Skimming the surface: a larval window on the biodiversity of nemertean worms

By Christopher Laumer, Department of Organismic & Evolutionary Biology, Harvard University

Go to the marina and gently lower a fine net into the frigid Pacific. Pace the docks a few times, trailing your net behind, just below the choppy water. In the small jar at the end of your net, you’ll find all the usual suspects – miniscule flitting crustaceans, kaleidoscopical green cells, perhaps a few fledgling fish. Look closely, however, and you may also see several of what for all the world appear to be tiny transparent helmets, blown off the heads of Lilliputian jockeys. The ocean can seem an alien universe.

Separate them, feed them, and watch them grow. Over time, from the surface of the helmet, small blobs of tissue coalesce to form a worm, the true helmsman of this UFO. Once complete, the worm will separate from its amniotic fetters, crawl about restlessly inside, and in a fateful minute, pierce through the edge of the helmet and writhe its way out, devouring the vehicle which was until now its entire world. Thus ends the life of a “pilidium” larva, and begins the life of an adult nemertean, or ribbon worm, a member of one of Earth’s most remarkable invertebrate groups.

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Dr. Svetlana Maslakova, Assistant Professor at the Oregon Institute of Marine Biology, is one of a handful of specialist researchers on the biology of nemerteans. Nemerteans are a mostly marine phylum with some 1300 named species, almost all of them voracious predators. Although they live in almost all marine habitats, and include some of the longest known animals (over 30 meters in one case), “nemerteans are a relatively understudied phylum”, Maslakova notes, in part because of inherent difficulties in identifying wild-caught nemerteans. Yet if putting a name on an adult nemertean is a difficult task, matching them to their distinctive larvae has been an almost Herculean labor. Employing the tools of molecular genetics, Maslakova has found an efficient shortcut to this endeavor, with exciting implications not only for nemertean biology, but also for the adult-centric sampling biases that constrain our understanding of the magnitude of marine invertebrate biodiversity globally.

Traditionally, to identify a nemertean, a researcher intimately familiar with their diversity has to examine both live animals and painstakingly prepared microscopic sections; identifying a single species could take months. But in the Northeast Pacific, this work has to a large extent already been done. “For other parts of the world, we just know that we don’t know anything.” To Maslakova, this baseline knowledge of regional nemertean diversity provided an opportunity to gain insight into the identity of their remarkable larvae. Nemertean pilidia are diverse, and include not only the jockey’s helmet, but also other forms that resemble hunting caps, papal mitres, quaker bonnets, and in a few cases, headwear you’d be hard-pressed to find analogies for outside an avant-garde Parisian runway. These various forms likely represent distinct species, but to formally link any particular pilidial type to adults of a named species, one must raise the larvae through their metamorphosis, which is so difficult that it’s been done in only a handful of cases. Thus, the identities of most pilidia remain, in the words of Thurston Lacalli, another nemertean researcher, “intriguingly anonymous”.

In Oregon, Maslakova, her husband Dr. George von Dassow (also a biologist), and her two Ph.D. students Laurel and Terra Hiebert (only distantly related), have been using genetic tools to reverse this ignorance. For the last five years, they have been picking pilidia and other nemertean larvae out of plankton tows, and sequencing a snippet of the same highly variable gene from each. By comparing each DNA sequence to a large database from identified adults, Maslakova hopes to strip these larvae of their anonymity.  If it seems straightforward, it’s only deceptively so.

“It’s very patchy,” Maslakova explains. “Some days we’ll go out and there won’t be any nemertean larvae, and some days we’ll find several dozen.” But even the lucky tows don’t yield every species. To exhaustively catalog all larvae in the area, it’s important “to sample regularly, as frequently as you can over a long period of time.” That is, over years. In Maslakova’s view, a complete survey was a realistic goal for her team only “because we are at the marine station, in residence.”

Furthermore, during Maslakova’s analyses of her DNA data, it became clear that the bounds of this project had expanded far beyond the envisioned connect-the-dots exercise to link pilidium A to known regional species B. While a few sequences were immediately recognizable, in many cases, they didn’t match any of the known regional species. Using an algorithm to place them in an evolutionary tree with other nemerteans from around the globe, Maslakova was able to discover their identities, or at least nearest relatives. One larva matched a species known only in Japanese waters. Another matched a European species. One matched Hubrechtella juliae from the Siberian coast, a species in a family hitherto unknown in the NE Pacific. Two species with a peculiar larval form – a kind of gelatinous Christmas stocking found in many parts of the world – appeared closely related to, but distinct from, another species known from the region, Riserius pugettensis. One of these species, when raised from its sock-like pilidium into full wormhood, turned out to be a voracious predator (and hence, possible biocontrol agent) of Carcinonemertes errans (yes, another nemertean), a devastating egg parasite of the commercially-important Dungeness crab. And at least eight larvae had sequences deeply unlike anything in the database – “orphans”. Maslakova suspects that populations of the adults may be living nearby, still undiscovered, despite occurring within miles of a major marine station.

Through her efforts, at least for the pilidium-bearing nemerteans, Maslakova has nearly doubled the apparent regional diversity with her larval sequencing (bringing it from 23 to 38 species). Hence, larvae clearly give a different glimpse of regional biodiversity than one would find if sampling only adults, some of which can be rare or inaccessible.  Indeed, Maslakova believes these results may suggest important advice for biodiversity surveys globally. “I think it’s a great practice if you’re trying to evaluate biodiversity in some unstudied region of the world, to sample both adults and larvae.” And globally, there’s certainly much sampling left to do: a recent quantitative study estimated that globally, perhaps less than one half of the real diversity of nemertean species have been given scientific names.

To identify them all, one must sample regularly, diligently. Even with molecular tools, it will take many decades. But Maslakova, like most in the small but devoted circle of nemertean biologists, is willing. “I consider myself a lifetime curator of this phylum,” she reflects. “It’s a mission of my life to know the diversity of these organisms.”

An invertebrate zoologist by training, Christopher Laumer currently works as a Ph.D. student in the Giribet laboratory at Harvard University's Museum of Comparative Zoology. His thesis research concerns the diversity, phylogeny, and genomics of free-living flatworms, though he maintains interests in many other microscopic invertebrate groups. When not pipetting at the bench, tapping at the terminal, or sifting sand for worms, he can usually be found buried in fiction, renaissance vocal music, or any of the other typical distractions of the perennial liberal arts student.