Elysia and related sacoglossans are beautiful and interesting, but their use of stolen chloroplasts puts them in a class above your average green sea slug. How the slugs accomplish this, and what function it serves, are coming into focus very slowly.
It has been known for a long time that kleptoplasts remain alive and functional in many Elysia species, but the idea that they can rely on the chloroplasts for all of their energy needs (the “crawling leaves” hypothesis) appears to be wrong, or at least overly simple. The inadequacy of the crawling leaf theory got me thinking about a possible role for kleptoplasts as factories for making defensive chemicals. There was support in the literature (e.g., Trench et al., 1972; Ireland and Scheuer, 1975), and I was interested enough to organize a student journal club on chemical ecology, and prepare experiments to test the connection between kleptoplasty and predation in Bahia de los Angeles in summer 2018.
To introduce the students to the journal club format, and to some important Elysia concepts, I presented the following paper by Baumgartner, Pavia and Toth from PLoS One, published in 2015. The paper has restored some of my faith that kleptoplasty provides a substantial metabolic benefit to the host slug.
They performed a series of experiments to ask a straightforward question: Do kleptoplasts provide usable energy to E. viridis?
E. viridis‘ gets kleptoplasts from at least two of its natural food plants, Codium and Cladophora, and they differ in their viability in the slug. Chloroplasts from Codium are highly functional, whereas those from Cladophora do not function well after being eaten. This provides a natural experiment, in which one can compare the effects of light on the growth of slugs fed Codium (highly productive kleptoplasts) versus those fed Cladophora (little contribution from photosynthesis). Importantly, the slugs in these experiments are not starved, so the authors are looking at the effects of photosynthesis over and above the energy and nutrients gained from feeding.
They tested the following hypotheses:
E. viridis were divided into four groups: Those eating Codium,under high light (~100 μmol quanta m-2 s-1) or low light (~6 μmol quanta m-2 s-1); and those eating Cladophora in high or low light
In the first experiment, they examined the effect of illumination on growth of the slugs. Rather than simply looking at growth rate, they used a measure called Growth Efficiency (GE), which calculated the amount of growth of the slugs as a function of how much the slugs ate.
Step 1: calculate slug Growth Rate (GR) = (Mend-Mbefore)/t, where M = mass and t = time
Step 2: calculate Consumption Rate (CR). For this they needed to either measure the change in mass over time (Codium), or the number of damaged cells (Cladophora). They had to use two different measures because the structures of the two species of algae posed different challenges. They used algae kept without slugs as a control for the effect of time.
Step 3: Calculate GE = GR/CR, yielding the number of milligrams (mg) of slug growth per gram of algae consumed (Codium) or per 1000 cells consumed (Cladophora). Because they were comparing the effect of light intensity on slugs fed the two algae, the different units used for consumption did not cause problems.
Figure 1A shows that slugs fed Codium show significantly better growth efficiency in high light than in low light. Those feeding on Cladophora show no difference in two conditions. The kleptoplasts from Codium contributed more to the growth of the slugs in bright light, which suggests that photosynthesis is producing energy that the slugs could use for growth.
However, they needed to be sure that the kelptoplasts from Codium were truly more functional than those from Cladophora, so they measured photosynthetic ability of chloroplasts and kleptoplasts using PAM fluorometry. The absorbance and fluorescence of chlorophyll can be used to measure the health of chloroplasts, and to calculate the relative electron transport rate (rETR), a measure of carbon fixation and therefore energy production.
Figure 2 shows that kleptoplasts from Codium responded to intense illumination with a strong increase in rETR, consistent with increased growth efficiency being caused by increased photosynthesis of healthy kleptoplasts. Kleptoplasts from Cladophora had low rETR regardless of lighting, correlating well with the lack of effect of intense light on growth efficiency of E. viridis. These observations support the idea that photosynthesis by kleptoplasts contributes to slug growth.
Just in case the increased growth rate of E. viridis was due to some other benefit provided by Codium, they measured nutritional quality of the two algal species.
In all cases, the nutritional value of Cladophora was as high or higher than that of Codium (Figure 3), so there was no evidence that Codium was intrinsically more nutritious. Importantly, Panel 3N shows chloroplasts in Cladophora increased rETR in response to more intense light. Therefore, the poor performance of Cladophora-derived kleptoplasts (Figure 2B) is likely to result from degradation inside the slug.
