A note about Thalamita crenata or "Eek! Chemistry!"

A little background on one of my study subjects. Thalamita crenata is a swimming crab and a member of the family Portunidae. Portunid crabs are named after Portunus, the Greek god of ports and harbors, because they are often found in nearshore estuarine habitats. Their final pair of legs are modified into paddles for swimming, which makes them quick predators (and also makes them really difficult to catch). If cornered, they pack a mean pinch. Their diet is what you would expect from any detritivore: they eat whatever is available. This includes microalgae, dead fish, zooplankton...even each other.

Photo credit: inuc0r0 @blogs.yahoo.co.jp

A few weeks ago, I got back tissue stable isotope data from 30 Thalamita crabs that I caught in the fishpond. Muscle tissue from this many crabs is not enough to make any sound conclusions about their feeding habits but I noticed one interesting thing. In order to describe it, however, I need to explain some chemistry.

Nitrogen isotopes are generally a signal of trophic level, or the position of an organism in the food web. Organisms at lower trophic levels, such as algae, have lower δ15N values than organisms at higher trophic levels, like shark or ulua. This is because the heavy nitrogen isotope (15N) accumulates in animal tissues, while the lighter one is excreted. In brief: when an animal eats something, it can either absorb or excrete molecules. Chemical reactions occur more often with light isotopes than heavy isotopes. The chemical reactions that are involved in excretion (transamination and deamination) are no exception: they occur far more frequently with 14N. Therefore, when the animal produces waste, 14N is removed from the "pool" of available nitrogen in the animal. What remains, and what is absorbed, is enriched in 15N, meaning there is more heavy N isotope in the animal's tissue than there was in the thing it ate (Gannes et al. 1998). The general rule is that δ15N increases by 3‰ with every trophic level.

Why the chemical digression? My description above paints a picture of a tidy trophic ladder, with producers at the bottom and a big predator at the top. Unfortunately for ecologists, food webs are almost never this simple. Here's what I noticed in the T. crenata data from the fishpond: the crabs I sampled have a large (~3-8‰) range in δ15N values. This suggests that crabs are eating at a number of different trophic levels. Since juvenile T. crenata have the same diet as adults (Cannicci et al. 1996), these differences are not likely to be due to differences in crab maturity (young crabs aren't nibbling on algae while larger ones feast on old fish carcasses).

T. crenata is an opportunistic, detritivorous cannibal. We don't see everything these crabs eat, we can only look at the chemical clues in their tissues. As you can imagine, it is hard it is to determine what exactly a crab has been snacking on if its diet and stable isotope signatures are both so variable. Questions like this come up often in stable isotope ecology, and require us to look further, either physically by looking at the animal's stomach contents*, or mathematically by using models to predict proportions of certain foods in the animal's diet (more on this later). The variability in my crab SI values tells me already that I will need some of these extra tools in order to make sense of my data. And I thought crabs were the biggest pain about working in the fishpond...


*This is a messy but often necessary process. The polite word for it in the sciences is "gut content analysis."

Gannes LZ, Martínez del Rio C, & Koch P (1998). Natural abundance variations in stable isotopes and their potential uses in animal physiological ecology. Comparative biochemistry and physiology. Part A, Molecular & integrative physiology, 119 (3), 725-37 PMID: 9683412

Big-Time Mahalos

LAIP summer interns counting and measuring seine contents.

New posts are coming soon, but in the meantime I would like to say Mahalo nui to the interns of Laulima A 'Ike Pono ("Working Together for the Collective Vision") for their help in the field this summer and fall. They have been helping me measure crab and fish abundances, catch Thalamita crabs for stable isotope analysis, and collect invasive limu and MPB. LAIP is a program created by Drs. Judy Lemus and Flo Thomas at HIMB to give students a comprehensive place-based research experience: in addition to training in contemporary oceanographic sampling techniques, LAIP interns receive a cultural and historical education on the fishpond. The LAIP interns have been awesome, and it's been an utter joy to work with them! We collected ~80 algae samples today, and next week we'll do seines for crabs and shrimp.

Native Hawai'ian seagrass in the fishpond

While looking for fish and crabs along the seawall this weekend, I found a few small plants like these growing among in cracks in the makaha. They belong to the genus Halophila, which is the only true seagrass found in the Hawai'ian Islands, and they are delicate plants usually found in sand flats. Patches of Halophila grow in other parts of Kaneohe Bay, so it's possible that fragments or seeds and pollen from these neighboring plants got caught in the seawall and took root. This seagrass is also found in completely marine waters (instead of estuarine environments, like some other seagrasses), which goes to show how much marine influence there is on the ocean side of the fishpond.

There are two species of Halophila present in Hawai'i: Halophila hawaiiana, which is native to Hawai'i, and Halophila decipiens, which is from Thailand. The only way to distinguish the two species from one another is by checking for the tiny spines that grow on the outside edge of the leaves of H. decipiens. It was hard to see these leaves very well, but it looks like this plant is H. hawaiiana: a rare endemic seagrass occupying the fishpond! This has exciting implications for pond rehabilitation: maybe if limu removal projects are successful, native plants like this can take over those niches. Seagrasses stabilize sediment and provide food for fish and turtles.

Life at the Sea Wall

Water rushing over the sea wall on a rising tide. You can see the orange sponge Mycale sp. growing on the bricks under the water.

Much of the fishpond has calm waters with low visibility. However, when the tide is rising, water rushes through the cracks in the seawall, bringing in saltwater, plankton, and small fish... and making it possible to see for more than a few feet underwater. Many filter- and suspension-feeding organisms take advantage of the high-flow environment. Fish gather here to eat plankton and other small invertebrates. Below are a few snapshots of life at the ocean break during a high tide. Many of these organisms are marine, and not found further in at the pond. Among these is the puffer fish Arothron hispidus, a species that has been mysteriously dying around the Hawai'ian Islands.

Feather duster worms (Sabellestarte spectabilis) growing among sediment-laden Gracilaria near the ocean break. These marine suspension feeders are found in areas of the pond with high flow, salty water, and access to marine plankton.

Two alien species side by side: a colony of orange sponge next to a feather duster worm.

A white-spotted puffer fish (Arothron hispidus) in the sea wall. This individual is healthy, but others have been found dead in Kaneohe Bay and other parts of the Hawai'ian Islands. Dr. Thierry Work, a wildlife disease specialist from USGS, is studying what may be a virus affecting these fish.

A portrait of life at the makaha. Featherduster worms extend their tentacles in the foreground. Convict tangs, or manini, are eating plankton brought in by the high tide, and picking microalgae from the rocks and seaweeds.

A swimming anemone (Boloceroides mcmurrichi) nestled in Gracilaria near the wall. This species can live on sand, mud, or seaweed, and can swim away from predators by paddling with its tentacles.

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.