Egg masses

Q: What do you do when you find yourself in a large, locked room with 29 other guys/girls and an abundant but finite amount of food?

A: Make plenty of babies, of course!

(This makes perfect sense if you are a 4 mm long snail)


I took down my herbivory experiment today and re-weighed all the algae after 4 days of snail grazing. It took about 10 hours in total. But other than grazing on the algae, the Lacuna have been busy making little Lacunas. There were egg masses in all of the snail mesocosms, and most of the algae had at least one egg mass attached. (This made my job a little more difficult because I had to remove the masses before weighing the algae.)

Lacuna eggs on the edge of a sugar kelp (Saccharina latissima) blade

Eggs on the branch of an articulated coralline alga, Corallina officinalis

Also, if you're not a big fan of the algae, you can lay your eggs on your neighbour's back instead. That's what friends with hard shells are for.

:)

Things that make my life easier

One of the projects I'm working on in this last month of summer is a repeat of an intertidal experiment we ran last year, with an additional control treatment. In our lab's various studies of rocky intertidal communities, a general method is to conduct regular surveys of the number and abundances of algae and invertebrates like snails.

Two of the most abundant intertidal herbivorous snails. The little snail is Littorina obtusata, the smooth periwinkle (and my favourite intertidal snail!) and the larger snail is Littorina littorea, the common periwinkle.

Survey method: 1. Place PVC quadrat on spot; 2. Record all algae and invertebrates found in quadrat

For field experiments, we set up permanent plot using bolts and washers so we can survey the same plot and track changes over time. However, the markers can be frustratingly hard to find again under all the algae, even if you know where the plot is.

One of my plot markers from last summer. When the tide comes in and out it vanishes under a canopy of algae.

So, the thing that has made my life better: pretty, eye-catching fluorescent zip-ties on the plot market bolts! They significantly reduce search time and make it possible to find and survey 40 plots in a low tide.

Bright yellow zip-tie tag on one corner of a permanent quadrat, labelled washer on the other.

They are so visible among the algae!! You can see more in the background...

Fun with herbivory

This blog has been awfully quiet as of late, mostly because I've been insanely busy. I've been back in Nahant for about two weeks now and am working hard to get all the planned research for my algae project done before class starts on the 7th of September and I need to be back in Providence. I've also been helping out with other projects in the lab. But I just got a big experiment up and running, so things will be a little less hectic for a couple of days - mostly simple maintenance and checking up on it.

When I last wrote about the algae project I talked about the big question of "what is it doing here?" We're trying to figure out how the invader is impacting the ecological community. In any introductory ecology class you learn that two of the main interactions between species are consumption (e.g. herbivory, predation) and competition (for space, light, food). My experiment is looking at the first interaction and asking, do the native herbivores in the community eat the invader? How much of it do they eat in comparison to other native algal species? Could this impact growth and survival of the invader relative to other species?

In this case, the herbivore is a tiny little snail, Lacuna vincta, that can occur in relatively high densities on the algae. It is found mostly at shallow subtidal depths, though I have seen it on algae in the low intertidal zone.

Here it is on a ruler. awww.

To see if it has significant preferences for the invader or for the native algae, I put a bunch of Lacuna in a little snail mesocosm (=a food container with holes drilled and mesh glue gunned on) and offer it a choice of the invader, plus five other common algal species. At the same time, I have other snails in other mesocosms that have only one species of alga, so I can compare how much of each species they eat when they have 6 choices vs. no choice over about 3 days.

Multiple-choice mesocosm. There are 6 species in each chamber, and the one on the left has 30 Lacuna snails. The one on the right is a control for any loss/gain of algal mass not due to herbivory.

I have 10 replicates of everything, so 10 x 6 species for single-choice + 10 multiple choice = 70 mesocosms. This means 240 pieces of algae which had to be collected and individually portioned/weighed and 2100 snails which had to be collected and counted. You can see why I've been busy.

Hours and hours of collecting and sorting snails...

Final experimental setup on two seawater tables at the MSC:

On Leslie matrices and ceviche

In the introductory ecology class I TA'd for last spring, the Leslie matrix is the bane of many students' lives. Many students come in expecting more saving the earth and/or charismatic megafauna and less mathematical models. About a month into the class, they are introduced to this:
which can be understood a little better by this, where each circle is one size class (1 is the smallest, 4 the biggest). P1, P2 and P3 are the probabilities of advancing (growing up) into the next class over a set time period and F1, F2, F3 and F4 are the probabilities of reproducing (making more little class 1's) for each respective size class. The number of individuals in each class after the time period is dependent on the number of individuals before the time period and all these probabilities - that relationship is described by the matrix equation.


So what does this look like in real life?

Let's say you have a snail about the size of your palm that is known to be a key predator of barnacles - this is important because barnacles act an 'entry point' of energy from the open-ocean (pelagic) to the ecological community that lives on/around the rocks and ledges (benthic). Barnacles eat plankton from the water column and use that energy to grow, making that energy available to predators like Hexaplex snails, which in turn get eaten by fish, etc. So there is a transfer of energy from the pelagic plankton to animals like fish which wouldn't happen without barnacles and their predators.

Hexaplex feeding on Megabalanus barnacles at about 8m depth.

At some sites in the Galapagos archipelago, Hexaplex is also fished to make the local dish ceviche. Presumably, the bigger snails are fished (this can be confirmed using fishing records and/or data from shell piles). How does that affect the way the population changes (grows/shrinks) at different sites? And from a bigger-picture view, how could fishing impact the flow of energy into subtidal Galapagos communities? If you can estimate the parameters of the model accurately enough, you can use the model to answer these questions. You could also model changes in the intensity of fishing to predict how the population would respond - information that can be useful if you'd like to set fishing limits or decide which population(s) are more worth protecting.

Leslie (who is very appropriately named for this research project) has been setting up to estimate class-specific Hexaplex growth rates (the P's) by measuring, tagging and releasing them at our study site. When we get back in January, we'll be looking out for these tagged snails and re-measuring them to track their growth.

Photographic record of size and shell morphology.

Newly freed Hexaplex at Isla Baltra. The bright green rock is the release point.

January is also supposed to be the reproductive season for Hexaplex, so we'll also be looking out for egg masses and laying Hexaplex in order to estimate reproductive outputs (the F's).

Hexaplex egg mass from last January, photo by JW.

Somewhat encouragingly, we recovered one of the Hexaplex that we did some preliminary tagging on in mid-June with 8-year old Z-spar epoxy. I think we only tagged 8 or 9 individuals then, so a return of one is pretty good news. We'll see what happens when we return to Baltra in 6 months.

Happy Leslie with the recaptured Hexaplex.