The evidence therefore supports the authors’ hypotheses, and they conclude that E. viridis derive measurable metabolic benefits from photosynthesizing kleptoplasts. Whereas Slugs feeding on Cladophora receive nutrients by digesting the cellular contents (including the chloroplasts), those feeding on Codium get an extra boost from products produced by active, healthy kleptoplasts.
Overall, the experiments seemed well-executed and convincing, with the only caveat being that the slugs fed Codium and Cladophora were collected on those species and not randomly assigned to the groups. There remains at least a formal possibility that the groups represent two populations of slugs that metabolize chloroplasts differently.
This brings us back to the the role of kleptoplasty in the biology of Elysia. It could be:
We should bear in mind that parsimony may not equal truth, and there is no reason to believe that only one of these answers is correct for all species and life stages.
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?
Welcome to the Roughly Annual Solar Sea Slug Journal Club.
Today’s paper came from the Proceeding of the National Academy of Sciences a few years ago (Rasher et al., 2015, Proc Natl Acad Sci 112:12110). I came across it again when I was updating records for this site, and, because it is germane to one of my pet theories, it seemed perfectly suited for an extended discussion. You’ll see how George Harrison fits into the story later. Yes, this post will meander a bit, but the fact that you are reading the Solar Sea Slug Blog suggests you may have some time on your hands.
The paper is a very nice exploration of the interactions between herbivores and their food plants. Up here on dry land, insects tend to specialize on particular food plants, and bugs and plants have evolved together in something of an arms race. Insects use volatile chemicals produced by the plants to locate them, plants produce defensive chemicals to keep from being eaten and from being infected by insect-borne pathogens, insects develop resistance to the plant chemicals, and sometimes use them for their own defense, and so on. The authors wondered if they could identify a similar web of interactions in the marine environment.
The algae Halimeda incrassata would seem to be rather unpalatable. It produces a collection of defensive chemicals, and is highly calcified, making it a crunchy, bad tasting mouthful. Despite the defenses, Elysia tuca, a tiny and distinctively-patterned species, is commonly found on Halimeda. The interaction between E. tuca and H. incrassata allowed the authors to ask how similar the relationship between a mollusc and a marine alga is to those of insects and terrestrial plants.
In order to compare the relationship between E. tuca and Halimeda to terrestrial plant-insect interactions, the study focused on five specific questions:
1) Is E. tuca really a specialist? This is important for the development of an intimate plant-herbivore relationship.
2) Does E. tuca find Halimeda based on chemical cues?
3) What are the cues that E. tuca uses?
4) What are the ecological consequences of E. tuca feeding on H. incrassata?
5) Does Halimeda use counter-defenses to limit the damage inflicted by E. tuca.
With regard to E. tuca being a specialist, the answer was a pretty resounding “yes.” They collected specimens of about 10 species of algae and marine plants at two sites, took them to the lab, and counted the numbers of E. tuca on each. With a few exceptions (also in the genus Halimeda), E. tuca were only found on H. incrassata. Further, when given the choice between many different algae species in the lab (Fig 1A, below), or three different species of Halimeda in the lab (Fig 1B), or in the field (Fig 1C) the slugs greatly preferred H. incrassata. To test whether the slugs were following chemical cues, the experimenters soaked cotton balls in water that had held H. incrassata (Fig 1D), and found that E. tuca much preferred these to cotton balls soaked in plain seawater. With regard to the questions posed by the paper, the results indicate that 1) E. tuca is a specialist, and 2) they find their host based on chemical cues.
The next question regarded the identity of the chemical attractants from H. incrassata (which will be henceforth referred to as “Halimeda”). Compounds were extracted from Halimeda with methanol, and the individual components of the extracts were separated as described in the supplementary methods. Each fraction was tested for attractiveness to slugs using the cotton ball colonization assay described in Figure 1, above. The first compound they described, 4-hydroxybenzoic acid (4-HBA; Figure 2A, left) is found in both “vegetative” Halimeda in the normal growing stage, and in “reproductive” Halimeda that are undergoing spawning events. When 4-HBA was placed on cloth patches next to Halimeda, the plants were colonized by significantly more Elysia than to controls with cloth soaked in the solvent but no 4-HBA (Figure 2B, left panel).
