Blastomere Separation: Part 3

This Sunday, June 6, my experimental purple urchins will have their two-month birthday!! In my opinion, this is quite a feat. I feel like a proud parent! Of the 10-12 urchins I performed surgery on at the 4-cell stage, 3 have survived and appear to be developing normally. Taking pictures is a little nerve-racking. It's not getting them to smile for the camera that's tricky, but getting them ON and OFF the slide safely and without too much heat exposure from the microscope light. For this reason I waited to take more pictures. (I wasn't just trying to keep you in suspense!) But I couldn't wait any longer to share this next developmental milestone.

The two pictures to the left are basically the same image, however, one is taken using bright field microscopy (top) and the other - using dark field technique (bottom). At this time the larva was just over 7 weeks old. To orient you, this is a ventral view with the anterior to the right, posterior to the left, right side at the top, and the left side on the bottom. The large non-transparent circle in the center is the stomach and right below the stomach is the juvenile rudiment. This is the developmental milestone I am referring to above. The juvenile rudiment usually develops on the left side, except in rare cases, and will go on to become the juvenile urchin. It will develop tube feet and spines and eventually protrude from the larval body. The juvenile will reabsorb most of the larval body in a process known as metamorphosis, and will walk way on its tiny tube feet. At the most posterior end of the larva there is a thick ciliated band called the epaulette. It assists the larva in locomotion, and it if you look closely at both ends you can see the cilia.

This picture is of a different experimental urchin. This is a dorsal view which is why the juvenile rudiment is above the stomach rather than below it. Also, this picture was taken with a 4x magnification lens while the previous were taken with a 10x. (The larvae were approximately the same size). The larval arms aren't quite as straight as on the previous pictures - maybe it was waving to the camera. However, what is interesting about this larva is the size of the juvenile rudiment. It's about the same size, if not larger, than the stomach. It is not uncommon for larvae of the same age to have rudiments of different sizes. Despite the fact that our Embryology class is ending on June 7th, I will continue to rear these larvae. I hope my next pictures are an illustration of urchin metamorphosis!

See also my earlier posts on blastomere separations: Part 1   Part 2

Field Trip to South Cove

On May 3rd after listening to a lecture on fertilization ecology by Dr. Craig Young, the Embryology class piled into OIMB’s 15-passenger van and headed out to Cape Arago. Decked out in our rubber boots and rain gear, we descended the winding path from the road at the hill top to the rocky intertidal of South Cove. The tide was a -0.4 that day, exposing thousands of rocks and boulders to our searching eyes. Specifically, we were hunting for bryozoans, colonial “moss animals” that can often be found growing under overhanging rocks or encrusted on them. But while we were down there, we certainly weren’t going to pass up the chance to explore and find as many amazing organisms as we could!

Although the entire trip was informative and entertaining, I think the highlight for everyone was watching our professor, Dr. Svetlana Maslakova, crack open a sea urchin and eat the roe right out of it!

“A little salty, but good,” she said. Several of the students followed her example.

As the tide began to turn, and the rocks were covered once more by the sea, we climbed back up the hill, bryozoans in tow, to return to our classroom and study the embryology of this unique and fascinating phylum of animals.

Mudflat Field Trip

The morning of April 19, 2010, was a rainy one, but that didn’t stop the Embryology class at OIMB from heading out to the mudflats of the Coos Bay estuary during the low tide. Our mission: to find several kinds of worms by digging in the muddy sand. To enhance our studies of spiralian development, we wanted to find nemertean worms (Micrura and Cerebratulus) as well as the tube-dwelling polychaete Owenia, which has a unique larva called the mitraria. We also were keeping a look out for a tube-dwelling worm from another phylum, the phoronid Phoronopsis harmeri.

The going was slow as we slogged through the muck and picked through the mud, but after a couple hours of searching and digging, we had found at least a few specimens of every one of our target species. These we took back to the lab at OIMB, placed in flowing seawater tables, and studied over the next few weeks.

Plankton Tow

On the morning of Monday, April 12, the class climbed on board the 20-foot R/V Pugettia and set out for the Pacific Ocean to collect some plankton. Larry Draper, our boat captain, took us about a mile beyond the Coos Bay jetties, turned north to escape the river plume, and then cut the boat engine. We lowered our 153 µm-mesh plankton net into the water and allowed it to drift for several minutes. After we pulled the net out of the water, we dumped the contents of the cod end into a glass jar. When you held this up to the light, you could easily see thousands of tiny plants and animals floating and zooming around: plankton! We repeated this procedure inside the Coos Bay estuary so that we could compare the plankton between the two sites. The rest of the day was spent in the lab sorting through our catch with microscopes, observing the strange and beautiful creatures of the plankton community.

Nudibranch veliger larva

Here are a few pictures of the veliger larva of the frosted nudibranch (a type of sea slug), Dirona albolineata. In the top picture, you can see two larvae inside an egg capsule. Many other capsules in the same egg mass held 6-8 larvae in each. The shell of the veliger protects the internal organs, e.g. the nervous system, digestive tract, and the retractor muscles, which allow the veliger to pull the velum and foot into the shell. A pair of statocysts (little capsules that work as balance organs) are visible as well, each with a statolith (a tiny granule) inside. One can also see the velum, which has two rounded lobes with long cilia used for locomotion in the water column once the veligers hatch from the egg capsules. 


The bottom picture shows a hatched veliger from the side, with a developed foot. On the back of the foot is the thin, visible operculum, which acts as a trap door, closing the shell opening when the velum and the foot are pulled in. When the velum is pulled in, the larva can’t swim and sinks to the bottom. Once the larval stage is complete, settlement cues in the environment induce metamorphosis  (transformation into juvenile stage). This includes permanent loss of the velum and shell (and other larval organs, like the muscle retractor), as the foot becomes the main locomotory organ of a sea slug.