In all my meanderings so far, I have never talked about my work in more than vague references to my connection to biominerals. Well, today won’t be the day I really start, but I would like to introduce the animals I work with. Because they are beautiful, awesome, and I love them (except when they’re sabotaging my experiments :-P). They are fan worms.
“Fan worm” is a bit of a loose term, and I’m still not entirely sure what group of worms it is/isn’t supposed to apply to. The group of fan worms I’d like to talk about today is family Serpulidae. (Call them “serps”. They won’t mind.)
Serpulidae are, if the latest phylogenetic research is to be believed, a subgroup of another “family”, the Sabellidae or feather duster worms (Kupriyanova and Rouse, 2008). All sabellids are sedentary filter-feeders. They live in tubes, putting a feathery crown of tentacles out into the water to catch their microscopic food. This is the fan in fan worm, and it’s all that most people ever see of these gorgeous creatures. It’s also today’s excuse to post some Nick Hobgood Christmas tree worms from Wikipedia, although their crazy spiralling tentacle crowns are not all that fan-like. (Bottle brush worms? :D)
These guys in the photo are mostly buried in a coral colony, with only their tentacle crowns sticking out.
Ancestrally, sabellid tubes are made of hard particles like sand and shell fragments glued together with mucus secreted by the worm. Serpulids are special in that they make their own hard material – calcium carbonate – instead of picking stuff up from the environment.
Serpulid tubes can have a highly organised structure that betrays sophisticated tube-building mechanisms (Vinn et al., 2008). Incidentally, some of them are pretty awesome if you look close enough. Below are the rather bland-looking tubes of Ditrupa arietina lying on the seafloor (from ten Hove and Kupriyanova ). Then an electron microscope image of the outer tube layer showing the cool jigsaw-like cross sections of the calcareous rods it’s made of (Olev Vinn via Wiki Commons).
The general anatomy of the animal inside the tube is demonstrated quite nicely in the photograph below, from ten Hove and Kupriyanova (2009):
This is Serpula vermicularis, the species that gave its name to the family. The head end, obviously, is the one with the tentacles. Below it is a rather elegant thorax wearing a jacket of skin flaps (technically, “thoracic membranes”), with a wide collar folding down over the top. The collar builds the tube: when the worm wants to expand its home, it pokes its head out, wraps its collar over the rim, and deposits a new layer of material from glands under the collar.
The weird funnel-shaped thingy sticking out Serpula‘s head above is called an operculum. It’s another speciality of (most) serpulids, functioning in defence against predators. It’s used to close off the tube, but – at least in my species – it’s also a sacrifice body part that pops off at a predetermined point if you tug or prod it too hard. A bit like a lizard’s tail. (Or a sea cucumber’s guts, because gross examples are always better.) Also like the lizard’s tail, the operculum regrows easily, but unlike lizards, serps can regenerate a perfect new operculum. Some serps, including mine, have upgraded their defences further by reinforcing the operculum with calcium carbonate. A calcified body part that you can make develop on demand. What more can you dream of? 😉
Serpulids are found all over the world. Most of the 300+ species live in the sea, all the way from tidal rock pools to deep sea vents. There are a few that can handle brackish water, and there’s a single species that somehow found its way into freshwater-filled limestone caves along the Adriatic coast. According to Kupriyanova et al. (2009), this little explorer is closely related to the brackish-water species, so serps probably only figured out how to deal with lower salinity once.
They are nowhere near as famous as corals, but a few serpulid species are prolific reef builders. Ficopomatus enigmaticus (one of the brackish serps) can grow in roundish reefs made of generations of worm tubes. Although the individual tubes are only a few cm long, reefs can reach several metres across.
F. enigmaticus is an invasive species. Hitchhiking from their European homeland on boats and spawning wherever they felt happy enough, the worms have spread across the warm, shallow, brackish waters of the world. Below, their reefs are shown polka dotting the Mar Chiquita lagoon over in Argentina (photos: Alejandro Bortolus, in Schwindt et al. ). Note the scale bar!
