Today a lot of my Dendronotus eggs had hatched on their own, swimming through the water as bona fide veliger larvae. Nudibranch larval culture has officially started!
These bad boys are much more spherical now--whew!-- which makes me think that pointy-shell thing I saw last week was an artifact of their premature hatching. Now they look like little swimming bubbles. Interestingly, their shells are mostly empty. My invertebrate larval culture guide says that planktotrophic larvae (those that feed while in the plankton) such as these hatch with relatively tiny bodies that grow as the larvae feed. We'll see if that holds for these guys.
I captured some video of the little veligers zooming around. Here they are at 10X magnification:
Here's another short video clip of some veligers that were conveniently squished under the coverslip. This kept them from swimming away and I was able to film them at higher magnification. You can see the little velum whirling away and then being retracted. See also how most of the shell space is empty?
So, now that these guys have hatched and have all that empty space inside their shells to fill up, they need to eat. What do I feed them, you ask? Well, because I was in a hurry to get them something, anything, to eat this morning, I fed them a bit of Isochrysis galbana, which is a haptophyte. Algal taxonomy is not well established yet, and there are many ways of classifying both micro- and macroalgae. I hesitate to wade into those murky waters, so suffice it to say that Isochrysis is a unicellular alga, golden-brown in color but neither a diatom nor a dinoflagellate.
This is what Isochrysis galbana looks like in culture. We grow it in 1000-mL flasks of sterilized seawater and nutrients.
According to the literature, veligers of Dendronotus frondosus can be raised on a mixture of Isochrysis galbana and a red alga called Rhodomonas salina. And it just so happens that we also have R. salina in culture, so starting tomorrow the veligers will get a mixture of algae for their breakfast.
The marine gastropods and bivalves go through a larval stage called a veliger. This larva gets its name from the ciliated structure, called a velum, that the animal uses for swimming. Veligers have shells--1 for gastropods and 2 for bivalves--and can withdraw the velum into the shell. Even gastropods that lack shells as adults, such as nudibranchs, have shells as larvae.
The egg mass from Dendronotus is still intact and the embryos are developing nicely. This morning when I looked at it through the microscope I could see the little larvae swimming around inside their egg capsules. I wanted to take a closer look under the compound scope, and when I teased apart the egg mass some of the larvae were forced to "hatch" prematurely. They're not yet ready for life on their own but now they're out in the real world swimming, for better or for worse.
Not being one to let an opportunity like this go to waste, I took some video of the almost-veligers.
You can see the cilia on their little velums whirling around. The larvae aren't as spherical as I had expected, based on what I've seen in other nudibranchs, and I think it'll be fun seeing how they develop. More as things unfold!
What better way to start a new blog than to talk about sex?
This morning at the Seymour Center I noticed a blob of what looked like nudibranch eggs on the wall of one of the tanks. Looking around for the likely culprit I saw three big nudibranchs on the tank. Ooh, cool!
This is Dendronotus iris, a large nudibranch, or sea slug. This bad boy/girl had a foot (the flat white bit that you see reflected in the aquarium glass) that was about 15 cm long. The brownish branched structures on the slug's back are its cerata, which function as gills. These animals do not have the ctenidium, or gill, that is typical of marine snails. Other nudibranchs carry their gills in a single plume that surrounds the anus.
There is one other big slug in this tank. It has a paler body color and cerata that are banded with orange and tipped with white.
Nudibranchs are among the rock stars of marine invertebrates--they are flamboyantly colored, have short adult lives with lots of sex, and leave beautiful corpses when they die. After a planktonic larval life of a few weeks, adult nudibranchs spend their time eating, copulating, and laying eggs. Each slug is a simultaneous hermaphrodite, capable of functioning as both male and female, and mating involves an exchange of sperm. In some other species of nudibranch the act of love can be followed by an act of cannibalism.
Nudibranchs lay egg masses in ribbons or strings that are characteristic of the species. It turns out that Dendronotus egg masses look like Top Ramen noodles:
Each of those individual little white blobs is an egg capsule that contains 10-30 developing embryos. These eggs were deposited yesterday (3 June) and the embryos have been developing but are not yet at any distinct stage. With water temperature at about 13C, I think they'll develop pretty quickly.