The reproductive stage of Halimeda is significantly more attractive than the vegetative stage, in part because the reproductive cells (gametes) are a rich source of nutrients. When patches soaked in extract from reproductive plants were placed next to Halimeda plants, they attracted more than twice as many slugs as those from vegetative plants (Figure 2B, right). This led the authors to identify halimedatetraacetate (HTA; Figure 2A, right) a chemical compound enriched in reproductive Halimeda. It was known that HTA deters feeding on Halimeda by other species, and that E. tuca sequesters HTA in its tissues. The authors went on to show that an extract from E. tuca that contained HTA deterred feeding by predatory wrasse.
This brings us to question #4, what are the ecological consequences of E. tuca grazing on Halimeda? Surprisingly, the effects of such tiny slugs are significant. The fact that the slugs feed on reproductive structures (which have the highest HTA content) is expected to substantially reduce the plants’ fecundity. Further, when the authors manipulated the numbers of Elysia on plants in the field, those with more slugs showed less growth (Figure 3A) and more branch loss (Figure 3B). Placing E. tuca in enclosures on branches (Figure 3D) also caused more branch loss compared with enclosures with no slugs. So E. tuca can cause significant damage to Halimeda. Because H. incrassata aids the development of seagrass beds, and generates the majority of carbonate sediments (a.k.a., nice white sand) in those areas, the authors suggest that grazing by E. tuca can have ecosystem-wide consequences.
How can a small slug that sucks sap cause such dramatic loss of plant tissue? One hypothesis is that the plant self-amputates segments that have been fed upon by Elysia. The model Rasher et al. propose is that the plants are trying to avoid the introduction of pathogens by the slugs by sacrificing segments. After culturing fungi from the slugs’ radullae, which they use to pierce the plants’ tissues, they tested one fungal species they referred to as Et-2. Halimeda innoculated with the fungus dropped segments above the injection site (Figure 4A). Injection of a fungus that is a pathogen of other species did not have the same effect (Figure 4B). The data are therefore consistent with the hypothesis that loss of segments is a defensive strategy in response to feeding by E. tuca, suggesting that the answer to question #5 is also yes.
The authors conclude that the answer to all of the questions they posed is “yes,” and that marine plant herbivore interaction described above strongly resembles those in terrestrial ecosystems, despite more than 400 million years of separation between the participating species.
At some point, this paper got me thinking of a potential alternative function for kleptoplasty. Shall we meander our way there?
By the end of last summer, I was finding most of the prevailing theories regarding kleptoplasty to be rather unsatisfying. While not every aspect of biology must have a function, kleptoplasty has costs that must be offset. It takes energy to segregate and store the chloroplasts, and they must be protected from the immune system. Plus, the animals that are active in the sunlight are exposed to predation and damage from UV light. Despite these costs, Elysia is a very successful genus, with species found worldwide in the shallows of tropical and temperate seas. Therefore kelptoplasty must provide a significant benefit.
So, what good is kleptoplasty? If you buy the arguments presented by deVries et al, photosynthesis by kleptoplasts do not supply a significant portion of the animals’ energy needs. Is the energy produced by photosynthesis used to make starch or fat for use during lean times? Maybe. Could the kleptoplasts be a “living larder,” being digested when food is scarce? The animals certainly become pale when they are starved, suggesting the kleptoplasts are being broken down, but why not just digest them at the time they are eaten and turn them into fat like the rest of us do?
One idea is that the kleptoplasts are merely used as camouflage. In the case of E. diomedea, which spends a lot of its time hidden in its food plant, this seems sensible. Not so much for E. crispata, which is easily visible against hard bottom reefs, which are generally not very green. Further, it seems like there are other, less complicated ways of making or storing green pigment to match one’s surroundings. However, let’s hold that thought for a minute.
Aside from making carbohydrate from sunlight and being green, chloroplasts produce important precursors for many biochemicals (e.g., Gould et al., 2008, Ann. Rev. Plant Biol. 59:491). These could be used by the slugs for the synthesis of fats or essential aromatic amino acids for their own nutrition, or to be used for their prodigious production of eggs. Given that there is absolutely no data regarding the role of photosynthesis in egg production by Elysia, this remains an attractive hypothesis.
However, an insight I thought was particularly brilliant was that chloroplasts synthesize isopentyl diphosphate (IPP), a precursor to a wide range of things, such as chlorophylls and terpenes. Some of these compounds are expected to be smelly, and, in principle, make the slugs smell like their food. Some of the chemicals may also taste bad, rendering the soft, slow animals less palatable.
