As described in the previous post, I had very modest goals for this summer in Bahia. Because I am getting more interested in the role of kleptoplasty in chemical defense, I thought it would be worth assessing the palatability of Elysia diomedea. Some Elysia species are known to taste bad because of chemicals assimilated from their food plants (see, e.g., Rasher et al., described in this post). E. diomedea is known to produce interesting derivatives of plant compounds (e.g., Ireland et al., 1978, J. Am. Chem. Soc. 100:1002), but, as far as I can tell, there is no evidence regarding the slugs’ palatability.
Fortunately for me, there is a relatively easy way to get a quick sense of their palatability. When snorkeling at the field station, one is generally followed by a small parade of large bullseye puffers (Sphoeroides annulatus) waiting for tasty morsels to be stirred up. What would happen if I dropped a slug in the water column and allowed the fish to eat it? One might expect a puffer to eat anything.
After a day in the field, I had time for a snorkel, so it was a perfect opportunity. After a short survey along the subtidal, I found a few Elysia in a small bunch of Codium (surprise!). I pulled out this little beauty, apologized to her and carried her to the surface.
If you click the link below for the short video (note: large-ish file), it is pretty clear that the puffer does not find the little Elysia to its liking.
Not only one, but three puffers rejected the Elysia. After the first spat out the slug, a second tried it, then a third. In no case did one of the puffers as much as chew, they rejected it as soon as it was in their mouths. Very good for the slug, and suggests that it may be something secreted in the mucus that repels the fish. One might also conclude that puffers don’t learn from their friends, since each had to try it.
Based on one slug (but three puffers), we can tentatively conclude that E. diomedea tastes bad. Are the bad tasting compounds derived from products made by the kleptoplasts?
Nature is perverse.
We’re back in Bahia de los Angeles, on the east coast of Baja California, Mexico. For the two previous years, I have had to suffer for a while before finding any Elysia diomedea. It was rather nerve wracking, because I needed them for research projects each of those years.
Because of time constraints, my goals this year are to help my friend, Dr. Drew Talley, with his long term research, and to discuss plans for student Elysia projects in summer 2018 with Ocean Discovery Institute.
We had some complications at the border, because the Mexican authorities had some reservations about some equipment that was being used by one of the other research groups. We were allowed to proceed after a few hours dealing with paperwork, but were delayed to the point that we had to stop along the way for the night.
We arrived without further incident, unloaded equipment and belongings, started setting up the station, had a great meal in town, we went to bed for the night. It was amazing to be back under all the stars, listening to the ocean and the occasional breathing of a marine mammal.
We woke up to a classic sunrise, and soon we were on the islands, setting traps for insect surveys and savoring the bay and the scenery.
After returning to the station, we ran some errands, followed by a little open time to get in the water. Although I may try a few extremely simple preliminary experiments, my work here does not depend on finding them. Naturally, that means they were abundant in the shallows in front of the station. I found the first within five minutes, and saw at least six within the half hour allotted for the survey.
They looked darker than the slugs we found last summer, but, as was true last summer, all were on or near Codium.
Keeping fingers crossed for a chance to test some ideas about chemical camouflage.
Just when I thought I had it all figured out.
After struggling for way too long, I finally got reliable growth of Bryopsis in the algae culture tanks by providing strong lighting, balanced nutrients, along with very intense water movement. Bryopsis grew well, although Derbesia also started to thrive and needed periodic removal, so I figured I was close to the magic formula.
Imagine my surprise when Bryopsis started to thrive, largely without contamination by dinoflagellates or Derbesia, in the slug culture tanks. In the 20 gallon long, the algae that was transferred on the plastic racks started to grow and spread, despite constant grazing from the resident E. clarki. In the 10 gallon, a few scraps growing on some of the macroalgae grew to fill almost half the tank.
This has made the single E. crispata very happy. She has grown considerably, and her color is amazing. Will post a photo when she comes out of the algae far enough to be photographed.
The tanks are all plumbed together, so they all receive the same nutrient input. The slug tanks receive somewhat less light than the algae culture tanks. The spectrum of the lights in the slug tanks is broader, with more green, yellow and UV, mostly because that is nicer for my eyes when I look at the tanks. Finally, the circulation is considerably less intense. The algae tanks are blasted with propeller pumps and wavemakers, while circulation to each of the slug tanks is only provided by a single Maxi-Jet 600 (600 liters per hour output), with the intake slug-proofed by a strainer and sponge, and the output directed through a Hydor Flo to provide swirling motion.
Because even these relatively small pumps give the slugs a bit of a wild ride, they are only turned on for 15 minutes of every hour. Quick summary: despite my beliefs to the contrary, it is possible for Bryopsis to grow strongly with modest water movement.
Although I am pleased that there is now abundant food for the slugs, it does bother me that I still do not understand all of the factors affecting growth of Bryopsis. Previously, Bryopsis struggled in conditions that were largely similar. If I get a chance before leaving for Baja this week, I’ll make yet another deep dive into the system logs to determine which parameters may have changed. The luxuriant growth has emboldened me to order a few more E. clarki, so that the colony can be going full steam by the end of the summer.