Sunday, May 17, 2009

The Strange Sexy Echinoblog/DSN Mystery this WEEK!

What is the Strange, Sexy Mystery of these two brittle stars?
But here it is again...

Because CraigM asked so nice, here is a special Echinoblog in honor of SEX WEEK at Deep-Sea News. (Info for this blog from Tominaga et al., 2004, Biol. Bull 206: 25)

I thought it would be fun to write about the oh-so rare occurrence of sexually dimorphic brittle stars? What does that mean?

Males and females look different!

A pretty fundamental difference among humans (and many vertebrates)-but among most animals-only THEY seem to really know for certain!
(The Echinoblog Art Department strikes again!)
In this case..our feature critter is a species of ophiuroid or brittle star, Ophiodaphne formata, which is found in the tropical Pacific. The population studied was from Tsuruga Bay in the Sea of Japan.

First, the essentials:
1. Males are MUCH smaller then females. (see picture)
2. Males live ON females. Mouth to mouth and bursae to bursae (see picture)
3. The female of the pair adheres to the oral surface of a host sand dollar! (Astriclypeus manni)
4. Spawning occurs WHILE they are paired ON this sand dollar!

Tominaga and his co-authors observed this behavior, including spawning and full
development of the babies up to metamorphosis (i.e., they settle into very tiny adult-like animals) including the very peculiar habitat of the male+female coupling which is believed to facilitate the reproductive success of the species.

They further observe that pairing in O. formata is observed throughout the year, including the “non-breeding” season. The position of “oral to oral” coincides with the respective locations of the “bursa” on the oral surfaces that open into the gonads of each individual.

Thus, they say it is most “efficient” for the males to have their arms holding tight to the larger female, mouth to mouth while he sheds sperm from his bursal slits. (sexy!)
The need to pair off together is important because of the low densities of this species relative to other, more common shallow-water species. They speculate that the “fertilization efficiency” would be low if males and females spawned separately and could not shack up on their host. Thus, the sand dollar provides the breeding site (aka the love shack!) and raises the level of “fertilization success”!!!


Anonymous said...

What is the "role" of the sand dollar as a host? Is it acting as a hard substrate or do these brittle stars parasitize the sand dollar? Also, super awesome blog, thanks!

ChrisM said...

Its likely that the sand dollar is likely just there as a substrate for the brittle star. The interaction isn't that well studied, but there's no clear benefit to the sand dollar in this team up...