Thursday, October 9, 2008

Brisingids pt1! Weird Deep-Sea Halloween Starfishyness!

As a summer student at the Hopkins Marine Station/Monterey Bay Aquarium Research Institute program back in the late 90s I was still deciding what I wanted to do with my life.

I was thinking that I would go into Entomology unless my summer student internship thingie worked out.

It did.

I fell hard for starfish. and naturally I went directly to the strangest of a strange group of animals living in a strange world. Starfish were odd and alien enough...but to see the REALLY bizarre ones?? Amzing.

I fell in love working with these animals and the first time I ever saw a living one was a HUGE day!!
I eventually went on to "cut my teeth" on brisingids, doing my Masters in Marine Biology at San Francisco State University and the California Academy of Sciences.

What's weird and AWESOME about brisingids? Simply put.... Everything. Some basic factoids:
  • Brisingids are proper STARFISH (Cl. Asteroidea) part of the Forcipulatida-which is the same group that includes the common starfish Asterias and Pisaster. The same general kind of pedicellariae (see below) found in brisingids are also found in common intertidal starfish.
  • Brisingids are deep-sea animals. Some are "shallower", occuring on the shelf (i.e., 100-700 m) but others, such as Freyella and its kin live in the "true" deep-sea-the Abyss and so forth (>1000 m!). Some of the world's deepest starfish (~5000-6000 m) are brisingids.
  • They live all over the world. Antarctica, Pacific, Indian, and Atlantic. Oddly, none in the Arctic.
  • Brisingids are diverse. Nominally 70 species, in 14 genera, divided into at least 2 families.
  • The name "brisingid" (order Brisingida) is derived from a story in Norse mythology which is so neat that it is the subject of next week's post!!
But what is the FUNDAMENTAL thing that makes brisingids so distinctive??

Brisingids have body forms that are specially modified for suspension feeding!

This affects nearly ALL aspects of brisingid body form and ecology. But how??

While you're looking at the next couple of lines..think about the sort of body form you see in other suspension feeding echinoderms such as crinoids or even ophiuroids! Brisingids look like a weird cross between crinoids, ophiuroids and asteroids.

Body Form
The endoskeleton in brisingids is closely tied to the suspension feeding lifestyle.
  • Disk skeleton is fused into a ring and braced to support the arms as they are held up in the water.
  • Tube feet "hold down" the animal as they raise their arms into desirible current flow. But movement is possible.
  • multiple arms are found in ALL brisingids. Some might have 6 arms..but most have about 8-20 arms. This is presumably to faciliate the suspension feeding.
  • Fully developed gut and stomach are missing. Possibly for secondary absorbtion of nutrients?
  • Gonads are in the arms but space for internal structures is minimized.
The modifications are even so specialized that the ambulacrals in brisingids are actually uniquely shaped as vertebrae! Presumably to allow the most flexure. Note: "normal" ambulacrals look more like this: Feeding Mechanism
Perhaps the MOST distinctive aspect of brisingids is HOW they feed.

Bear in mind, that it was only in/around 1976 that we even KNEW that brisingids held their arms up in the water!! When they were first was thought they just dragged their arms along the bottom!

A paper by Roland Emson (at King's College in London) and Craig Young (then at Harbor Branch, now at Oregon Institue of Marine Biology) presented a detailed study of how feeding happened. (Interestingly, I had later discovered that the Russian deep-sea biologists S. Galkin and N. Korovchinsky had documented feeding in a paper from 1984. Not as detailed as Roland and Craig's work and in Russian-but earlier nonethless..) Here's how it works:

1. Brisingids have lots of spines.
Spines come off the lateral sides. They project off the tube foot furrow. They are almost everywhere! In some species-spines are even present on the surface of the body! All arranged in a familiar "cruciform" arrangement that you see over and over again in other echinoderm suspension/filter feeders. 2. Spines are covered by a "sock" of pedicellariae. Each one of those spines is covered by a sheath of tissue, like a sock. This "sock" is covered by literally THOUSANDS of Frakkin' little pedicellariae!! These are little jaw-like structures that cover the surface. Think of them like a bunch of little bear-traps.
3. The little bear-traps (pedicellariae) densely cover ALL of these spines.
4. The pedicellariae essentially "go off" when food hits them. So, some small shrimp, krill or other tasty bit of organic, edible goodness?? BAM! Snagged by the spine/pedicellariae!! Prey are held fast by the pedicellariae similar to velcro (to use Emson & Young's terminology). 5. Once food is is then moved via the tube feet to the mouth and devoured. According to various accounts, this is typically small amphipods, and other hapless crustaceans...(ha! take that small hapless crustaceans!)

Whew! More on brisingids next week!!


Mary Ellen said...

That is pretty freaky. So, they're all covered with velcro?

ChrisM said...

More the spines, but the general body surface has them too.