Better know The Asterinidae: Familiar & Unfamiliar!

A lot of times on the blog, I talk about weird and often unfamiliar groups of echinoderms. But today I thought I would talk a little bit about a family of sea stars, the Asterinidae, which its most likely that most people have seen at some point. They're familiar and as a result, we know quite a lot about some of them..

Members of the Asterinidae include some 150 species in over 25 genera spread out all across the world. Everywhere from the deep-sea to under rocks in the tropical Atlantic and Pacific. Most have five rays, whereas others, such as Meridiastra calcar from Australia can have up to 8 or 9..

The many species go by several familiar common names: Bat star, cushion star, Knitted star..

This is actually a good place to identify the very distinctive patterns in asterinids.. namely, these crescent or "knitted" looking plate patterns on the body surface..

               

In fact, the Japanese word for sea star.. hitode, which means palm, may actually be BASED on an asterinid (Patiria pectinifera).

But for once, there's quite a lot known about them! So here's five subjectively interesting facts about them!
1. They live everywhere
Asterinids occur all over the world with many in shallow water habitats, including under rocks in places as diverse as the North Atlantic. They often live under rocks, or hidden away in cryptic habitats..

There is a HUGE diversity of these tiny little guys. Some are able to reproduce asexually but others just seem to get around.. Being small and easily transported...

to tropical habitats in Singapore...

This Callopatiria granifera was a species I observed in Cape Town South Africa!

But honestly, one of the places you are MOST likely to encounter them is here.. in a tropical reef aquarium. These tiny ones are most likely in the genus Meridiastra or Aquilonastra.

Again, this is one of those species which is small and easily transported.. Living rock is a GREAT place for them to turn up..

This aquarium species is asexual and once one gets into your aquarium, you're most likely going to have a bunch of them after too long...

So, you can encounter shallow water asterinids almost all over the world! Australia, South Africa, South America, Antarctica, North America, etc, etc.

But as with most starfish groups, there are often weird deep-sea members... Both of these get to be pretty big sized animals.

I've mentioned these briefly before.. the flat one on the left is called Anseropoda, which is SO flat that it feels like a cloth rag when you pick it up!  We know very little about it.. Sometimes, small ones are so thin that light shines through them! Its name literally means Ansero for "goose" and "poda" for foot.. So, Goosefoot starfish! These can get to be almost 1.5 feet across!

The one on the right, is called Tremaster mirabilis. Katie Gale and her colleagues found that these will feed on coral in the North Atlantic. As a species it occurs widely around the world including the North Atlantic, the South Pacific and near Antarctica.

 2. Funky Feeding
Asterinids practically EMBODY the classic feeding mode of stomach eversion in sea stars! Here's a classic video I've been showing since the blog started!  It shows the stomach extended out onto the glass and feeding on the algae and other good stuff.  Most other asterindis feed in a similar fashion...

BUT some are neat ambush predators!! One species in New Zealand.. Stegnaster inflatus pretends to be a cave and can actually AMBUSH tiny swimming crustaceans!!

One of these days, somebody needs to actually make a video of this thing feeding!

This image from SeaFriends in New Zealand

3. Reproduction! Lots of it!  Asterinids are one of the most heavily studied sea stars because of their many reproductive strategies...

Some, such as this "Asterina panceri"(Asterina gibbosa) actually BROOD their young

From Byrne 1996, Fig. 4h

and I've reported in the past on this other brooding species, which live inside their mother and whose babies will actually EAT one ANOTHER! 











And of course when all else fails, there's always dividing yourself!  Asexual or fissiparous reproduction is why those tiny aquarium stars are so numerous! 

3.  Commensal Worms! In the Pacific Northwest species, Patiria miniata, its been know for quite some time that there's actually a species of polychaete worm (Ophiodromus pugettensis) that actually lives on the underside!!

This is one of the better studied species of asterinids, but there's many other species of Patiria and other species such as Patiriella and Meridiastra which have a similar surface morphology and are conceivably "habitats" for other animals...

4. They are important to the Evolution of Sea Stars!

A few years ago, I and my colleague Dave Foltz published this DNA tree of one of the most diverse clusters of the Asteroidea- the Valvatacea, which included several members of the Asterinidae. 

Here's figure 2 from our paper...

Yeah, I know the image is hard to read, but basically, what I wanted to show was how the Asterinidae seems to show close relations with several other groups of sea stars, including the big multi-armed predators, the Solasteridae

and the enigmatic Antarctic Perknaster.. 

Also, the aforementioned Tremaster mirabilis? Sea stars which resemble this species can be found from the Jurassic!  And its position in the tree seems to suggest it might be... interesting..

and of course, there's more I haven't summarized here.. their phylogentic history relative to their various interesting features, etc.. but that's another post!

Echinoderms..So What Good Are They?

Image by John White

A typical conversation I've had with interested parties:

me: ...and THAT is why echinoderms are cool!
other: So?
me: What do you mean?
other: What good are they? Why should I care? How do people use them?

Between this fun little exchange and it bein' the summer grant for writing NSF grants, the whole notion of importance has been a lot on my mind. The answer to the question above, obviously is A LOT.

