Friday, June 16, 2017

The elephant

"You can eat an elephant in small bites." - Ed O'Brien

This week was an elephant. A big, heavy, brute of a week, full of data collection and field work and counting and counting and counting.

Two species of ascidians on my fouling panels:
Botryllus schlosseri (black and yellow), and Botrylloides
(red). Photographed with a dissecting microscope
at 6.5x magnification.
To put it succinctly, my fouling panels in Eel Pond have been taken over by ascidians. Also known as sea squirts, they're squishy, gel-like, blob-shaped animals, and they have absolutely covered my fouling panels in Eel Pond!

Some of you might remember the "mystery blobs" I started finding about a month ago. At the time, I was pretty sure the blobs were ascidians, but I couldn't identify them to species. Well, now I can confidently tell you that the blobs are ascidians, but they are not one species. There are four. Four species of ascidians. All. Over. My. Panels.

As you can imagine, the copious ascidians took a lot of time to count. It was a mammoth task, but I'm no stranger to long hours at the microscope. I actually removed the ascidians from a sub-set of my panels to see if the fouling community would develop any differently in their absence, and as you might imagine, that took a ton of time too. I kept reminding myself to take it in small bits, to take breaks and pace myself. This week was a beast - an elephant I had to eat in small bites.

A species of solitary ascidian, Ascidiella aspersa (white
arrow), on my fouling panels. You'll notice Ascidiella
is much bigger than individuals in the colonial ascidian
next to it. Because it's so clear, most of the time I only
noticed Ascidiella on my panels because of its colorful
digestive tract.
Ascidians are fascinating creatures, though, and I was happy to see them recruiting to my panels. Ascidians have two different life-styles - some species are solitary, while some live in colonies of clones - but despite this difference, they all have the same basic anatomy. Each ascidian individual has a incurrent and excurrent siphon to draw water into and push water out of their body. The water gets filtered through a mesh structure inside the ascidian's body called the pharyngeal basket, which catches any particles that could be used for food. The outside of an ascidian's body is covered in a fleshy, skin-like layer called a tunic. They're actually pretty fun to dissect.

Eel Pond is notorious for its diverse and abundant ascidians, so their dominance on the panels actually fits one of my hypotheses. I think their high numbers on the Eel Pond panels shows the influence of local retention and larval supply. It will be interesting to see how the fouling communities continue to develop this summer!

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