Along with most of the working world, the Solar Slug team was summarily ejected from the lab on March 19, 2020.  I have been allowed to return occasionally to perform maintenance on the tanks, but was otherwise expected to stay home and continue teaching and other work remotely.  The next generation of slugs had been laying eggs, but I would not be able to devote any form of loving attention to the offspring.

As a devoted follower of the blog, you will remember that it took several years to work out conditions for rearing baby slugs.  The hatchlings need to be protected from pathogens and predators, and must have access to a clean supply of Bryopsis plumosa in order to take their first meals.

Some of the baby slugs did not get the memo.  A few weeks after we began isolation, I saw a few tiny slugs in one of the tanks.  It was a bit of a relief, because I had begun to wonder if I would be able to rear the next generation before the adults became too old to produce eggs.  

Baby slug, reared without effort. Note the large cluster of Valonia at the right of the photo.  4/14/20

The photo below gives a better sense of scale. The youngster, probably already a few weeks old, is less than a centimeter, and is dwarfed by one of the parents. It is impossible to be certain whether it is feeding on the red, filamentous algae, or finding green algae in there somewhere. Their overall color makes it clear that they are both full of green chloroplasts.

As I mentioned above, hatchling Elysia can be delicate, and are very particular about their first meals. If Bryopsis plumosa is not available, then they will starve. During the period of neglect, Bryopsis pennata (excellent food for the adults) has been growing well. Other algae, such as Ulva and Valonia (bubble algae) have also been thriving, along with the unidentified filamentous red algae. Although I have not seen it, there very well may be adequate growth of B. plumosa hidden among the other algae.

Early mortality must have been pretty high, because there are a few dozen youngsters (rather than hundreds) cruising the tank. That is more than enough slugs to keep the colony going for the year, but would not be adequate for experiments. Fortunately/unfortunately the campus is unlikely to return to normal by the fall, so student Slug Club will probably be postponed. Meantime, the new kids will grow into the next generation of parents.

Elysia diomedea in field station tank. 6/30/18

Having the permits meant that we could get straight to work.  The first order of business was to get the tanks ready and find some slugs.  As a happy coincidence, the tides were very low, allowing us to explore the tide pools for interesting organisms.  Even better, we could collect rocks and plants for the slugs tanks by simply picking them up and putting them into a bucket, rather than having to dive down to get them.  We quickly had the tanks ready for sluggy inhabitants.

Tanks ready for slugs. Each contains a small collection of rocks and algae. The tank on the right is illuminated by a high-output LED fixture, while the one on the left is shaded by black felt, resulting in a 100-fold difference in intensity. 6/30/18.

The tanks were also ready for experiments.  One of the hypotheses we wanted to test was that photosynthesis by the slugs’ kleptoplasts contributes to the presence of bad tasting compounds in their mucus and/or tissues.  To test this hypothesis, half of the slugs would be kept in the dark for a week, while the others would be live under lighting adequate for photosynthesis.  For the experiment, one tank would be lit by strong LED lighting, with photosynthetically active radiation (PAR) above 100 µmol photons per square meter per second (plenty for photosynthesis), while the other received less than 1% of that amount.  The tanks were connected to the same life support system and had similar amounts of algae and rocks, so the conditions in the two tanks were nearly identical.

The plants and rocks were an excellent start, but our luck was even better.  We managed to find seven relatively large Elysia in the tide pools just in front of the station.  Even though I have not found Elysia in the same locations during hundreds of hours of snorkeling, they were present in abundance during low tide. It looked as thought the forces controlling the bay were smiling upon us after thumbing their noses at us for a week.

The slugs settled in well, exploring their new habitat and lounging on the algae.

Elysia diomedea in tank at BLA station. 6/30/18.

Elysia diomedea exploring rocks in new home. 6/30/18.

There was even reproductive activity.  As can be seen below, slugs appeared to court each other.

Elysia diomedea courting in a tank at the BLA station. 7/2/18.

They deposited multiple egg masses.  The masses contained thousands of tiny white eggs without extra-embryonic yolk, which is consistent with what others have observed for the species (see e.g., the Sea Slug Forum) .   

Egg mass of Elysia diomedea. 7/4/18.

Egg mass of Elysia diomedea in BLA station tank. 7/3/18.

Although, I did not have a camera with sufficient resolution to show the details of the masses,  I would agree with others that E. diomedea embryos are smaller than those of E. clarki and E. crispata. The relatively small size of the embryos, and resulting smaller amount of yolk, means that E. diomedea are probably “planktotrophic,” hatching earlier and feeding on plankton in order to finish development.  Larger embryos, such as those of E. clarki and E. crispata, are “lecithotrophic” living on plentiful yolk stores  until it is time for the veligers to settle and start feeding on Bryopsis.

Having slugs and algae also gave us the chance to do some DNA extraction.  The students took small samples from a couple of slugs, and some local algae, then extracted the DNA using the DNeasy Plant kit from Qiagen.

Codium specimen. A small sample was removed for DNA extraction and amplification. 7/3/18.

Unidentified green alga. DNA was sampled for identification. 7/3/18.

After a few rounds of incubations and separations, the students had generated tubes of clear liquid that presumably held DNA.

Elizabeth, Maria, Keyla, and Lily holding tubes of DNA extracts. 7/3/18.

Meantime, there was lots of other stuff going on.  Ric had been leading the other half of the group in troubleshooting and starting the feeding experiments.  Because we wanted to test the palatability of the tissues from slugs kept in the dark, we needed to modify the feeding assay we used in tanks at USG for use with fish in the bay.  It seemed simple enough to hang food cubes on fishing line that is anchored by lead weights at one end and held in the water column by a float at the other.  Figuring out the right thickness of fishing line, and how best to secure the lines to the weights and floats, required a good bit of trial and error.

The students were also working on their scientific presentation skills.  In one exercise, Ric had them give short summaries of the work while standing in the bay, in order to have them project their voices in a noisy, distracting environment.

Students giving talks while standing in the bay. 
7/2/18.

Unfortunately, two weeks had passed, which meant that it was time for me to leave the students, the great people from Ocean Discovery, my friend Drew, the bay, and the slugs, and return to Maryland.  The students were doing great, the assays had started to work (although the first round of PCR amplification of the DNA extracts was not successful), Ric had everything well in hand, and there would be a real molecular biologist arriving in a few days to act as my substitute.

I watched my last sunrise at Bahia for the season.

Sunrise over the bay.

I grabbed some of the DNA, packed my things, said my goodbyes, and headed north to San Diego.

Slug Life! The Photobiology group on my last morning. From left: Keyla, Bennie, Zaira, Me, Maria, Ric, Lily, Diana, Melanie, Elizabeth.  7/6/18.

At this point, we were about halfway through the field season.  After all the hard work and adaptive management, will there be results?