Saturday, May 26, 2012

Brachiolaria larva of Pisaster ochraceus

Pictured here is a dark field image of a four-week old Pisaster ochraceus (a kind of sea star) brachiolaria larva that is 2.35 mm long. This is a ventral view with the anterior end up. The central V-shaped structure is the mouth, which leads into esophagus (a clear tube just below), which in turn leads to the stomach (light pink oval shape below), and finally the intestine (dark pink tube), which opens outside via an anus (out of focus) on the ventral side. The convoluted circumoral ciliary band is particularly obvious (clearly in focus) along the sides of the larval body in the middle part of the image. It is divided into the pre-oral (anterior to the mouth) and the post-oral (posterior to the mouth) portions.  

The second image is a rare side view (left side) of the larva. Located near the anterior portion of the larva is the attachment complex that characterizes the difference between the brachiolaria and the preceding bipinnaria larval stage. Below is a close up of this attachment complex. There are three brachiolar arms capped by the dark colored suckers that test the substratum and provide temporary adhesion during settlement. These arms are differentiated from other larval arms by containing an extension of the anterior larval coelom (which
one can clearly see in the second picture). The dark circle between the two bottom arms is the adhesive disk which contains glands that secrete a cement-like substance for more permanent attachment during metamorphosis.

The last image is a close up of a developing spicule located just near the larval stomach. See an earlier post by Lara Macheriotou for context. This is interesting because asteroids lack the ability to make larval spicules, unlike the pluteus larvae of echinoids and ophiuroids that have a calcareous skeleton.  See a post by Nick Hayman about the underlying developmental mechanism. The spicule pictured here is part of the developing juvenile rudiment, and will be incorporated into the adult skeleton.

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