Although it looks innocent enough, this little guy sent us on a bit of an investigative journey.
When I first looked at it, the long arms and the pointed shape reminded me of an echinoderm larva, for example, an ophiopluteus (the larva of brittle star). But when I described it to Eric, he knew that was wrong — it lacked skeletal rods.
Next we considered a hydrozoan larva, although we hadn't seen one like this before.
While looking under the microscope, Eric noticed nematocysts (stinging structures) in the tentacles!
Here's a view under higher magnification (below). Look for the long, cylindrical shapes. The nematocysts confirmed that this is a cnidarian.
We continued wondering about some type of hydrozoan, but we weren't sure. So we sent a picture to Richard Strathmann (Friday Harbor Laboratories) and Richard Emlet (Oregon Institute of Marine Biology).
With Richard Emlet's response we struck gold, and he sent us down yet another path — and a very fun one!
Richard suggested that this could be a cerianthid (tube anemone) larva. We started looking for images on the Internet. We could only find a few, but it seemed promising. Then when I searched for a picture of the larva of the local tube anemone found in Bodega Harbor (Pachycerianthus) I found one picture (from Australia!) that looked like a perfect match. It was a different species, but the similarity was unmistakable.
We had been looking more closely at this larva and noticed shorter lobes around what appeared to be a mouth opening on the under surface:
These oral lobes are another important characteristic that helps identify this as a tube anemone larva.
It's actually quite difficult to find more information about this type of larva. Eventually we found one description that said they typically swim with the oral surface facing upward. So we went back to the microscope, let the larva orient itself, and then took some more pictures and filmed some video.
From what we can tell, these could be the first tube anemone larva pictures and video from California (the Pacific Coast?) shared on the Internet. Although adult tube anemones are encountered by scuba divers (see example from Monterey), the larvae are seldom seen. Enjoy!
In this video, there are a couple of close-ups — starting around 43 seconds, in addition to the long tentacles, look for the very small tentacle bud (a new tentacle just starting to form)....and around 55 seconds and the 1-minute mark, look for the active cilia along the tentacle edges that the larva uses to swim.
P.S. Here's a little bonus information for those of you interested in invertebrate taxonomy. We had no idea an anthozoan larva could look like this. Generally we think of anemones and corals having simple planula larvae (see the post from 23 July 2013 for an example of a sea anemone larva). To see one with a very jellyfish-looking larva (which is apparently carnivorous) was eye-opening to us.
So we had to do some extra research about tube anemones. This group is unusual — they used to be considered an order within the subclass Hexacorallia. [Other orders in the Hexacorallia include Actiniaria (sea anemones), Corallimorpharia (coral anemones), Zoanthidea (zoanthids), and Scleractinia (hard corals).] But recent evidence suggests that tube anemones have an evolutionary history quite distinct from other anemones and corals. In fact, genetic data suggest that tube anemones likely belong to their own subclass = Ceriantharia! This would result in the Anthozoa including three subclasses: Hexacorallia (as above), Octocorallia (soft corals), and Ceriantharia (tube anemones). That tells you how different the tube anemones are, and perhaps this unusual and distinctive larva reveals some of that history!
Stampar, S.N., M.M. Maronna, M.V. Kitahara, J.D. Reimer, and A.C. Morandini. 2014. Fast-evolving mitochondrial DNA in Ceriantharia: A reflection of Hexacorallia paraphyly? PLoS ONE 9(1): e86612. doi:10.1371/journal.pone.0086612
P.P.S. Of course we want to know *which* species of tube anemone this is...but that has turned out to be a very difficult problem. I can't even find out how many species of tube anemones occur in California. I know there's one species in Bodega Harbor (Pachycerianthus fimbriatus). But Intertidal Invertebrates of California says, "Several species of the order Ceriantharia occur along the Pacific Coast." I found one reference listing three other species in southern California (Cerianthus aestuari, C. benedeni, and C. johnsoni). I also found a different reference listing one other genus in the Southern California Bight: Arachnanthus. If you can help with more information about tube anemones along the West Coast, I'd appreciate any pointers!
For now, we'll just have to describe this beautiful drifting larva as an unidentified cerianthid.