But for good or bad, echinoderms do not evoke the same need for study that say, various pesty mammals or scavenging, nocturnal insects seem to generate. They do not attack people with rabies nor do echinoderms reside in your sink waiting for your dinner to go bad.
So, aside from their intrinsic interest, what makes echinoderms "worth" studying in the professional world? 

1. Ecology
Far and away the most important reason. A great many near-shore echinoderms have demonstrated critical roles in marine ecosystems. Echinoderms occupy critical roles in those systems, without them those ecosystems would be radically altered. Potentially with very deleterious affects on human systems. Examples:

• Pisaster ochraceus-keystone species in intertidal ecosystems-feeding on and interacting with mollusks of various types.
• Asterias amurensis. Introduced from the North Pacific to Southern Australia, where it is currently running amok and apparently wreaking havok with Australian shellfish.
• Strongylocentrotus and/or Diadema. Purple sea urchins in kelp forests or Black needle urchins in coral reefs. Remove them or increase their numbers and the balance of food is lost.
• Acanthaster planci. I've written about these earlier. But the short version? They eat coral. A LOT of it.

(from New Scientist)

• Biomass. Echinoderms are also probably very important in deep-sea and other cold-water ecosystems. But that role remains poorly studied. The presence of deep-sea echinoderms: sea cucumbers, ophiuroids, etc. is substantial and can constitute a majority (up to 90%) of the TOTAL deep-sea biomass. You don't see them, but by the pound, there's a LOT of them spread out on the ocean floor!
• Plus, they process the benthic biomatter like giant deep-sea earthworms. apparently quite a bit of it.

2. Geology: Index Fossils

Image by Paul Lamble

In geology, echinoderms are important as index fossils. These are fossil members of a particular kind that are used to determine or indicate a specific strata/age of rock. Helicoplacoids, for example occur only in the Cambrian.

 These fossils correlate with occurrence for specific types of organisms in the fossil record and are usually common enough that they can be found readily and make immediate identifiers for the age/layer you are attempting to identify.

 MANY echinoderms find their way towards use in this fashion: sea urchins (including sand dollars, sea biscuits, and "regular" sea urchins), crinoids, blastoids, and even asteroid ossicles can be useful at specific horizons. Fossils can also be used to help reconstruct the paleoecology of a specific area.

Some could only have lived in unconsolidated sandy bottoms. Others only on hard bottoms. Paleozoic fossils can be surprisingly data rich.

3. Food Its always weird for me to think that ANY echinoderms are eaten as food.
But there they are.

Really, only two groups of echinoderms have ANY kind of real market.
To my knowledge, people don't eat crinoids or ophiuroids and only marginally devour asteroids... 

Sea Cucumbers. aka trepang, gamat, or beche-de-mer. Eaten throughout Asia and believed by some to have various medicinal qualities, including tissue repair (some support) and as an aphrodesiac (not well supported).

Holothurians from several different regions, including Alaska, British Columbia, Australia, Madagascar and areas throughout the Indo-Pacific tropics are supported. An update can be found here. Sea cucumber fishery politics can be very contentious. and sustainability of the fishery remains a hot button issue with several species perceived as potentially endangered from overfishing.

Image by Wyld Ginger

Image by Photobat

Sea Urchins. Sea urchin gonads are eaten by the Japanese and now throughout the world. Several taxa, including Strongylocentrotus, are sought out for their tasty innards... Sea urchin fisheries appear to have organization. The North Pacific Strongylocentrotus is represented by the Pacific Urchin Harvesters Association.

Image by goodbyesunday

Image by Yusheng

4. Genetics & Development Recent years have found echinoderms occupying increasingly important roles as experimental animals throughout biology. 
Although nearly all of the classes have been studied in minor ways , three conspicuous taxa have emerged at the forefront.

The Purple Sea Urchin-Strongylocentrotus purpuratus. 

By far one of the MOST heavily studied echinoderms in the world. A search on Google Scholar revealed 8,730 hits for "Strongylocentrotus" and "development" with only some 2,740 hits for "Strongylocentrotus" and "genetics". All that plus a recent issue of Science from 2006 which announced the 814 megabase genome of the purple sea urchin (Strongylocentrotus purpuratus). Honestly, how many single echinoderm SPECIES get a whole FRAKKIN' issue of Science devoted to them????? The Asterinidae (Cl. Asteroidea)-particularly Asterina miniata. This odd little group of starfish occurs quite commonly in several nearshore and easily collected habitats. That, plus the keen developmental patterns observed have made several bat star species VERY heavily studied.

When last I checked Google Scholar..some 1850 citations were recorded from JUST "Asterina" (in part a synonym of Patiria) and "development" and some 940 for "Patiria" and "development" with only some 524 for "Asterina" and "genetics". If the "development" hits are combined, that makes some 2790 total.

Asterias spp. (Asteriidae). While not quite in the league of the two groups above. Running "Asterias" and "development" scores an impressive 5, 110 hits but only 979 if run against "genetics" but still....