Let's Learn About Multi-armed OPHIUROIDS!

I've often talked of my love of multi-armed sea stars. BUT brittle stars can have more than five rays (arms) also!!

So wait, there are ophiuroids (brittle stars, basket stars, etc.) that have MORE than 5 rays?? 
Yes.
Most sea stars (close relatives to ophiuroids) have 5 rays, but a significant number of them have more than five rays and there are some entire groups of sea stars (e.g., the Brisingida) which are characterized by having multiple arms (six to 20ish). When sea stars have more than 5 rays there tend to be about 7 to 15 of them. But some, such as the Antarctic Labidiaster can have up to 50.

What one sees in most  starfish species is that multi-armed sea stars tend to be large, highly mobile and predatory. This tropical, shallow water Luidia maculata for example demonstrates all of those qualities.

In contrast, Multi-armed ophiuroids have different patterns than what sea stars demonstrate. Ophiuroids tend to be either six to seven rayed or 10 or more. Generally none with intermediate arm numbers (i.e., 8 to 9) and no species are known with more than 12 rays.

Certainly no 20-50 rayed beasts like the sea star Labidiaster. And as we'll see, there's no apparent ecological association with having more than 5 arms in ophiuroids.

The multi-armed condiiton occurs  in many, many different groups of asteroids throughout evolutionary time (here) and has evolved multiple times throughout the group. This is also the case for brittle stars...
(My thanks to Dr. Sabine Stohr for assistance with names!)

Diversity in Multi-armed Ophiuroids
Most brittle stars have 5 rays. Having more, implies that the extra arms are somehow adaptive or suggest that the added investment to develop one further body part is somehow consistent with the life mode of the animal. Most ophiuroids appear to have a much more invested skeleton compared to asteroids, so it would seem that adding more arms is much more of an investment in development,


Low numbered multi-rayed brittle star species, those with about six to eight arms are not uncommon and seem to show a fairly diverse range of habitats. But the ones with more than 10 rays are more seldom seen.

This one for example is Ophiothela danae. 6 rays. These occur in tropical-shallow water habitats and live with their arms wrapped around the stalks of sea fans and sea whips.  Does the sixth arm help facilitate this lifestyle??

This species appears to be successful. It occurs widely in the tropics and has "invaded" the tropical Atlantic from the Pacific. 

Here is Ophiactis savignyi, a brittle star that reproduces both sexually and asexually (i.e. it divides in half) brittle star which occurs globally all around the world (read more here). Perhaps its sixth arm is part of the asexual condition? Or maybe its cryptic life style?

Here is Ophionotus hexactis. Six arms. Adaptive? Predatory? Highly Mobile? An animal I'm not sure much is known about it.

But on the OTHER hand, here are some Antarctic brittle stars, such as Ophiacantha vivipara which have SEVEN arms. Its a brooding species (i.e., the adults raise babies on their bodies). Not sure how this benefits the animal. Maybe in feeding?  Aids in parental care somehow? Or maybe its just an incidental feature?

From Antarctic ophiuroid blog https://brittlestars.wordpress.com/2010/01/27/some-peculiarities-of-the-antarctic-ophiuroids/

There are a few more species of ophiuroids that show six to seven arms than I've shown here. But for various reasons (i.e,  I either didn't have pictures of them, space, etc) I haven't included them. Several species of serpent stars in the genus Asteromorpha for example can demonstrate six or seven arms.

Species with more than 10 arms

Ophiacantha enneactis & O. decaactis. The genus Ophiacantha, is kind of unusual because it includes several species which possess more than 5 arms (see O. vivipara above).

Two of them, O. enneactis (illustration-top) and O. decaactis (below) have been collected from the Aleutian Islands in deep-water. O. enneactis is from 549-881 m. And O. decaactis is from comparable depths.

These are similar species and both are pretty small. What they do down there in the deeps with all their arms is "poorly understood." (i.e. we don't know). Perhaps suspension feeding? Predation?

Ophiacantha decaactis Belyaev & Litvinova 1976

Image from Belyaev & Litvinova 1976

Astrochlamys sol Probably the winner of ALL the multi-armed ophiuroids though is this species: Astrochlamys sol, a bizarre Antarctic form, which is a member of the Gorgonocephalidae, so its actually a modified basket star.


I thought Basket Stars already had more than five arms?? 
Basket stars are a subdivision of ophiuroids called the Euryalida. They tend to have big, thick arms with fleshy tissue covering over their skeleton. These ophiuroids extend their arms into the water column and usually have hooks or long arms to feed on food or prey as it swims by..

I've written about one particular basket star, Gorgonocephalus here.   But there are a fair number of basket stars in cold water and tropical habitats.

Note that although there seem to be many arms, there are in fact ONLY five. But they branch...

Below is the underside of Gorgonocephalus, the cold-water basket star from the Arctic/subArctic. Note that only five rays radiate away from its mouth. The  arms bifurcate or split for several iterations away from the five primary rays on the disk..

















