Peter Gawne, Belize Expedition

Randi and I have now been looking into the diets of reef fish for some time now. Most of our dives are spent with our noses in the reef, not really paying attention to what's happening over our heads.

This particular dive was too deep for us to get two full sets of data. When working underwater, the deeper one goes the less time that you can spend down there. It's a complicated physiological process that involves the accumulation of tiny nitrogen bubbles in the bloodstream, but it will suffice to say that we did not have time to record another data set. On this dive we knew that we would have a little time to lift our heads out of the reef, take in some of the sights, and hopefully snap off a few pictures.


Thousands of Creole wrasses engaged in an elaborate mating dance over our heads.

On a peninsula of reef, extending up from the sand channel, we found Creole wrasses (Clepticus parrae) in the thousands. In the days surrounding a full moon Creole wrasses gather in huge numbers in order to spawn. These small fish aggregate in a spawning rush in order to play nature's numbers game. Creole wrasses spawn in large schools, all at once, so that a huge number of fertilized eggs are in the water at once. Predators, disease, and lack of food are but a few of the obstacles facing the young Creole wrasses on their journey toward adulthood. If even a fraction of these fertilized eggs survive, the Creole wrasses have accomplished their mission.

Atop our thin finger of reef, one of these obstacles is very apparent. The spawning rush attracts predators of all shapes and sizes. Egg-predators, such as damselfish, are drawn from the cover of the reef in order to feed on the plentiful eggs suspended in the water. The Creole wrasses stick to their numbers gamble, and make no attempt to protect their spawn or to deter the egg-predators.


Large, fast-moving horse-eye jacks patrolled the periphery of the reef.

Egg-eaters are not the only predators to join the party. Permit, groupers, cero mackerel, horse-eye jacks, barracuda and skipjack tuna all patrol the blue water surrounding the peninsula. These large predatory fish are present in great numbers, presumably to take advantage of the many fish that have been drawn into the open by the prospect of a free meal.


Two large tiger groupers hold in the distance as horse-eye jacks encircle the reef.

Large in both size and numbers on this particular dive, it was unusual to see so many groupers. Tiger, black and yellowfin groupers all made an appearance. These animals have been hunted extensively as their firm white flesh and great size makes them especially prized by fishermen. One of the groupers on this dive allowed Randi and I to get close enough to see an old wound on its head, probably from an unsuccessful spear-diver.

This dive has been one of the highlights of our trip. It has proven difficult for me to capture the scale of Creole wrasse spawning, so I hope the pictures provide some glimpse into the magnitude of the event. It was very nice to take a quick break in data collection to just sit back and enjoy the show that nature sometimes puts on.

-Pete

Randi Rotjan, Belize Expedition

Pete and I are not slogging through the mud in the mangroves like Dan and Alex. Instead, we face different challenges in the lab and on the reef. Every morning, we rush to check our fish feeding experiments, siphon, replace food cubes, and do whatever else needs doing in the wet lab. We then prep our data sheets and decide on the dive agenda for the day. Then, it's dive-dive-dive all day long, followed by a night of sample processing, microscope work, data transcription. Midnight comes, we crash, and then start again.

Pete in the field; Randi at the scope.

The fish could save us a lot of trouble if they could just talk to us directly (though then we might be out of a job!). All of our effort is aimed towards a single goal: why are fish eating what they are eating? Simple question, perhaps, but it’s important. Together with collaborators Dr. Michael Berumen (KAUST, Saudi Arabia) and Dr. David Raubenheimer (Massey University, New Zealand), we're studying some of the most important and charismatic fish on the reef -- parrotfish and butterflyfish. It is a deceptively simple question to ask why these fish are eating what they are eating, but we first have to figure out what they're eating. To that end, we follow fish around the reef, while counting the number of bites and recording the prey items. Then, we collect samples to bring home for nutritional analysis.

Chaetodon capistratus butterflyfish (L) and Sparisoma viride parrotfish (R)

But that's not enough – are the fish really eating what we think they are eating? It may look to us, for example, like a fish took a bite of macroalgae. But remember that coral reefs are among the most complicated ecosystems on the planet and algae is NOT just algae… it can be encrusted with sponges and hydroids, covered with epiphytes, or be host to lots of tiny crustaceans and other organisms that find refuge in the dense foliage. So, did the fish eat algae, or something ON the algae? The mystery continues.

