In the image at left is a metatrochophore larva of
the polychaete Nephtys. I caught this larva on February 14, 2013 in a plankton tow taken off a dock in Charleston, OR. It has two ciliary bands, the anterior prototroch and the posterior telotroch, which help in locomotion, and 10
body segments. I wanted to observe the internal structures of this larva, so
I fixed it with paraformaldehyde, stained it with fluorescent phalloidin, and cleared
it in a mixture of benzyl benzoate and benzyl alcohol (Murray Clear). Because I stained it with phalloidin, which binds to
filamentous actin, I was able to observe the details of muscular anatomy quite well.
You’ll notice two prominent, bright muscular bands along the
sides of the larva. Many invertebrates have longitudinal and circumferential
muscular bands that antagonize each other and control the shape and size of the
animal; however, Nephtys lacks
circumferential muscular bands (Clark and Clark 1960). Lateral to the bright
longitudinal muscle band and running perpendicular to it are smaller muscles of the parapodia. These muscles control the lateral paddle-like projections of the polychaete body.
As a larva grows, its size (volume) will increase at a
greater rate than its surface area, so the efficiency of locomotion by ciliary
action alone decreases (Chia et al. 1984). My larva was about
700 µm long. The development of musculature can improve larval locomotion and
extend pelagic larval residence times (Chia et al. 1984).
Chia F-S, Buckland-Nicks J, and Young CM. 1984. Locomotion
of marine invertebrate larvae: a review. Can J Zool 62: 1205-1222.
Clark RB and Clark ME. 1960. The ligamentary system and
segmental musculature of Nephtys. Q J
Microsc Sci 101(2): 149-176.
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