Feeding mechanism of the actinotroch larva

This picture shows an unidentified actinotroch larva of a phoronid worm that I caught in a plankton tow taken off a dock in Charleston, OR on February 20, 2013. The photo is a lateral view, and the anterior end of the larva is up. Actinotroch larvae have an anterior hood that covers the mouth (a lobe-like structure seen at upper left) as well as a crown of tentacles located posterior to the mouth. You can see that the tentacles are longer ventrally and shorter dorsally. This is because tentacle pairs are added progressively with new tentacles forming mid-dorsally, so the mid-ventral tentacles are the oldest. The number of tentacles varies between species. This larva had ~ 20 tentacles. Tentacles are involved in feeding. A posterior ciliated band, called the telotroch (at about 6 o-clock), helps in locomotion.

The second picture is a close up of the tentacles showing the cilia. The tentacles are covered by motile cilia, which create the water flow, but also bear a row of non-motile latero-frontal cilia. These stiff latero-frontal cilia apparently serve as a mechanical sieve, detaining food particles as they flow past the larva (Riisgård, 2002). Once caught on the upstream side of the tentacle, a food particle can be transported up to the mouth by the beating frontal cilia (Riisgård 2002), a tentacle flick (Strathmann and Bone 1997), or rapid lifting of the preoral hood which creates negative pressure (Strathmann and Bone 1997).

I gave my actinotroch some unicellular algae Rhodomonas (from a lab culture), which is a good food for many ciliated marine invertebrate larvae. This picture was taken a day after food was added, and it appears that the larvae fed on Rhodomonas, judging from the accumulation of the reddish pigment (the color of Rhodomonas) in the stomach.

Riisgård HU. 2002. Methods of ciliary filter feeding in adult Phoronis muelleri (phylum Phoronida) and its actinotroch larva. Mar Biol 141: 75-87.

Strathmann RR and Bone Q. 1997. Ciliary feeding assisted by suction from the muscular oral hood of phoronid larvae. Biol Bull 193: 153-162.