F. enigmaticus reefs have a pretty big ecological impact in their new territory. Their filter-feeding makes the water less murky (Bruschetti et al., 2008), which is good for the seafloor community, not so great for the phytoplankton that caused the murkiness. The reefs provide hiding places for native predators, changing the composition of the seafloor community (Schwindt et al., 2001), and they can also serve as resting stops and hunting grounds for birds (Bruschetti et al., 2009).
And finally, let’s talk a bit about serpulid babies, because baby worms are the best. I don’t know about other serps, but my species has very stylish BABY PINK EGGS. The moment you remove an adult worm from its tube, it panic-spawns all over the place. If you mix the pink eggs with the boring white sperm in some seawater, by the next day the dish will be full of tiny, zipping white balls. (At this point you’d better feed them, since unlike some other baby polychaetes, they don’t get a lot of food from mum. In nature, they’d swim off and live in the plankton, hunting tiny algae until they are ready to settle.)
In another day or two, the little balls grow quite a bit and turn into textbook examples of the type of larva known as the trochophore. If you’re good to them and give them enough food, they’ll keep growing like crazy. You can always see whether they’re hungry or not, since they are transparent and the colourful algae they like to eat show through their skin. This one, from McDougall et al. (2006) via Wiki Commons, was clearly well-fed when it fell victim to science:
They look all hairy around the broadest part – those are the cilia they use to swim. They are very good at swimming! Within a couple of weeks, they’ve transformed into a more mature form with three newfangled segments and a lovely pair of eyes, like this other one from the same paper:
They are now sniffing along the bottom, looking for a place to settle. When they find a spot they like, they lie down, secrete a tiny tube (made of just mucus at first), and metamorphose into transparent baby worms complete with an operculum and everything. This is what Pomatoceros lamarckii looks like mid-metamorphosis (again from McDougall et al.):
At this point, they are a bit ugly, but don’t worry, the ugly wormling stage doesn’t last long. I’ll finish off with one of my own photos what they turn into:
These are slightly over three weeks old, and they have tiny, iridescent tentacles and minute, transparent opercula. Their now-calcified baby tubes are just a few mm long.
Aren’t they lovely? 😀
Bruschetti M et al. (2008) Grazing effect of the invasive reef-forming polychaete Ficopomatus enigmaticus (Fauvel) on phytoplankton biomass in a SW Atlantic coastal lagoon. Journal of Experimental Marine Biology and Ecology 354:212-219
Bruschetti M et al. (2009) An invasive intertidal reef-forming polychaete affect habitat use and feeding behavior of migratory and locals birds in a SW Atlantic coastal lagoon. Journal of Experimental Marine Biology and Ecology 375:76-83
Kupriyanova EK & Rouse GW (2008) Yet another example of paraphyly in Annelida: molecular evidence that Sabellidae contains Serpulidae. Molecular Phylogenetics and Evolution 46:1174-1181
Kupriyanova EK et al. (2009) Evolution of the unique freshwater cave-dwelling tube worm Marifugia cavatica (Annelida: Serpulidae). Systematics and Biodiversity 7:389-401
McDougall C et al. (2006) The development of the larval nervous system, musculature and ciliary bands of Pomatoceros lamarckii (Annelida): heterochrony in polychaetes. Frontiers in Zoology 3:16
Schwindt E et al. (2001) Invasion of a reef-builder polychaete: direct and indirect impacts on the native benthic community structure. Biological Invasions 3:137-149
ten Hove HA & Kupriyanova EK (2009) Taxonomy of Serpulidae (Annelida, Polychaeta): The state of affairs. Zootaxa 2036:1-126
Vinn O et al. (2008) Ultrastructure and mineral composition of serpulid tubes (Polychaeta, Annelida). Zoological Journal of the Linnean Society 154:633-650