That's what makes A. sol, so unusual. It actually has 10 to 11 arms!! Making it the ophiuroid (not technically a "brittle star") with the MOST number of arms!
Its species name "sol" refers to sun, which likely alludes to its arms radiating away from the mouth.

This species occurs in the Antarctic in fairly deep depths 300-1200 meters. It was described by the famous echinoderm taxonomist/biologist Theodor Mortensen in 1936 in his HMS Discovery monograph. We apparently don't know much about its biology aside from the fact that its a brooding species and tightly hugs its substrate...

Image by Igor Smirnov via WoRMS

On a personal note I'm always fascinated by these forms. Unusual in appearance but also kind of mysterious.  What does it do with all those arms? What is so unusual about deep-sea/cold-water/Antarctic species that they have so many arms? Is brooding related? 

Know your Deep-Sea Urchins! (Okeanos echinoid edition!)

Leg 4 of the Hohonu Moana Okeanos Explorer Expedition has begun! (here is the link to live feed!)

We have new investigators Scott France (on invertebrates) and MacKenzie Gerringer (on fishes) studying the huge diversity of deep-sea corals, sponges and other life in the northern Hawaiian Islands region.

The Hawaiian region brings with it a very different fauna from the one we saw in the Atlantic Okeanos cruises. There's actually quite a bit more diversity in the Pacific in sea stars, and many other echinoderms. So I thought I would offer a guide to some of the more prominent groups of echinoderms that we are likely to encounter in these deep-sea habitats: the sea urchins!!

Most everyone is familiar with the spiny balls that one observes in the intertidal or while SCUBA diving. 

And basically, sea urchins in the deep appear the same.. BUT many of them are unusual in some way.  Different adaptations or different evolutionary histories and there's much more to them than the "typical" urchins most people encounter in shallower depths. 

Here are some of the more commonly encountered sea urchins we will likely encounter in the Hawaiian region (below about 1000 m). These are also pretty typical of deep-sea settings throughout the Indo-Pacific.

There ARE a few that aren't noted here. Mostly the "irregular" urchins such as sand dollars, or sea biscuits. 

My thanks in advance to the Facebook screengrab group and Carina M. Gsottbauer

1. Aspidodiadema:  This is a genus of unusual deep-sea urchins that is represented in Hawaii and in many tropical deep-sea habitats in the Pacific as well as one species in the Atlantic. These have really long spines that it uses for locomotion in conjunction with its tube feet.

There's apparently more than one species of Aspidodiadema in Hawaii, but so far we've been calling the one seen by Okeanos, Aspidodiadema hawaiiense. Upon looking at both images, there do seem to be some differences but its unclear without a specimen to examine..

Here's a nice moving GIF of this Aspidodiadema species on the move! 

Rewatching yesterday's #okeanos dive: sea #urchin moving slowly (Aspidodiadema hawaiiensis) #Hawaii, Lone Cone ~2060m pic.twitter.com/jDatlCvp7S

— Carina M. Gsottbauer (@CarinaDSLR) September 22, 2015

This image shows the same type of animal as the one above but with a mysterious bag like extension emerging from the top. Possibly an anal sac as we see in diadematid urchins?? Unclear.

In comparing them, this second one seems to be lighter in color... 

2. Caenopedina! This genus is a member of the family Pedinidae, of which it is the only living member. This genus is widely occurring mostly in deep-sea habitats. 

According to the Hawaiian Undersea Research Lab's Animal ID guide, there are two recognized species in the region: 

Caenopedina pulchella This species shows these very thick spines which are brightly colored green and purple!

The small individual above seems to have much smaller spines relative to this larger one.. but the colors patterns appear consistent..

This second species is Caenopedina hawaiiensis and it seems to show a very different appearance... 

3. Echinothuriid Urchins! These are one of the most frequently encountered sea urchins in deep-sea settings. Commonly referred to as "pancake urchins" or "tam o shanter" urchins, their body shape is suspended by water pressure, so when removed from the ocean into say, a bucket on the deck of a ship, their very soft skeletons collapse into a flat "pancake" like shape.

I've written on these animals plenty of times here and here and their shallow-water relatives here. 

One caveat about this section: I'm not sure that any of these can be correctly identified..so I'm keeping it vague. At least for now. 

But salient features of the urchins as a group include: 

1. Needle sharp spines which can be pretty dang painful!

2. Cute little walking legs with special "hooves" on their spines 

We've seen at least 2 species.. this purple species..

Rewatching yesterday's #okeanos dive: sea #urchin w/ mittens on its spines #Hawaii, Lone Cone ~1800m pic.twitter.com/kEL4cTtBIn

— Carina M. Gsottbauer (@CarinaDSLR) September 22, 2015

There's this slightly different pink echinothuriid

 this grey species                            

Echinothuriids are a frequently encountered group in the deep-sea. We will likely encounter more of them..