Predators of Elysia are expected to include nudibranchs, which are largely blind and find their food by smell, or fish, many of which find their prey by sight. If the chloroplasts were pumping out chemicals that gave the slugs a smell of their food, it would make it much more difficult to find them by scent. One bonus is that the green color of the kleptoplasts will also make it more difficult for visual predators, such as fish, to find the slugs. On top of that, any noxious taste would protect the slugs from predators, regardless of their hunting methods. Overall, this model seemed to have fewer caveats than any of the others.
I thought I had come up with this idea on my own. Then I rediscovered the above paper by Rasher et al. while I was updating a saved search in the Scopus database. PNAS is my Wednesday lunchtime reading, and I am sure that I was excited to come across a paper about Elysia, so I am certain that I read it when it came out. I am saddened by the fact that I forgot that I had read the paper, and assume that the paper got me thinking about kleptoplasty and chemical camouflage.
Have you figured out the connection to George Harrison yet? You have to be getting on in years or love music trivia to remember, but he produced a popular song “My Sweet Lord,” during his post-Beatles solo career. He ran into some legal trouble when the Chiffons’ record label sued him for appropriating the melody from their highly popular “He’s So Fine.” If you go online and listen to both of them, it won’t take you long to think “dang, he stole their melody.” Harrison admitted that he was very familiar with the melody, and the judge ruled that he had committed “subconscious plagiarism.” In the same way, I had no memory of even having read the Rasher paper when I was formulating ideas and researching the biosynthetic capabilities of chloroplasts. I just thought I was being terribly clever. Nonetheless, it is highly likely that the paper was somewhere in the recesses of my mind during the process. Are there any truly new ideas?
But, more importantly, how does one test this? The first step is to make some predictions, and here are a few possibilities:
There are likely to be more, better experiments, but the above provide a start.
Back from Bonaire, with a fresh puzzle.
In research, as in life, there are things that don’t make sense. Often these things make enough sense that you ignore them, choosing to focus on other mysteries. One such little small, nagging issue is the question of what draws Elysia crispata to hard-bottom coral reefs, which lack obvious growth of green algae known to be their food. Based on observations of many years, the slugs are not in transit, most are just sitting there.
My knowledge of the habits of Elysia in the wild is far from encyclopedic, but the species I know best have hearty appetites and stay close to their food. E. diomedea are found on or near Codium in Bahia de los Angeles, and E. clarki spend most of their time face down in their food in aquaria. This tends to hold true in the literature as well. For example, E. tuca is generally found on its favorite food, Halimeda incrassata (Rasher et al., 2015, PNAS 112: 12110). As a counter example, Middlebrooks et al. (2014) found that E. clarki were often found at sites that contained few or no specimens of their food plants (Penicillus, Halimeda, Bryopsis) determined via DNA barcoding.
In any case, I think I am justified in being puzzled by the lack of an obvious food source on the reef. The photos in this post are all from a single dive at The Cliff, a site in the north-ish part of Bonaire. We found maybe a dozen slugs, most in the face-down posture, which makes them look like large blobs of colorful frosting on the rocks. The area had a lot of dead coral, which possibly serves as a substrate for the growth of food algae. However, there were no obvious growths of green algae anywhere nearby, although algae such as Halimeda and Caulerpa are plentiful in mangroves on the island.
Rather than snap a few photos of the more photogenic slugs, I thought it might be useful to document as many of the slugs as I could, with emphasis on the substrate. Honestly, what you see is what you get; there are no large clumps of Bryopsis or Halimeda hiding around the corner.
What are these gals eating? The most prominent alga is Dictyota, a brown alga which, based on known feeding habits, is an unlikely food.
Are they grazing on the little strands of green algae that can be seen if one expands the photos and looks really hard? Is this a late life stage that does not feed as much? Are E. crispata truly crawling leaves, getting their energy from photosynthesis? Is the much lighter color of E. crispata, compared to related species, like E. clarki and E. diomedea, a clue?
As I mentioned in the previous post, sometimes it is not so easy to identify an alga. In this case, it is a species that bloomed spectacularly when a local reefkeeper set up a new tank. The rock had been thoroughly cleaned and bleached, and no corals or fish had been added, so Alan did not expect the growth of nuisance algae. He was rather surprised to see a rapid, spectacular bloom of long, furry green algae.
