Tuesday, February 24, 2009

The Cushion star Culcita novaeguineae: An exercise in growth-related taxonomic confusion!

So, people have seen some of the high-profile accounts of how some weird deep-sea fish classified into three families, were actually shown to be the males, females, and larvae of one species!
Well, not to be outdone, we at the Echinoblog have a similar (if somewhat less extreme) version of that...

Ontogeny and growth of course can be quite a pickle of a problem when trying to understand the diversity within a species.

For example, here is the juvenile form of one individual.
And here is the adult!! They look NOTHING alike!!
Now, if we look at Culcita novaeguineae, a starfish which inhabits the tropical Indo-Pacific region where it feeds primarily on coral .
We have a parallel where the JUVENILE is VASTLY different from the ADULT!!

Can you possibly believe that THIS?
Is the juvenile form of THIS????
I actually get a whole bunch of images from curious aquarists, divers and even scientists who have seen the big cushion stars and the tinier, "cookie" stars but NEVER realized that they were the SAME!!

How does that happen?
As it turns out, this species..as well as all species within the OREASTERIDAE have this particular developmental quirk.. That is there is what's called a "goniasterid" phase. the babies look like members of a different FAMILY!!
The small "cookies" were called "Goniodiscus sebae" (among about 12 other synonyms) and were members of the Goniasteridae until they were "raised out" to the adult forms.
(Goniasteridae: Peltaster placenta for comparison)
Whereas the adults were called Culcita novaeguineae and were members of the Oreasteridae.

and what happens? Quite simply, they just grow out, becoming more "inflated" as they reach an "adult" size, making cushions out of cookies!!

As it turns out, MANY members of the Oreasteridae have a "juvenile stage" and this complicates our understanding of just how many prominent oreasterids are the older, bigger stages of smaller cookie-or otherwise tiny starfish in other groups?

6 comments:

BEHNAZ said...

Thanks for your information
good luck

poopie pants said...

Do you know why Culcita novaeguineae have such a prominent "cushion" morphology? It seems like it would hinder locomotion and feeding. (SFSU Biology 555 student)

Marine_Team said...

Hey I was just wondering if anyone knew the average radius of Culcita novaeguineae? I'm doing a size to depth ratio. Also the culcita's I was looking at appear to be sub-adults, as they're approximately adult size but still have slightly defined rays...
Any help would be appreciated :)

ChrisM said...

Generally speaking, Culcita is about 7to 10 inches across but I'm not sure that's going to be helpful. Obviously, I would say "average radius" would need to be assessed based on where you are and any number of other factors.

Culcita occurs over a wide range and the "average" size may vary depending on where in the Indo-Pacific you happen to be.

Rays can be more or less defined, depending on how relaxed the animal is and how much of the body is splayed out. There's a lot of flexibility in Culcita that isn't apparent from first glance.

In fact, having "slightly defined rays" may NOT be a feature of subadults because juveniles are more cookie like in shape.

I would tread carefully.

That's not much..but it something.

Marine_Team said...

Thanks,
I'm looking at culcita at the GBR, the average sizes i've found are between 4 to 5 inches, which is what leads me to believe they're not fully developed. But as you said different areas could have different sizes.
I'm having trouble finding literature on sizes, is there anything you can suggest?
thanks again

ChrisM said...

the best place to start for finding literature on Culcita is in the taxonomic monography and etc.

You might try

Livingstone, AA. 1932.Notes on some representatives of the asteroid genus Culcita. Australian Zoologist 7(3):265-273.

the oreasterid monographs of L. Doderlein.

but really, ANY taxonomic monographs that cover Culcita (e.g., the works of H.L. Clark on Australia,etc) should have measurements. Esp. given how many synonyms of C. novaeguineae there are-(n. sp. descriptions usually have measurements)

Look for the R/r ratios. big R=center of disk or anus to arm tip.
r=center of disk or anus to interradius

that should get you started.