Brooded Juvenile Brittle Star

I found this juvenile by dissecting the central disk of an adult brittle star Amphipholis squamata. It was a little less than 1 mm in diameter. In this picture you can discern the developing ossicles, or small calcareous plates, that will cover the central disk and the rays, and form the skeleton. The central disk is visible as the dark pentagon in the center.

These viviparous brittle stars give birth to live young that are brooded internally. The parent can nourish the juveniles until they reach 2 mm and are large enough to crawl away. In the second picture you can see 10 tube feet on the juvenile brittle star, these will be used for locomotion as it crawls away.

Due to the small size of the adults (only 3-5 mm), they utilize a different reproductive strategy than larger sea stars. Instead of investing energy in producing large numbers of small eggs to free-spawn into the water column (and allow them to develop into feeding ophiopluteus larvae - as pictured in the next post), these small animals produce only a few small eggs (about 100 μm) in each gonad, and invest energy in brooding them internally until they are large enough to crawl away. Although it is a larger investment per egg, this direct development strategy ensures that the young develop to the juvenile stage. Amphipholis squamata can brood multiple cohorts simultaneously, and in our dissections we found brooded young in multiple stages of development within the same adult brittle star.

Amphipholis squamata is also interesting because it is a simultaneous hermaphrodite and appears to be capable of self-fertilization. The grey colored adult brittle stars feed on diatoms and detritus, and can be found under small rocks on sand or gravel in intertidal zones worldwide (Kozloff 1974).

Kozloff, E.N. 1974. Seashore Life of the Northern Pacific Coast; an illustrated guide to northern California, Oregon, Washington and British Columbia. U of Washington P: Seattle.