At first we thought is might be Bryopsis (yay!), so it seemed worth trying to feed to the slugs. Once I saw and felt it, it was clearly something else. It was soft, like Derbesia, but longer and had branches that extended radially (like a bottle brush) from the main stem. Bryopsis feels coarser, and the branches extend in a single plane (like a fan). So, it was not one of the usual suspects. Nonetheless, it was worth throwing some into a tank to test whether the gals would eat it. They did not immediately plunge into it, as they would have for Bryopsis, but they seemed to find it palatable enough. Note the fine structure of the branches in the photos below.
The plant has some characteristics of the order Bryopsidales, such as the lack of clear cellularization. It looks like the plant is made up of a continuous, single cell.
I thought a quick look at the DNA sequence would clear things up, but that was not the case. The closest match, Acrosiphonia, with 88% sequence identity. That’s not a very good match, and even though it looks somewhat like Acrosiphonia, the unidentified alga lacks several key features, such as the hooks on the branches (which cause mature plants to develop a dreadlocked appearance) and clear cellularization of Acrosiphonia. Plus, Acrosiphonia is a cold water species, unlikely to thrive in a warm reef aquarium.
The closest visual match so far is Trichosolen, which does have warm water species. The only species with rbcL sequence in the database (T. myura) is only an 86% match for DNA, so it’s probably not the one either.
By way of comparison, the usual pest algae (various species of Bryopsis and Derbesia) were only 82% – 83% identical, so we can at least rule out the possibility that it is an oddball species of one of those.
The hunt continues for a match. Not very satisfying, but some days are like that.
Happy Holidays to all of you fans of slugs!
Although the site and the project are devoted to adorable molluscs, we would be nowhere without algae. These days, I spend more time and resources trying to acquire, grow, and identify algae than I do attending to Elysia. It should not be a surprise, given the outsize role of algae in the biology of the slugs, but, until this project was underway, I had never given a lot of thought to the care and diversity of algae. Subsequent posts will describe some of the progress in algae care, but today we’ll focus on some systematics and molecular biology.
The plant in the photo above has been nagging at me for well over a year. I can’t remember exactly how it came up, but KP Aquatics mentioned that they had a species of algae they called “spongy sea pansy,” which was like Udotea, but larger and squishier. They were quite a bit taller than Udotea, grew in clumps, and were indeed quite spongy. Their biology is somewhat different from other algae in the order, in that the thallus (the body of the alga) dies back periodically, and a new one grows from the rhizoid (the rootlike part). In my experience, species like Udotea or Penicillus send out runners that produce new thalli, and the old ones just die off.
I have been referring to them as Avrainvillea, because they fit the description reasonably well, but had never done the hard work of verifying that it was not a similarly squishy genus, such as Rhipilia or Cladocephalus.
A real phycologist (algae specialist) would have probably started with a good microscope and species key. I took the molecular route, since I was already using PCR to amplify DNA from the rbcL gene in a few other species, and sending it off for sequencing.
Because I was testing new PCR machines, I had set up three independent reactions, and the results were the same. The screenshot below shows the results of a BLAST search for one of the sequences through the NCBI database, with the closest match at the top. The second best, with 98% of the nucleotides being identical, is Avrainvillea nigricans. The best match (99%) is to an “uncultured Ulvophyceae” clone from a study by Christa, Gould, Wagele, and their collaborators. If I read the entry correctly, the sequence is from kleptoplasts extracted from Costasiella, a Caribbean slug that feeds on…did you guess…Avrainvillea. To provide a little context, Cladocephalus and Rhipilia, the genera that were possible candidates based on appearance, were only 93% and 82% identical, respectively.
That is a pretty clear-cut result. It looks like Avrainvillea, it is squishy like Avrainvillea, and its DNA is essentially an exact match for Avrainvillea nigricans. It is Avrainvillea.
As you’ll see in the next post, the results aren’t always so easy to interpret.
As the summer winds down, it looks as though the project worked better than I had hoped. There is a lot left to do, so this is far from the end, but what a great beginning!
To remind you of the the primary goal of the summer’s project, we wanted to use the DNA contained in the slugs’ kleptoplasts to identify their primary food plant(s). The previous posts described how we worked out methods, collected slugs and candidate food algae, extracted the DNA, amplified the rbcL gene from the chloroplasts, and sent it off for sequencing.
The first sequence that came back from Macrogen did not look very good, which was disheartening. The chromatograms looked awful, and the sequence was gibberish, causing concern that our extractions or PCR reactions were contaminated.
