For Eric’s summer class, we used a small plankton net to collect some plankton near the mouth of Bodega Harbor during a flooding tide last week. The students sorted through the plankton sample in lab to see what larval forms they could find. One of the students (Zack) spotted an amazing larval form.
This is a brachiolaria, a late-stage larva of a seastar, that appears to be the Ochre Seastar (Pisaster ochraceus). The photo above was taken under the microscope, but the specimen is quite large for a planktonic larval stage, close to 2 mm long and visible with the naked eye. This larval form is beautiful and alien in appearance and doesn't look much like an adult seastar!
A surprising feature of the brachiolaria larva is that the juvenile seastar begins developing within the transparent larval body. In the photo above, the juvenile body is visible near the bottom of the larva and is shiny white in color. With a closer view, you can see that the developing juvenile body already includes the start of calcareous ossicles that will become the internal skeleton of the juvenile and adult seastar! In the photo below, the developing ossicles look like intricate snowflakes (white arrow).
When the brachiolaria returns to a rocky shore and finds
suitable habitat, it will initially attach to the rock with a short stalk. Remarkably, the transparent larval body is
reabsorbed into the juvenile body during a dramatic metamorphosis. Soon after
settlement, the juvenile sea star breaks off its stalk and crawls away as a
tiny sea star.
The larval phases of a sea star. Illustrations compiled and modified from various sources.
Eric was able to capture some video under the microscope of this beautiful sea star larva. In the video, notice the long ciliated larval arms used for swimming and gathering food from the plankton. You will also see a close-up of the developing juvenile body (white in color) and the snowflake-like ossicles. (If you can't see the video player below in the e-mail message, click here to go directly to the video.)
With many thanks to Zack for spotting this planktonic wanderer! And to Eric for his microscope camera skills and the helpful diagram.
P.S. For more information about development in sea stars, see earlier posts on the larvae of the Bat Star (Patiria miniata) — "A new star(t)" in 2013 and "A profound change" in 2023.
P.P.S. As a reminder, when juvenile Pisaster are first visible to human eyes in rocky shore areas, it's about 4-6 months after metamorphosis. Here's an example of one that's about 4.5 mm across (between 1/8 inch and 3/16 inch), photographed in November 2014.
