trait. But as Wetzer explains, “That question can't be answered without a phylogeny first.”



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