Spiny, Dangerous Sea Urchins and the Fish that Love them!

(Figure 2 from Moore & Auster, 2009)

Today, a neat bit of natural history from a short paper by Jon Moore at Florida Atlantic University and Peter Auster at the University of Connecticut published in the Peabody Museum of Natural History 50(2): 381-386 in 2009!

The paper is based on submersible (ROV) observations by the authors on three North Atlantic seamounts in the deep-sea (1410 to 1475 meters). The paper highlights an association that you simply wouldn't have seen if you had dredged these up in a net.

The observations are focused on deep-sea "pancake" urchins aka members of the Echinothurioidea! I wrote up a post on these years ago as one of Deep-Sea News' "best" Deep Sea species.. here. But the short version is this:

-Body Soft and collapses into flat shape when out of water (hence the term "pancake" urchin).

-Poisonous spines on top!

-Very old and were recognized from the fossil record before they were identified from living animals!

check ? Okay...

It turns out that in addition to all of that? Some FISH actually like to live on and around them! Specifically, baby CUSK eels! (family Ophidiidae).

(Figure 2 from Moore & Auster, 2009)

This pic shows the juvenile eel identified as Barathrites living in relationship with the echinothurioid urchin Hygrosoma petersi at 1488 m at Yakutat Seamount.

The urchin here is about 17 cm in diameter (fish is about 9 cm long).

Usually, these associations were of single eels living in and around the urchins, pecking at food and bits in the sediment-while swimming in and out around the spines and such.

 

(Figure 3 from Moore & Auster, 2009)

Moore and Auster believe that this is sort of a commensal relationship. The fish presumably gets some protection while it safely eats and forages. The urchin doesn't seem to get any obvious benefit..but doesn't seem too perturbed by the fish either...

Not every urchin gets some freeloading fish living in and around them!

Moore and Auster report that of the 37 urchins they observed, 89% of them (i.e., 33 urchins) did NOT have commensals.

(Figure 4 from Moore & Auster, 2009)

However there WERE different taxa that played host to baby cusk eels.. Their figure 4 shows a tiny little fish in association with another genus, Araeosoma fenestratum. Note that this is the proper name for the animal in their Fig. 4! Thanks to Dave Pawson. for the information!

Its important to point out that its only the juvenile cusk eels that seem to practice this "hiding among the sea urchins" type of behavior. Adult cusk eels apparently get quite big (62 cm in length! that's about 2 feet long!)..and yeah, no one's seen them hiding behind a 17 cm sea urchin!

What's kind of interesting about this behavior is how you see a similar KIND of behavior among completely unrelated fish in shallow water! Poisonous sea urchin spines just got that somethin' somethin'!!!

Shallow Water fish relationships

To compare, here is Diademichthys lineatus a relatively familiar shallow-water clingfish that lives in and around the spines of the well-known Diadema.

How closely associated are these fish? The name "diadem" refers to a spiny crown (i.e., the sea urchin) whereas "ichthys" refers to fish and "lineata" refers to the pattern on the body.. But their name is literally tied to their habitat!

According to a paper by Hiroko Sakashita in 1992, which was written about them here the relationship between this shallow-water species and their hosts are a bit more exploitive with juveniles feeding on body parts (pedicellariae and sphaeridia) as well as copepods that also lived as commensals..

As the animals grow into adults, their diet changes and they go after food that is more independent of the sea urchin.. clams and other small invertebrates elsewhere...

So..in both cases..it seems as if the fish get what they will out of it but then "outgrow" their hosts as they mature.

Bah! Isn't that just like a fish?

Hawaiian Deep Sea-Urchins!!! Below the Surface of a Tropical Paradise!

So, following up on the Hawaiian deep-sea starfish post from a few weeks ago...I thought it would be cool to show some other echinoderm diversity...and who doesn't love sea urchins???
A brief background on where these pictures came from... These were ALL taken by the Hawai'i Undersea Research Laboratory (HURL) at the University of Hawai'i at Manoa and are all species that live in deep-water in and around the Hawaiian Islands. HURL operates two manned submersibles..the Pisces V and the Pisces VI.
I went down and deep-sea stalked my echinoderm loves in the Pisces V back in 2000. The following are not images from that expedition but a mix of pix from past voyages of HURL's Pisces V submersible... Thanks to Chris Kelley at HURL for allowing me to use them!

1. Aspidodiadema hawaiiensis. Here's a neat species that is also observed in the Bahamas. It has these long freaky spines, which they use to MOVE.

These urchins remind me of the giant black spy spider robot from Johnny Quest....


2. Phoromosoma bursarium. What survey of deep-sea echinoderms is complete without some echinothuriids?? I wrote up a blog on these for Deep-Sea News awhile back as one of the 27 BEST Deep-Sea species (it made the top 10!).

