Gambusia affinis: A Professional Invader

Photo credit: J Centavo @Flickr

I'm far away from the fishpond right now, taking a summer modeling course through Michigan State University. However, talk of some fishpond species has crossed the Pacific Ocean and reached me here. Dr. Andy Sih, a behavioral ecologist from The University of California Davis is visiting the campus and just gave a talk on "behavioral syndromes" (a technical name for personalities) and their role in mate choice, population dispersal, and invasion of new habitats. Interestingly, one of the animals his lab has used to study behavioral effects on invasion is Gambusia affinis, the western mosquitofish, which is invasive to Hawaii and very plentiful in the fishpond. According to the Global Invasive Species Database, G. affinis is one of the Top 100 Invasive Pests in the world. Due to its generalist diet, it competes with other fish for resources and often also eats the eggs of other fish (including economically desirable ones, though it's unknown whether it eats awa'awa or moi eggs). The interesting thing Dr. Sih said about G. affinis, though, is that among the members of the Gambusia genus, G. affinis is uniquely invasive and aggressive. Its feeding voracity contributes to its ability to spread, establish in new locations, and have heavy impacts on the local species once it establishes. This makes it a good study organism for behavioral syndromes that may be characteristic of invasive animal species. Dr. Sih's research suggests that behavioral syndromes such as feeding voracity may make some species (and some individuals within species) better invaders.

Isolating MPB from sediment: taking the new protocol for a spin

The final step: a glass fiber filter containing cleaned, filtered MPB ready for stable isotope analysis.

This week I tried a new diatom separation protocol. This may not sound exciting, but visually, it is. The MPB collected from the pond is a foamy, brown layer of detritus, diatoms, cyanobacteria, microbes, and even some tiny invertebrates mixed with very fine sediment (See "Mystery of the Fuzz," May 31 2010). Sediments and detritus, however, can contribute to noise in measurements of carbon isotopic composition (because they too contain carbon, sometimes with very different signatures than whatever they're mixed with). SI analysis also requires a certain weight of the compound of interest, and having sediment mixed in with a sample will also make it heavier with material that is not important.
Because of these two issues, people have come up with a few different ways of separating sediment from their samples. The first protocol I'm testing employs very fine mesh filters and a high-density liquid made with silica.
Diatoms and cyanobacteria are luckily smaller than most sediment particles, so the first step is to run the sludge through a mesh to remove large pieces of sediment and little inverts. The resulting foggy water sample is filtered again through a smaller mesh which retains the diatoms, and these are in turn rinsed into a beaker with a small amount of water. Now comes the fun part: the contents of this little beaker are added to a plastic centrifuge tube, which contains a silica liquid that is denser than water. We then centrifuge the diatom/silica/small sediment mixture. Since any remaining sediment particles are heavier than the silica, they spin to the bottom.

Diatoms and cyanos, however, have a lower density than the silica, and they remain at the top of the tube. Centrifuge tube containing colloidal silica and MPB/sediment. The MPB is the bright green layer floating at the top, and there's a lump of sediment at the bottom.

We can then scoop the green-brown layer off the top of the silica, rinse it, and end up with a small, concentrated sample of MPB algae free of sediment. We'll see how these samples turn out!

Unusual (Diseased?) Crab Tissue

A side note from today's crab dissections: I came across a crab with interesting and different muscle tissue from the others. The crab was the same species as the others, Thalamita crenata, but its muscle tissue had a foul smell and was white and gooey, unlike the normal clear greyish tissue. I have no idea what this is, but I thought it was intriguing and want to share. Was it something he ate?

Crab leg with foamy white muscle tissue.

"Diseased" muscle tissue, at left, next to a normal sample.

Getting Crabby

My previous attempts to make homemade crab traps for the small crabs I'm using (Thalamita crenata) did not work. Today Hi'ilei (the executive director of Paepae o He'eia) showed me how to use crab nets to catch them. We used chopped up uku head (though awa'awa skins are the best because they have a strong smell and are easy to tie down) and tied the pieces of fish into circular crab nets. Once we dropped the nets into the water, it took about 15 minutes for the little Thalamita to start showing up. I removed each crab from the net, measured the width of its carapace, took one of its claws, and released it. Their claws grow back, and one claw contains more than enough muscle tissue for stable isotope analysis.