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?