Hydrocoel in 8-Armed Ophiopluteus

On April 12, 2010, my fellow students and I went on a boat trip in and outside the mouth of Coos Bay, Oregon. We did a plankton tow with a 153-micron plankton net in the bay near Buoy #10, and in the open ocean (about a mile off shore) near Buoy #1. I wasn’t able to help collect the sample in the open ocean because the boat was rocking a lot, even though it was a very nice day for Oregon. Don’t worry; I didn’t throw up.

While using a dissecting microscope to sort through the sample from the ocean later that day, I stumbled on this larva nestled within many diatoms. I was surprised that I was able to see the larva at all, because it was nearly translucent, though it was a relatively big larva, with a “wingspan” over 2 mm. The larva had 8 long, slender arms, which were set at a wide angle. I determined that it was an 8-armed ophiopluteus (a larval brittle star). My Invertebrate Zoology professor Richard Emlet, who happens to know a lot about echinoderm larval development, suggested that it might belong to one of three local species of brittle stars, Ophiopholis aculeata, Ophiura luetkeni, or Ophiura sarsii.

The coolest part about this larva was the 5-lobed hydrocoel, which is the large coelomic sack on the left side of the esophagus, well visible on this photo. This stage suggests that the larva was around a month old (R. Emlet, personal communication). Each lobe of the hydrocoel will become an arm of the water vascular system in the adult brittle star. The hydrocoel will migrate around and surround the esophagus before metamorphosis.