Eric knew what it was, and called me to take a look because he knew this was one of my favorite types of larvae. But there was a catch.
We recognized this as an actinotroch — the larval form of a phoronid. (I've talked about adult phoronids a couple of times — see the post from 27 June 2013.) However, it looked different than actinotrochs we'd seen before.
The tentacles were very long and slender, and it appeared to have more tentacles than we were used to. When you have a chance to see a new species of phoronid, it's pretty exciting, because there are only about 14 species in the world! So we embarked on a journey to identify this actinotroch.
Eric read that we had to look for several features. To help you as we talk about them, here's a general illustration of actinotroch anatomy:
Modified from An Identification Guide to the Larval Marine Invertebrates of the Pacific Northwest (edited by Alan Shanks)
The number of tentacles is important. In this case, we counted 26. Next — the presence and shape of the secondary sense organ. This was fun, because we'd never looked for one before. And there it was!
In the picture below, the arrow is showing the secondary sense organ — the pointed projection on the hood of the actinotroch.
Eric got an even better picture of it under high magnification (next image). The actinotroch uses the ciliated sense organ to identify a good place to settle.
Another important characteristic is overall size. Although small, this larva is relatively large for an actinotroch — about 1.2 mm long (next photo).
We could also see adult tentacles starting to form on this actinotroch. Importantly, not all actinotrochs develop adult tentacles. In the picture below, the adult tentacles are the small, stubby ones forming a fringe.
Given all of these characteristics, we think this is Phoronis architecta. Phoronis architecta is common in southern California. There are also records in Washington, but apparently not much is known about it north of Point Conception.
Here are a couple of other really fun things about this amazing larva.
The posterior end consists of a very active band of cilia (see below). This is called a telotroch and the larva uses it for swimming (see video!).
Earlier I mentioned that the actinotroch has a hood and lots of tentacles. Both are involved in feeding. Strathmann and Bone studied this in 1997. They discovered that the ciliated tentacles generate currents that draw food particles towards the tentacles. When a particle contacts a tentacle, the hood lifts and creates suction, drawing the food particle toward the mouth (which is under the hood).
If you watch this video clip, you can see particles being drawn towards the tentacles, the tentacles flicking, and the hood lifting (occasionally).
Introducing Phoronis architecta!
P.S. Information about feeding from Strathmann, R.R. and Q. Bone. 1997. Ciliary feeding assisted by suction from the muscular oral hood of phoronid larvae. Biological Bulletin 193: 153-162.