Midterm exam

Chromodoris by David Doubilet

Well done, everyone!  I have a thick stack of exams to grade, and will get a good start on them this weekend.  What follows is a minor rewrite of what I posted at this point in the term last time I taught this class.  As my students recover from turning in their midterms and from the various viruses that seem to be sweeping campus, this slideshow of photos taken by David Doubilet  might be therapeutic.  But don't get so dazzled by those lurid colors and such that you stop thinking altogether. Remember that one of the special things about gastropods is that they exhibit torsion, a 180-degree twist of the visceral mass (including most internal organs and nervous system) relative to the foot. Among many consequences of torsion is the loss of the left post-torsional gonad and displacement of all the pipes that empty into the mantle cavity over to the right, downstream of the gills, which is a better place to dump your urine, feces, and gametes, after all. While nudibranchs and their sea-sluggy opisthobranch kin are detorted (untwisted) as adults, notice they all retain a penis that emerges from the right side. Yes, they ALL have a penis, being simultaneous hermaphrodites, and it's on the right side for all of them. A remnant of torsion past, both ancestrally and developmentally.

Bivalves are busier than they look

COA students census clams at Hadley Point

The processes of respiration, feeding, and locomotion in bivalves all rely on the animal moving water through the mantle cavity.  To burrow, bivalves rely on muscle action to squeeze the shells closed, which puts pressure on the blood sinuses in the visceral mass (body) and moves blood into the foot, causing it to extend.  The closing shells also squirt water anteriorly out of the mantle cavity, helping to soften the sediments in front of the burrowing clam, easing their work.  Ed Yong at Not Exactly Rocket Science has a great description of the burrowing process along with photos and links to videos.  The water movement for feeding and respiration is more subtle, and it relies on the action of the cilia that cover the extensive ctenidia (gills) of most bivalves.  Water is swept into the mantle cavity through a larger, ventral incurrent opening at the posterior of the animal.  The oxygen-rich water, laden with food particles, moves anteriorly and dorsally across the ctenidia, which both capture food and extract oxygen from the water current.  The cleared water then passes back posteriorly and out through a smaller dorsal excurrent opening.  You can sometimes see this process in the lab by carefully marking the water near the incurrent opening using food coloring or milk.  You can also visualize water flow through mussels, using the spiffy technique of Schlieren imaging. 

The inspiring Vi Hart

Having just finished my midterm check-in with the class on how the course is going, I’ve been reflecting on my teaching, particularly the lectures for invertebrate zoology.  I like to think my lectures are content-rich and characterized by enthusiasm for the material and that the energy I bring serves as an invitation for students to engage more deeply with the class content.  My speaking style has been influenced by many people, and I am amused to sometimes hear echoes of their voices coming through in my lectures.  Sometimes channeling my inner Diver Ed is a very effective teaching tool.  Vi Hart is someone else whose expertise and exuberance I greatly admire.  I’ll never be able to match her speed of presentation, probably to my students’ relief.  This post was really just an excuse to share these cool videos on hexaflexagons.

tiny camera captures internal view of a starfish eating a mussel

There are some really nice echinoderm videos out there, but this one featured at Deep Sea News is unique.  The beginning of the video is dramatic enough, with an advancing front of starfish creeping up a mussel-encrusted pier piling.  Notice how fluid the starfish look in the time lapse.  Mutable collagenous tissue doing its thing!  Then the predation begins.  The starfish’s stomach should look somewhat familiar if you’ve ever interrupted a feeding starfish.  But seeing it from the perspective inside a mussel shell is a thrilling novelty.  Not really a “mussel’s eye view” as they have no eyes, but still!  As the stomach slides in, you can see the mussel’s gills, the ctenidia, on the right.  The slender, ciliated filaments that make up the gill show up as blurry striations in the foreground.  On the left is the mussel’s mantle, the organ that lays down the shell.  The dark orange gonad can be seen extending into the mantle tissue.  It’s all just starfish food.