Saturday, February 7, 2009

The Sex Life of Archaster! Pseudocopulation in Sand Stars!

Since this week leads up to Valentine's Day, I thought this would make a great time to talk about something that's been much on my mind.....

Yes! That's right! Pseudocopulation!!!

Wikipedia defines as:
Pseudocopulation describes behaviors similar to copulation that serve a reproductive function for one or both participants but do not involve actual sexual union between the individuals.
I mean, who doesn't think about pseudocopulating every so often?? it turns, out a bunch of echinoderms..particularly starfish and brittle stars do all the time!

But today, let's look at one of the best known of those asteroids which engage in this farce of faux fornication!
Archaster typicus!
Archaster is the single member of the Archasteridae, a group of starfish that live in the tropical Indo-Pacific, occurring from the east coast of Africa to New Caledonia, Australia, and the tropical regions of Asia, including Japan in shallow-water environments.

Archaster is a frequently encountered species, which closely resembles the other sand-star Astropecten. But both live in sandy, unconsolidated sediments..and are often called "sand stars" or "sand-sifting stars."

One of the more remarkable behavior aspects about these animals is that they are often found in pairs.

The Sex Life of Archaster typicus!

Since, their earliest discovery in 19th Century, this has been a point of interest in this species. Its since been documented as a case of pseudocopulation. Behavior that optimizes fertiliziation of the species.

A classic study by Run et al. (1988) in Marine Biology based on one species, A. typicus from a Taiwanese population revealed many SEXY starfish dynamics!

1. The pairing behavior occurs during a separate season from spawning season. Pairing occurs in early May, peaks in May-June and decreases in July, ending in August.

Gonads are REALLY swollen during this period! (just before spawning!).

Pairing leads to fertilization...the whole pairing/fertilization process takes about 2 minutes.

2. The actual SPAWNING occurs about two months after the pairing!
3. Density and movement of populations in Archaster both increase during "pairing season".

show a higher tendency for movement under these circumstances.

4. Archaster shows immediate sex recognition when individuals are side by side. Probably
owing to chemotactic recognition.

5. During the PAIRING: When a male FINDS a female..there is a behavior pattern that looks like this (aka the Archaster Kama Sutra!!):

6. The female is ALWAYS on the bottom! Males are always on top!

Oddly enough, the "male on top" urge is SO strong that male on male pairings, including male-on-male-on female "piles" of Archaster are encountered!

One of the interesting points is the measure of something called the Gonad Index, which is a measure of the ratio of the weight of the gonad to the body shows that the male GI is LOWER for males then for females!

There is thus the suggestion that a great deal of energy is used by the male relative to the female.. Possible uses of that energy?
  • The movement and "mounting and pairing" behavior itself is more energetically expensive
  • The females on the bottom can eat and store energy from the food. Males on top are isolated from their bottom food sources and can only feed on whatever happens to be on the aboral surface of the female.
  • While females are covered (i.e., protected) by males, being on top, males are exposed to air, sun, and the low tide (i.e., dessication) which is more stressful.
Whew! I'm all sweaty!

Go here for MORE Pseudocopulation! Ophiuroids! Asteroids!

1 comment:

Ron Yeo said...

Thanks for this very informative entry! :)