4. Cidaroid Urchins! One of the other "typical" urchins one encounters in the deep-sea is that cidaroids.  This is a fairly old group of urchins, which branched off early in the history of urchins and has a fairly good Mesozoic fossil record (see here).   

Cidaroids are unusual for urchins in that they lack skin on their spines, which results in an overabundance of "fouling" animals which can settle and grow on them. I've briefly talked about the ecological importance of this here. 

We've also seen cidaroids with a CRAZY range of spine shapes and morphologies, such as what I summarized here. 
                           
Cidaroids are not just predators on corals as shown below, but are also likely predators on stalked and other crinoids as I've posted about previously..

A nice diversity of cidaroid urchins in the deep-sea Hawaiian Islands.. More at the HURL gallery here.   but sadly we've really only seen a few at the deep depths Okeanos has been exploring..

Histocidaris variabilis.. note the barnacles growing on the spines...

Stereocidaris hawaiiensis

Possibly Stylocidaris calacantha.  This one was observed high up in the branches of this bamboo coral, likely feeding on the polyps..

Goniasterid Starfish LOVE to eat Octocorals!

Okeanos Explorer has begun Leg 4 into the Northern Hawaiian Islands and their last stream on Sunday was chock full of amazing vistas full of deep-sea (about 400-500 m) sea pens and corals! 

Probably one of the most exciting parts of these cruises is that I'm able to actually watch my favorite starfish species, some of which I described, in ACTION! Feeding on what turns out is their favorite food: octocorals!!

I've documented sea stars feeding on octocorals from the Atlantic Okeanos expeditions before (here).  and many of the species are the same.

Sea pens are big fleshy octocoral colonies that look sort of like an aquatic "quill" or pen. Here's one called Ptilosarcus, which occurs in shallow water from about SCUBA depth. 

Octocorals are specific type of cnidarian, which have eight arms on each polyp. They have a whole diversity of body forms and can be more colonial, such as this gorgonian or more like a big fleshy stick like the sea pens seen above..

The majority of the sea stars that we have observed feeding on octocorals are in the family Goniasteridae. As a generality, they look like this.

                                    

A big peripheral ring of plates frames their body. They can be anything from completely pentagonal to more star shaped. Many are covered by spines whereas others are not..

Among the best known of shallow-water goniasterid octocoral predators is the so called "spiny star" aka Hippasteria phrygiana (aka Hippasteria spinosa, etc.), which used to be known by MANY species before being recognized as one wide-spread species which occurs in three oceans! (here). 

This species occurs at SCUBA depth (and deeper) and many, involved studies have documented how these animals are important parts of ecosystems due to their predation on sea pens.

Thus, it makes a lot of sense that several of the deeper water species of Hippasteria, such as this Hippasteria imperials seen by the Okeanos Explorer should be a fairly active predator on deep-sea sea pens! 

Disks are swollen with water...

Here is just a gorgeous shot showing a seemingly endless vista of sea pens with these two large, Hippasteria imperialis roaming around like they own the place! 

BUT, Hippasteria imperialis isn't the only goniasterid starfish which feeds on octocorals. Many of its relatives, both close and far are also pretty active predators on deep-sea octocorals..

This big guy is Hippasteria muscipula! A species I just described last year! (here) It would seem to occur over a pretty wide depth range..from several hundreds to several THOUSANDS of meters!

They seem to be primarily predators on bamboo "corals" which are octocorals in the family Isididae.

and there was THIS stunning image showing H. muscipula but with 3 polynoid worms crawling around on its body surface. This is unusual, since polynoid worms usually crawl on the oral surface in the tube foot grooves..

Another closely related Hippasteria-like sea star which we've seen in the Hawaii region is the genus Evoplosoma.  

We've seen plenty of examples of Evoplosoma feeding in the Atlantic as well as parts of the Pacific. 

This shot is just awesome. We can see its stomach and tube feet hard at work clearing off the meat from the bamboo coral's stalk..

BUT that's not all! We've encountered multiple goniasterid species apparently feeding on coral! 

1. Astroceramus eldredgei

This was a new species I described recently and named in honor of Dr. Lu Eldredge formerly of the Bishop Museum. Observations by the Hawaiian Undersea Research Laboratory showed this very handsome animal munching on the blue plexaurid octocoral Astromuricea theophilasi.

Its a relatively shallow water animal (about 266-429 meters) compared to some of the other goniasterids here... None known below 500 m.

2. Calliaster pedicellaris

This species was described by Walter K. Fisher in 1906 and VERY rarely seen since then. 

3. Circeaster pullus

Here's another species I described back in 2006 and named in honor of Dr. Craig Young at the Oregon Institute of Marine Biology. It was observed perched on this coral skeleton where it was presumably feeding..

Have not seen any of these on the Okeanos dives as yet though.

4. Circeaster arandae? 

This one was a surprise because it very much looks like a species I described in 2006 from Madagascar!!  But it lives in deep, DEEP water (>1000 m) which is consistent with my earlier account. But weird to find it so far off..

How many more octocoral predators will we see??