Getting to the heart of urchin spine attachment

by Amanda Kahn

One of the really neat things about BMSC is that it’s got a lot of resources on-site for us to use.  A freeze drier, autoclave, fume hoods, field equipment, scientific diving program, drying ovens, distilled water, wet labs, dry labs, and so much more fill the buildings of the research station, allowing so many cool and interesting projects to happen.  However, since Bamfield is remote and doesn’t have the same infrastructure as our home universities, many of us take our samples back to our home campuses to use more energy-intensive or specialized equipment there.

During a course at the University of Alberta that specialized in microscopical techniques and advanced invertebrate zoology, I studied an irregular, or heart, urchin that had been collected from BMSC.  Unlike most sea urchins, which have conspicuously long spines projecting from a round, pentaradially symmetrical body (pentaradial means there are five planes of symmetry through an animal), heart urchins do not show pentaradial symmetry.  Instead, they have a mouth on one end of their body and they burrow through sediments in a definite direction (namely, forwards), extracting edible organic bits from the inorganic mud of the seafloor.  Both types of urchins–regular and irregular–use spines on the downward-facing (ventral) side of their body as paddles or stilts to move around.

The heart urchin in the video above, Echinocrepis rostrata, looks like a giant nose sniffling around the seafloor.  Others look like ovals, but all have a side that’s primarily for pushing through the sediments and another side that is not.  For my class project, I used scanning electron microscopy to compare the attachment sites for spines on the downward-facing (ventral) side, which pushes through the mud, with those on the upward-facing (dorsal) side, to see if all that paddling through the sediments makes a difference for the ventral side.

Credit: A Kahn

SEM image. Close-up of the base, showing the milled ring (MR) and central ligament (CL) attachment site. Credit: A Kahn

Above are two SEM images of an urchin spine, taken at the Advanced Microscopy Facility at the University of Alberta.  At the base, there’s a milled ring (MR), which muscles and the central ligament (CL) attach to.  Those muscles and ligaments then attach to tubercles and attachment sites on the skeleton (called a “test”) of the urchin.

View of the dorsal side of the heart urchin. Those round peaks are the attachment sites for the spines. AD – adradial sutures, connecting different plates of the urchin test together. Credit: A Kahn

Above now is a picture of attachment sites on the dorsal, non-burrowing side of the heart urchin. Notice that they’re circular, and that, according to the scale bar in the upper right, each attachment site on the dorsal side has a diameter of 200 microns or less.  Now check out the attachment site from the ventral (downward-facing) side of the urchin in the SEM image below–the attachment sites are no longer circular, but ovoid, and they are HUGE!  Much longer than 200 microns along the widest axis!

CL – central ligament. T – tubercle.  ta – tendon attachment.  ST – stereom trabecula…a fancy name for the porous-looking part of the urchin’s skeleton. Credit: A Kahn

I wondered why this was, and I hypothesized that since spines on the ventral side are the ones responsible for the urchin to zip (err, relatively speaking) through the mud, they need to have more, or stronger, muscles, that would require bigger attachment sites.  Heart urchins have a front end and a back end they move primarily in one direction, so the attachment sites might be ovoid because the spines move mainly in one direction and they’d need more muscles for that direction of movement.  It’s all hypotheses at the moment–to actually see if that’s the case, I’d have to study more than just an urchin test, and actually observe live urchins, look at the muscles that are attached and how strong they are…sounds like another trip out to Bamfield!  Still, it’s a neat idea.

Do you have other ideas for why the attachment sites are so much larger on the ventral versus the dorsal side?  Or why they’re shaped like ovals?  How would you go about testing your hypothesis?  Let’s brainstorm in the comments below.