Basically, these are weird urchins that walk around on hoof-like spines. Some genera have these big puffy sacs of unknown function.
But one thing I HAVE experienced from firsthand observation-those spines on these critters? They STING. A colleague of mine at MBARI experienced this during the North Pacific Expedition last year.

3. Chaetodiadema pallidum. There's not very much known about this species, but in and around the Hawaiian region between 50 and 402 m on fine sediment.
These can be VERY abundant... and bringing up a bunch of them doesn't do them justice when they are observed on video arranged in these almost unnatural distribution patterns... spooky!
Who needs science fiction when you've got reality?

4. Chondrocidaris gigantea..Speaking of freaky... Here's one of the shallower-water urchins that you see in deeper waters.. Note that the spines are completely covered over by overgrowth..sponges or other encrusting invertebrates, perhaps?

In this species and other cidaroid urchins (you'll be seeing these below) the spines LACK epidermis and you get all kinds of weird things growing on them..

Here's a little bit I wrote up on them last year...(a bit dated by the holiday theme)
5. Prionocidaris hawaiiensis. Another urchin about which we know very little...
Except that we know where they occur in 92-214 meters? They are VERY abundant!
6. Histocidaris variabilis. What's weird with this one? It has BARNACLES that grow on the spines!! Similar in some ways to the way that Chondrocidaris has sponges and etc. on the spines...
7. Acanthocidaris hastigera. Not much known about this one..but dang, its cool-looking ain't it?
8. Caenopedina pulchella These are neat because the coloration on the spines is actually EMBEDDED in the spine calcite. So, even the preserved ones are red and green!
9. Phrissocystis multispina This is one of the weirder ones... This is a spatangoid urchin (i.e., a sea biscuit) and the group is known primarily from the fossil record.

But the living ones are VERY fragile. I've held dry specimens..and the sediment in the gut can literally cause the bottom of these to fall out from under them.
Thanks to Craig Young, Kevin Ecklebarger and J.L. Cameron we also know a little about its very strange-looking sperm and reproductive biology.

Enjoy! Seeya guys next week!

The 27 Best Deep-Sea Species #10: Echinothurioid Sea Urchins

(Araeosoma fenestratum from Will Jaekle's cruise blog, Illinois Wesleyan University)

Who, What, Where: Sea Urchins in the Echinothuroida. Most echinothuroids are deep-sea with some shallow-water tropical members (aka the Fire Urchins-see prior post here). They are thought to eat bottom detritus and "macroplant" materials. They have yolky (lecithotrophic)eggs.

How? Sometimes there's cool deep-sea echinoderms that are cool because just KNOWING them is like a cool story. They DO interesting things and HAVE interesting parts!

1. They have little "hooved" walking spines. That's right..echinothuroids often occur on soft, unconsolidated mud..and they have unique little spines that they walk around on with these odd little flat "hooves".

(Araeosoma belli, image courtesy of Dave Pawson, NMNH)

2. The top spines in some species have big, puffy, balloon-like sacs with an offensive substance. Some echinothurid urchins have big fleshy sacs which enclose the spines emerging from the top or abactinal surface.
Based on an study by Emson & Young 1998 these spines are functionally similar to hypodermic needles and may emit an offensive fluid to ward off unwanted visitors. This is similar to the tropical species which have toxic poisons in these spines.

(Phoromosa sp., image courtesy of Dave Pawson, NMNH)

3. Echinothurids were described FIRST as fossils and THEN found alive later!! Echinothurid urchin fossils were described FIRST as Jurassic fossils and then discovered later on by early deep-sea biologists! Echinothurids are also thought to be relatively "basal" on the big evolutionary tree of Echinoids. They are "Living Fossils". Where paleontology and deep-sea biology meet and greet!

(the Cretaceous Echinothuria shown here from the World Echinoid Directory)

4. The test is decalcified and held up by water pressure. When pulled up on deck? these animals go by the name "Pancake Urchins"

Why? Because echinothuriid urchin tests collapse like a big empty bag of water. That's because in the wild their bodies are HELD up by the positive water buoyance in delicate paper-like skeleton.

Fire Urchin Video!

I have for some reason encountered a trio of tropical Fire Urchin videos I feel compelled to share with everyone this week! Maybe its because they remind me of Fall?

Two of the species below belong to the Echinothuriidae..which are primarily deep-sea (I will follow up with a post on this later)..but the species featured today live in the shallow waters of the tropical Indo-Pacific!

The species at the very bottom is Astropyga which is a member of the Diadematidae...not that closely related-but similar in appearance...(sorry to be paraphyletic here..but they do all carry a theme!)

Their bright colors announce the very toxic poisons that their spines and other structures produce (more on this later when I fully write up echinothuriids later on!)

Asthenosoma varium

Asthenosoma sp.?


Astropyga sp. (Diadematidae)