Nonetheless, Paul Kim at Macrogen promised to optimize the reaction and sequencing conditions, and worked hard to provide interpretable data. Patience and persistence have finally paid off, and we can make some simple, declarative statements about the slugs and their food plants.
Statement 1: We obtained usable rbcL DNA sequence from Codium, Ulva and Elysia.
Statement 2: Elysia diomedea steals most, if not all of its kleptoplasts from Codium.
To flesh out these statements a bit:
From Bahia, we now have DNA sequence for Codium simulans and for Ulva. The Codium data is the first for the species. Although rbcL sequence for related species (such as C. isabelae) can be found in the NCBI database, there is currently nothing for C. simulans. We’re not sure which species of Ulva we used, although it is likely to be Ulva californica. In theory the DNA sequence could have told us which species it was, but the region of the rbcL gene that we amplified and sequenced is identical to that in many of the species in the database, so we would need to try another gene, or a different region of rbcL. An important lesson from this year’s work was that we need to preserve samples of the algae we sequenced.
The most exciting result was that we got sequence from E. diomedea kleptoplasts! Overall, we extracted DNA from two individual slugs at different times, and performed at least three separate PCR amplifications (both in BLA and at USG when I got back), and they all came back matching Codium! In retrospect, it is not a shock that slugs that we found in close association with Codium, and which spend a lot of their free time on Codium, actually eat Codium.
The figure above shows a small portion of the sequence, highlighting a few of the sites at which Elysia and Codium differ from Ulva. Overall, the DNA sequence from Elysia was 99% identical with that of Codium, and those few sites that differed appeared to be locations at which there was variation between individuals. Ulva showed about 81% identity to Codium and to kleptoplasts from Elysia.
Despite how it sounds, this is not a trivial result.
First off, Codium has been suspected, but never confirmed as a the food plant. Back in 1969, Trench and colleagues said that E. diomedea fed on green algae, possibly C. simulans, based on the chlorophylls found in the slugs and the morphology of the kleptoplasts, but their methods could not reliably distinguish between green algae species.
As a corollary, there is no evidence that they eat Ulva or Padina, despite being surrounded by them. We did not get rbcL sequence from Padina this year, but it is not closely related to Codium, and the sequence in the database for P. durvillei (the most common species in our study area) shows roughly 70% identity to that from Codium and E. diomedea. Had there been significant Padina or Ulva DNA in the slug sample, the presence of multiple divergent sequences are likely to have made interpreting the results impossible. In other words, we got lucky that there was one dominant species of kleptoplasts. Having sampled only two slugs, we can’t rule out other food plants. Another caveat is that the result shows that chloroplasts from Codium persist in the slugs’ tissues, but the slugs could be eating other species for which the chloroplasts do not last as long inside the slugs.
Another important conclusion is that our methods actually worked. As a neurophysiologist setting up a molecular lab in a dusty, hot garage in an isolated location, there were no guarantees that we would get any usable data. In addition, we used degenerate primers for PCR, to amplify rbcL sequences from all potential algae species, counting on DNA sequencing to tell us which species were present. Our choice of Sanger sequencing, which is much less expensive but prone to problems if the amplified DNA comes from more than one species, could have also caused complications. Planning, persistence, and some luck all worked in our favor.
With these data in hand, there is lots more to do. To fill in some of the gaps discussed above, we need to sample from more slugs in more locations. At the same time, we need to more systematically collect specimens and DNA from algae at different sites around the bay, especially C. simulans. If we are going to generate DNA sequences, we may as well do it in such a way that we can add them to the database.
There is also a lot to be done to understand the big picture of kleptoplasty and how E. diomedea fits into the ecology of the bay. Because of delays in receiving equipment, we had very little time to prepare the behavioral experiments before we left Maryland. On top of that, the losses and stress caused to the slugs by the extreme heat this year, resulted in essentially no data regarding the slugs’ preferences for light. The I-mazes are build and ready, and we plan to add a chiller to the holding system, so procedures should be perfected before the next field season. We also still don’t know much about their environmental requirements. They eat Codium, and live on Codium, but do they have other requirements in terms of water movement, temperature, nutrients, or turbidity?
That the project worked can be chalked up to a lot of planning, hard work, and generosity on the part of a great group of people. At the risk of sounding like an Academy Award acceptance speech…
There would have been no Photobiology group without the “Angels,” Cristal, Rosalia, Nancy, Allison, and Susan. It was so much fun to watch them work and learn. They will be giving their presentation during the Report to the Community for Ocean Discovery this week, and it will be great.
