Photo of Melibe leonine, copyright of Monterey Bay Aquarium
Sites and Critters

The hooded nudibranch

Two things are occurring right now that justify a posting about my favorite of all sea slugs, the hooded nudibranch (Melibe leonina). The first is that they have been all over Seattle waters in recent months.  The second is that they are the inspiration for my Halloween costume, which interns will have to put up with tomorrow in the lab.

Rhoda Green, my diving partner in crime, recently posted this great medley of Melbe leonina video footage:

In Rhoda’s video, you can see their interesting movements when they’re swimming.  It is common to see Melibe in doing these side to side movements in the water column, but eventually they settle in seagrass or kelp and get to their real purpose in life: feeding and mating. Melibe doesn’t have a radula like most other sea slugs. Instead, their large oral hood, which is lined with tentacles, closes around food particles like you see in the video. Like many gastropods, Melibe is hermaphroditic. Mating Melibes will reciprocally fertilize one another and then each lay coils of cream-colored eggs on the surface of eelgrass or kelp.

One of my favorite things about Melibe is that when you pull them out of the water and smell them, they have the undeniable scent of watermelon.  It’s not even an “essence” of watermelon or a “hint” of watermelon.  It’s like chewing Bubbalicious watermelon gum. Oddly, this smell at least in part results from the production of 2,6-dimethyl-5-heptenal and 2,6-dimethyl-5-heptonic acid, two chemicals the organism produces for defense purposes. I’m not sure why smelling like Bubbalicious would deter predators from gobbling you up, but apparently predators of the marine environment have different tastes than I do.

Photo of Eliza's Melibe leonina Halloween costume

My Halloween costume…

Clip art of rat
Sites and Critters

Rats in the intertidal

On many occasions, I see rats scurrying through riprap habitat in the intertidal. It’s kind of gross, particularly when you’re climbing through these boulder fields to get to your dive sites. Maybe it is because of this “gross” factor that I don’t usually mention rat sightings when I’m talking about what I do.  But the fact is that rats have been an undeniable part of urban ecosystems throughout human history.  In the intertidal riprap, they form extensive networks of burrows that I suspect are relatively undisturbed by human activities. What do they eat in their little waterfront colonies?  Is it possible that they rely on food resources from the marine environment?

Food web diagram from Kurle et al. (2008)

Food web from Kurle et al. (2008) of rats in the Aleutian Islands. Rats reduced seabird populations, which reduced predation pressure on grazing intertidal invertebrates, decreasing the abundance of algae.

I should say that while I’ve been seeing rats in the intertidal in Seattle for some time, it didn’t really occur to me to ask these questions until a recent conversation with Dr. Bob Paine at the University of Washington. He has been taking note of their presence in the intertidal for some time, and it was based on his suggestion that I began to consider rats as potential predators in urban marine ecosystems.

Indeed, there is some precedent for considering the role of rats in marine ecosystems. In 1993, Sergio Navarrete and Juan Castilla at Estación Costera de Investigaciones Marinas published a paper documenting predation by invasive Norway rats (Rattus norvegicus) on intertidal invertebrates in a marine reserve (link to Navarrete and Castilla 1993). They found that rats were particularly keen on keyhole limpets and crabs. In a more recent paper, Carolyn Kurle and colleagues documented the effects of predation by Norway rats on seabird eggs and chicks in the Aleutian Islands (link to Kurle et al. 2008).  Because seabirds feed on intertidal invertebrates, the presence of rats facilitated greater abundances of invertebrates and less dominance by algae, which invertebrate grazers controlled.

While I don’t see a lot of evidence of seabirds nesting in urban settings, the abundance of rats in the intertidal raises the question of how they may be impacting intertidal communities. Limpets and crabs are certainly potential prey items here, as are mussels, seaweeds, and a variety of other types of species.  Of course, it is possible that the plethora of food options provided to city rats by humans make intertidal organisms a minimal component of their diets despite how much time some of them may spend in intertidal habitats. For now, we can’t be sure.  Time permitting, it’s something I’d like to look at in future work.


This Week In The Lab

Urchins on hunger strike

The results are in.  After several months of tethering urchins and measuring their feeding rates, it seems that we can now conclusively say that tethered urchins go on hunger strike. Given that the whole operation was rather comical (try putting urchins on leashes, building little urchin boxing rings, and feeling normal!), I am tempted to present this conclusion jokingly and without the context you probably need to understand why it matters.  The bottom line is that I am still without a means of quantifying the effect of urchins on urban marine ecosystems in the field. So that’s unfortunate.

Tethered urchin

Tethered urchin

A huge thanks to the folks at MaST aquarium for letting me set up kiddie pools in on their dock and use their flow through seawater system. Throughout August and September, I used these tanks to test whether tethering impacted the way that urchins feed. Urchins exhibit an extremely patchy distribution in urban marine ecosystems. When they are present, the algal community appears to be considerably different, with less foliose red algae and a different suite of sessile invertebrates. My overarching question is whether urchins alter the community structure on rocky habitats when they are present in urban marine ecosystems, or whether these differences are the result of some other process. In order to do this, I ideally would conduct a transplant experiment, moving urchins to sites where they currently are absent and measuring any changes in community structure that result.  But pilot studies demonstrated that transplanted urchins are not easy to keep track of – they move away from transplant sites quickly, often disappearing into deep crevices between the rocks.  If they don’t stay on experimental plots where they’re transplanted, I can’t effectively quantify their effect.  Tethering was the last of several attempts to contain the urchins within experimental plots and would only have been effective if they continued to feed once tethered. Since they did not continue to feed, we can rule it out as an approach for measuring the impact of urchin feeding in the field.

What was striking about the results of the experiment was that the differences in feeding rates between tethered and non-tethered urchins were consistently so significant. I conducted the experiment with three different types of algae: Ulva sp. (fleshy green algae), Chondracanthus exaperatus (a red alga known as Turkish towel), and Laminaria saccharina (“sugar kelp”). Urchins that had not been tethered consistently ate 2-3 grams of algae per day, while tethered urchins ate less than a gram or nothing at all. Statistically, this led to highly significant differences in feeding rates.  Before-and-after weights of algae in stalls with tethered urchins did not, on the other hand, differ significantly from empty stalls where urchins were left out as an experimental control.

For now, I’m taking some time to regroup and reconsider how we can test the effect of urchins on algal communities. It is an issue that we’d love to understand better, particularly since urchins play such an important role in temperate marine ecosystems in less urbanized environments. The lesson of the day is that designing effective field experiments can be more challenging than one might expect. We’ll keep working at it though, and will let you know develops.