Playing in the sand, for science

This semester I am teaching a lab for a General Biology course for non-majors. I polled my students on the first day of lab, and their academic plans are quite varied: several want to major in psychology (always a popular major), some want to go into business, a few said they hope to go into politics or public policy, and some haven’t yet selected a field of study. I think only one or two are even considering a STEM field. Which is all just to say that I have a group of students whose academic goals don’t have much in common except to study something other than science. Several of them are the first in their families to go to college, which is very exciting for them and for me.

Most of the activities we do in this class are lab studies. Last week, for example, the students extracted DNA from a strawberry (100% success rate for my class, thank you very much) and then used puzzles and 3-dimensional models to understand the structure of DNA. We do have a couple of field trips scheduled, though, which are the days that students really look forward to. Outside the classroom is where most of the fun stuff happens.

Today I took my class to the beach! We were there to do some monitoring for LiMPETS (Long term Monitoring Program and Experiential Training for Students). For the past few years I’ve taken my Ecology students out to the intertidal to do the rocky intertidal monitoring. The General Bio students don’t have the background needed for the intertidal monitoring and I don’t have the classroom time to train them, so we take them to do sand crab monitoring instead. This is a simpler activity for the students, although the clean-up on my end is a lot more intensive even though I get them to help me.

Emerita analoga is a small anomuran crab, more closely related to hermit and porcelain crabs than to the more typical brachyuran crabs such as kelp and rock crabs. It lives in the swash zone on sandy beaches and migrates up and down the beach with the tide. Its ovoid body is perfectly shaped to burrow into the sand, which this crab does with much alacrity. The crabs use their big thoracic legs to push sand forward and burrow backwards into the sand until they are entirely covered. They feed on outgoing waves, sticking out their long second antennae (which can be almost as long as the entire body) and swivel them around to capture suspended particles.

We went out to Seacliff State Beach to count, measure, and sex sand crabs. The protocol is to lay out a 50 m transect along the beach, roughly parallel to the shore where the sand remains wet but isn’t constantly covered by waves. Students draw random numbers to determine their position along the horizontal transect and venture out into the ocean, measuring the distance between the transect and the point where they are getting wet to the knees. Then they divide that distance by 9 to yield a total of 10 evenly spaced sampling points along a line running perpendicular to the transect.

The corer is a PVC tube with a handle. It is submerged into the sand to a specified depth and collects a plug of sand that is dumped into a mesh bag. Sand is rinsed out of the bag and the crabs remain behind. Students then have to measure and sex each of the crabs.

Each crab is classified as either a recruit (carapace length ≤9 mm) or a juvenile/adult (carapace length >9 mm). Students get to use calipers to measure carapace length, which they enjoy. Adult crabs are sexed, and females are examined for the presence of eggs.

A sand crab’s sex is determined by the presence or absence of pleopods, abdominal appendages that females use to hold onto eggs. If a female is gravid, the eggs are visible as either bright orange or dull tannish masses tucked underneath the telson (see below):

The pointed structure in the photo above is the telson. You can see the tan eggs beneath the telson. They look like they would fall off, but they adhere together in a sticky mass until they are ready to be released. Adult females have pleopods whether or not they are gravid, making it easy to sex the crabs even when they are not reproductive.

Most of the larger crabs today were gravid females and could be sexed with a quick glance at the ventral surface. Sexing the smaller individuals requires a lot more effort. The crab’s telson has to be gently pulled back to expose the abdomen, which isn’t easy because the crab doesn’t like having its parts messed with. In fact, one of the ways to determine whether or not a crab playing dead is really dead is to pry up its telson–a dead crab will let you without making a fuss, while a live one will start thrashing about.

It was a good day to spend time at the beach. The weather got better as we worked and the water wasn’t very cold. The students had a good time splashing around in the waves, and they all fell in love with the crabs. There were a few sad moments when crabs got chopped in half by the edge of the corer, but the vast majority were released back to the ocean unharmed. From a teaching perspective, I was happy to give the students an opportunity to do some outdoor learning. After all, the world is our biggest and best classroom. Most students learn best when they get to actually ‘do’ science, and even though most of this group will not go on to complete a science major, they hopefully have a better appreciation of what it is like to collect real data as citizen scientists.