Pterobranchs, oh my!

Latitude: -62 56.3150 Longitude: -58 14.6020

There’s no other way to put it, pterobranchs are strange little critters and there’s a reason you’ve probably never heard of them. They’re small, cryptic, and to be quite honest, don’t appear to look like anything but a ball of mud or algae. It takes a keen eye to pick one out from the smorgasbord of emptied Blake trawl – especially when your entire field-of-view is filled with sea stars, crustaceans, and bryozoans. Once you spot one, however, you’re likely to see quite a few more of these little friends here in Antarctica!

Pterobranchs, along with their more conspicuous acorn worm cousins, belong to a group called Hemichordata. Whereas there are approximately 200 species of described acorn worms worldwide, there are fewer than 30 described species of pterobranchs extant today. If you’re familiar with fossils, pterobranchs are close allies to the highly diverse graptolites. The differences between pterobranchs and acorn worms extend past the number of species too. Whereas acorn worms are solitary deposit feeders occupying the soft sediments of many coastlines (and the deep sea), pterobranchs are small, colonial filter feeders which are very difficult to collect. There are so few reliable locations to obtain Pterobranchs, that Antarctica is at the top of the list of locales where we can obtain sufficient quantities and diversity for these little critters!

Hemichordates possess several traits that make them particularly interesting from the perspective of deuterostome evolution. Deuterostomes are the group of animals that were first named (and unified) by the developmental trait of forming the anus before the mouth, a trait that we, as chordates, also share with the hemichordates and echinoderms (i.e., urchins, sea stars, and their allies). The last common ancestor of deuterostomes is also where pharyngeal gill slits arose – the same gill slits present in hemichordates, sea squirts, fish, and gave rise to components of our inner ear and jaw. Though these deuterostome traits are comparatively conspicuous and well-studied in development and body plans of acorn worms (providing critical information for our understanding of the last common ancestor to deuterostomes), far less is known about pterobranchs.

Because pterobranchs are colonial animals, they possess many strange and interesting traits, like reproducing both sexually and asexually, having tissue conduits (called stolons) with which they maintain physical connections to their neighboring clones, and fortifying the philosophical difficulties of drawing a line between “the individual” and “the whole organism.” Admittedly, that last quality can be discussed over a beverage at length (for fun), the earlier qualities are part of what make pterobranchs so interesting – particularly when one frames questions from the perspective of how these dang weirdos arose from the same ancestor that gave rise to acorn worms (or taking another step back, how pterobranchs arose from the last common ancestor of all deuterostomes). To help address these questions (and others), we will be sequencing the genome of the pterobranch, Cephalodiscus hodgsoni, closing the last major gap in available genomes for the major deuterostome body plans.

By comparing the genome of pterobranch to those of, for example, acorn worms, urchins, sea stars, sea squirts (both solitary and colonial), and vertebrates, we will be able to address questions like, “What are the genetic consequences/modifications to becoming colonial?” and, “Are there genomic components which are comparatively conserved, or immutable, among the major deuterostome body plans; and, if so, what are they and what role do they play?” Not to mention my favorite inquiry, “What make you tick, you friendly little weirdo?” But until we get back onto terra firma where we can start addressing these questions, I’ll be here here sorting through mud for more pterobranchs.

Michael Tassia
Ph.D. student in the Halanych Lab
Auburn University