Field Trip to South Cove

The morning of April 20, 2011, we, students in the Embryology class at OIMB, made our way to the rocky intertidal at South Cove, located at the southernmost end of the Cape Arago Highway, near Charleston, OR.   We unloaded from the van with buckets, butter knives and small tubes, donned our rain gear, and descended the trail towards the beach.

Our task was to search throughout and beneath the boulder field, exposed by the low tide, for several kinds of organisms; specifically, several types of bryozoans. These included Crisia sp., Flustrellidra corniculata, and Dendrobaenia lichenoides.   Bryozoans are colonial “moss-animals” that can often be found growing under overhanging rocks or encrusted upon them. To enhance our study of mollusc development, we also looked for chitons, and the gastropod Calliostoma ligatum.

We tipped over rocks (and put them back where we found them), and after a couple hours of searching, we had found at least a few specimens of every one of our target species including one colony of the unusual, and rather uncommon on the intertidal, bryozoan Flustrellidra corniculata, which has a brooded lecithotrophic pseudocyphonautes larva (see post by Tony Dores).

Echinus rudiment formation

Shown here is the echinopluteus larva of the purple sea urchin, Stronglyocentrotus purpuratus (top image), and a sand dollar Dendraster excentricus (bottom image). Larval anterior is up. In both pictures, you can see the juvenile sea urchin or sand dollar developing inside the larval body. The rudiment of the juvenile, called the echinus rudiment develops in a pouch called a vestibule, which forms as an unpaired epidermal invagination on the left side of the larval body. The left coelomic sac contributes to the formation of the juvenile. The top picture shows the three-week old larva of S. purpuratus from the ventral side. This larva does not yet have a well-developed echinus rudiment, but the invagination can be seen clearly to the right and above the stomach, which is the large dark oval shape in the middle of the larva.

The three-week old D. excentricus larva shown here has a much more advanced juvenile rudiment than the S. purpuratus larva. The larva is shown from ventro-lateral view, so that the juvenile rudiment is facing you. The juvenile rudiment is quite large and takes up most of the posterior portion of the larva, obscuring the larval stomach, which barely peaks out from behind the juvenile rudiment on the left. By the late pluteus stage, the juvenile is almost fully developed within the vestibule, and will have acquired juvenile spines and skeletal structures. The juvenile also has five podia, or tube feet, on the oral side of the juvenile, which is the side facing away from the larval stomach. Juvenile tube feet can be extended out of the vestibule and retracted again. This D. excentricus juvenile has developed podia, which appear as five lobes on the right. When ready to settle, the juvenile extends its podia from the vestibule, contacts the substrate, and walks away, having re-absorbed much of the larval body.

Coelom formation in bipinnaria

We are raising bipinnaria larvae of the sea star, Pisaster ochraceus in our Embryology class at OIMB! I was particularly interested in the development of coelomic sacs in the bipinnaria. These pictures show three different stages of development of coelomic sacks with larval anterior oriented up.

The top picture is of a 2-day old late gastrula, and the elongated cylinder inside the gastrula is the archenteron, or primary gut. You may notice that the tip of the archenteron is slightly T-shaped. This is because during gastrulation, before the archenteron makes contact with the oral epithelium, two coeloms form as pouches from the tip of the archenteron, during a process called enterocoely.

The coeloms bud off from the archenteron forming a sac on the left and right side of the gut as you can see in 4-day old bipinnaria larva in the middle picture.

The bottom picture is a dorsal view of a 12-day old bipinnaria with well-developed coelomic sacs. It is interesting to note the larger size of the left coelom because the two coeloms have different roles in development. The left coelom, called the hydrocoel, is connected to the dorsal epithelium via a hydropore canal, and opens to the environment via a small round hole, called the hydropore, which you can see as a small dark shape to the upper left of the larval stomach (the upside-down pear-shaped structure occupying the posterior portion of the larva). The hydrocoel will form the water-vascular system of the adult sea star. I look forward to continuing to watch the development of the Pisaster ochraceus larvae as well as the development of their coelomic sacs!

Regeneration in bipinnaria

The top picture on the left shows a bipinnaria larva of the starfish Pisaster ochraceus. One interesting characteristic of this organism, and the starfish in general, is its ability to regenerate missing body parts both as a larva and as an adult. I read that bisected starfish larvae have been observed to regenerate to form complete larvae within 12-14 days (Vickery and McClintock, 1998). I wanted to see it myself, so on April 13, 2011 I surgically bisected several bipinnarias across the middle, separating the anterior from the posterior portion. The two bottom pictures show the divorced anterior and posterior portions of the bisected larva. I took these pictures within 5 minutes of the surgery. If you look closely you can already see that each fragment is closing the wound! Amazing!

Cutting the larvae was difficult, because these bipinnarias keep swimming around. In addition to tracking and anticipating their movements, I had to be careful not to break the tip of the glass needle I used to make a cut. I pulled pipettes using a Sutter Micropipette Puller. They are so sharp, their ends so miniscule, that they cannot be seen with the naked eye. I broke 5 needles while cutting 15 individuals!

Finally, towards the end of the procedure, I started to get the hang of trapping the larva and cutting it without poking myself or breaking the needle! After the bisection, the two pieces of the larva swam away as if nothing had happened! I will follow the regeneration of these larvae, taking pictures as they develop.  I hope to witness organogenesis, the formation of organs, until the larvae are complete once again!

Vickery, MS and McClintock, JB. 1998. Regeneration in metazoan larvae. Nature. 394: 140