Tuesday, August 17, 2010

A sea urchin that eats.... WOOD???

Today there is weird deep-sea sea-urchin fun! This great but kind of tucked away paper by Pierre Becker et al. published in 2009 in Les Cahiers de Biologie Marine (50: 343-352) reporting on Ind0-Pacific sea urchins from deep-sea wood falls!

What is Asterechinus elegans??!!

In my experience, the first indication that you've got an interesting story is when you've got an unusual critter that no one's ever heard of...

Case in point: According to the NHM echinoid site, Asterechinus is a member of the Trigonocidaridae, a family SO frakkin' weird that I had never even heard of it or Asterechinus before!
And since the database picture is based on scanned images from the original description and figures by Theodor Mortensen in 1942, the species is rare enough that pictures were NOT immediatley available! And for the NHM database...that's sayin' something!

So, from the paper's Figure 1A comes what is likely to be the FIRST live picture of this animal since 1942 (over 65 years!). Its a small species with the largest never exceeding 25 mm. And look! It lives on...wood!
(Figure 1A from Becker et al. 2009)

These were specimens collected by the French BOA I and SANTOBOA cruises in 2005 and 2006 respectively to the Vanuatu Archipelago. Apparently, a great deal of woodfall organisms were obtained...One of which is written up here..

Here's a deep-sea log to give you kind of an idea of what deep-sea wood looks like..

So before we start on the next section just a brief bit of background- "Wood fall ecology" is part of a relatively new part of marine ecology, including the study of whale falls that studies the influx of massive amounts of organic nutrients to the seafloor's bottom. More on whale falls can be had here.

Because the deep-seafloor lies so far below the sun's immediate influence, it can be relatively poor in nutrients, making ANY kind of potential nutrient deposit, such as a dead whale, unrooted kelp, trees, etc. a BIG event.

A succession of unusual faunas, composed of both fishes and invertebrates, usually springs up around these deep-sea oases when they form. As it turns out, this urchin forms a member of this fauna.
Other wood-related echinoderms include the weird-enigmatic sea daisy Xyloplax!
It Eats Wood!!! (aka Xylophagy!)So, upon collection and preservation, they opened the specimens up and looked at the guts-and lo and behold! The guts of these critters were FILLED with wood!! To quote the paper
Observations on the gut content of all individuals (n=20) revealed that they were mainly composed of numerous wood fragments of different size, shape and colour. For instance some were small light cubes, while others were large dark twigs of up to 7mm long.(!)
Although most sea urchins are known primarily as herbivores, they-like many other animals- naturally LACK the ability to innately digest cellulose from trees. And so, Becker et al. cultured and analyzed the microbes in the gut and came up with this...
There was FILAMENTOUS BACTERIA in their guts!! And all of it was in contact with the woody food in the urchin's intestine...
(from Fig. 2C from Becker et al. 2009)

Tests of these bacteria revealed them to be composed of a variety of bacterial types, including Proteobacteria, Planctomycete, Firmicutes, Cytophaga-Flexibacter-Bacteroides, and Actinobacteria. These were all compared against other known marine bacteria using DNA phylogenetic analysis. And it turns out that the bacteria had CLOSE relationship to a bunch of veeerrryyy interesting other bacteria including
  1. bacteria from the gut of other xylophagous (wood-eating) animals
  2. bacteria from sulphide-rich environments (such as big dead whale falls)
  3. mangrove soils
  4. marine sediments from hydrothermal vents & cold seeps!!
Why is # 4 important? Bacteria function in vent and seep animals to help process toxic substances, such as sulfide, into digestible form in order for the host to process the nutrients. Closely related organisms often share similar qualities...

Although the authors did not have the exact and complete story, the data above, in addition to other elements of the intestinal bacteria flora all STRONGLY suggests that Asterechinus elegans may host a bacterial community in their guts which they use to aid in wood digestion!!

What other animals use a microbial flora in their guts to aid in digestion?

and People!
(this image from Nature.com)

Finally...if these deep-sea urchins EAT wood..Do they also eat....WITCHES?

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