Richy Alvarez, the intelligent and talented Directed Research Fellow, was another reason this project came together. There are so many big and small things that he did to make sure equipment was ready and that the students were prepared, I can’t thank him enough. Big thanks also to Thiago Lima, for generously taking time away from his postdoc at Scripps to work with the students in the field, and for giving advice on the project (he is an actual molecular biologist) along the way.
Huge thanks to all of the staff at Ocean Discovery Institute, especially Joel Barkan, who coordinated the process of turning the plan into a reality when I was 3,000 miles away. I can’t say enough good things about the support I received from everyone at Ocean Discovery, at all levels, and how easy it was to work so closely with so many people. Bahia de los Angeles is a magical place, but doing science there can be a hot, tiring affair. Working with this group makes the process so much more fun.
The experiments also required equipment. Some, like the PCR machine and centrifuge, were generously loaned (thanks ThermoFisher and USD!). Others, such as the tanks and DNA sequencing were purchased from vendors who went the extra mile to do things well and on time (Glasscages and Macrogen).
None of this could have happened without permission from the Comisión Nacional de Áreas Naturales Protegidas (CONANP), which administers the Biosphere Reserve at BLA, and the support of Jose Mercado, who owns and operates the Casa Caguama field station in BLA.
Finally, I owe an enormous debt to Drew Talley, my best friend for over 40 years. He introduced me to Bahia many years ago, and worked tirelessly this year to secure loans of equipment, permits, and who has been incredibly supportive of the development of this project. He has the right to call himself the Captain.
Things were looking great. We had almost 20 slugs, protocols seemed to be working, and the students were becoming comfortable with all of the procedures.
It was time to get some Elysia chloroplast DNA from. Fortunately for the slugs, we did not need a lot of tissue. All we had to do was knock one out, and remove a piece of parapodium. As we showed before, it’s easy to paralyze a slug by soaking it in a magnesium chloride solution that matches the ionic strength (i.e., is isotonic with) of their bodily fluids. This solution rapidly enters their bodies and stops all neural signaling. After 15 minutes, the selected E. diomedea was relaxed and flat as a pancake.
After a quick snip, she was back in the tank, and roaming around within a few hours.
After that,it was time to extract the DNA. The crew got started, extracting DNA from the slimy slug piece, along with a fresh piece of Ulva. There was no time for PCR, but we did have a chance to do one more survey of the area in front of the station.
The conditions were not great, in that the water was somewhat cloudy and surgy by the time we got in. Nonetheless, we got a chance to explore and enjoy the sea life. We also found a few more slugs, which was definitely a bonus.
After that, it was time to pack up and get ready to be on the road. It was sad to be leaving the beautiful place and the people, but time, tides, and summer school wait for no one. We said our goodbyes after dinner. They continued the work for a few more weeks after I left, and I have been getting regular progress reports from Richy.
Hard to say goodbye to the slugs as well.
As always, we were up with the sun. We got on the road early, with tubes of DNA on ice.
The trip north was uneventful, and we arrived at the border in Mexicali on schedule. The wait at the border was about 1.5 hours, made somewhat less pleasant by the 112 degree F heat. We managed to get ourselves and the DNA across, and I was on my way home.
Summer classes started the day after I arrived back in Maryland, so it took a few days to find time to amplify the DNA we extracted in Bahia.It was worth it, though. Very nice bands for Elysia, Codium, and the second sample of Ulva. There were faint bands for the first sample as well, suggesting that the extraction was not a complete bust. With the DNA that was sent last week from the group, we now have a significant number of samples for sequencing, and, with luck, a nice story to tell. After the last round of sequencing did not produce usable data, I gave Macrogen a call. They have been amazing, and are in the process of troubleshooting the last samples I sent them. Keeping fingers crossed.
There was some sad news. The day after we left the station, temperatures shot up to a record 120 degrees F. With those kinds of temperatures, it was impossible to keep the holding tanks cool enough, and most of the slugs were lost. That was sad for the slugs, and meant that there would not be enough animals to finish the behavioral assays this year.
Nonetheless, as the Bahia program winds down this week, we can look back on a lots of success in terms of working out protocols, laying the groundwork for future population surveys, and acquiring DNA samples.