A forest of Turbinaria macroalgae on a calm day

Next step: bring the fish into the lab and offer them more isolated food choices. This algae or that algae? With sediment or without? With extra protein? Or perhaps the fish prefer a fatty diet? Lean? Getting closer – fish make very deliberate decisions that we dutifully record in our lab notebook.

Sparisoma aurofrenatum parrotfish choosing different diets (cubes on chains) in a tank

But, what are the fish actually digesting? To answer that question, dear readers, we carefully collect fish excrement, place it in tiny vials, examine it under the microscope, and then preserve it to bring home for nutritional analysis. We will then do some basic arithmetic: food in - food out = food digested.

A photo of the lab at Carrie Bow

And just when you think you finally have the answer.... some smart blog reader will ask: Do juveniles and adults have the same diets? What if the preferred food type isn't available? Do fish in Belize have the same preferences as the same fish species elsewhere in the Caribbean? Do different fish species prefer different food types? Does food preference change with season? Reproductive period? Lunar cycle? What if food quality changes? What if fish density changes and competition regulates access to preferred prey? Does metabolic need change in higher temperatures? Lower temperatures? Etc etc etc.

The Belizean barrier reef

If fish could talk, I'm sure they'd laugh at us (after all, they probably know the answers!). Then again, they might also tell fish tales seeded only with a grain of truth. Thus, we are trying to find clever, indirect ways to understand how coral reef ecosystems function in a natural environment, and how reefs will change in changing environments. We'll keep you posted, but it's back to the lab for now since the fish are still silent. That's okay though, in the end, data speak louder than words. :-)

Guest post by Alexander J. Forde, University of Maryland, from the Belize Expedition

It is hot. I wipe droplets of sweat from my forehead and squint as I feel the burn of sunscreen in the corners of my eyes. I am standing knee deep in clear water surrounded by tiny mangrove trees about a meter high. In the distance, large mangroves rise up and block my view of the ocean, obscuring the fact that I am on a small, remote island in the Caribbean.

As I trudge across the flooded peat swamp, I am distracted for a moment by the "chirp chirp, tweet tweet" of the Yucatan Vireo, a gray bird about the shape and size of a robin. As usual, the bird is invisible in the foliage and my weak attempts to coax it out with my own rendition of its call are unsuccessful. I continue my shuffle across the soft peat and, off to my left, an entire school of fry suddenly leap out of the water, having been disturbed by a hungry pack of larger fish. Later, I hear a huge splash followed by the screaming and flapping of birds, which I chalk up to the foraging of a crocodile (as mentioned in Daniel's previous post).


Vireo

Most people could guess that the latter two sounds were death-knells of predation, but few would associate bird song with mortal attacks on prey. Interestingly, it is exactly the association between birds and predation that is motivating my work on the island. Predation by birds can have very important effects on herbivorous arthropods, such as driving the evolution of warning colors, or decreasing damage to plants and consequently increasing plant productivity.

The mangrove forest is quiet when I reach my destination: a pair of cubic frames made out of PVC pipe that are built around small mangrove trees. One frame has netting over it to keep out birds and the other has no net so that the birds can enter. I measure how much the trees have grown since the frames were built and record the arthropods and damage present on the leaves. I notice serpentine leaf-damage, which indicates the presence of a tiny leaf-mining caterpillar, irregularly shaped areas scraped by tree-crabs, and circular areas chewed by free-living caterpillars. The tree inside the frame with netting even has a borehole into one of its branches through which a wood-eating larva recently entered.



Frames

With help from my collaborators I had established 20 pairs of these frames on the island 3 months previously. This simple experimental design involving bird "exclosures" has a rich history of detecting long-term impacts of bird predation in other habitats and after this study is completed, it will help us understand the ecosystem function of birds in mangroves. If birds do have strong ecological effects on arthropod assemblages and on mangrove growth, then they will need to receive more attention from land managers who seek to conserve and restore mangrove forests.

As I set out in the motorboat that takes me from the Carrie Bow field station to my field site, I make a mental check list of activities for the day. I have finished surveying the bird-exclusion frames for this trip, so now I can shift my attention to side-projects. Today all I need are plastic bags, water and a lunch, because I am going to be collecting the pupal cases of a caterpillar that eats mangrove leaves. The last time I was on the island, I found that all sorts of insects invade these cases, sometimes killing the developing moth inside and sometimes setting up shop after the moth has already left.

I tie up the boat to the trunk of a mangrove on the edge of the forest and work my way through the dense stand of large trees that extends along the exterior periphery of the island. I finally enter the more open central domain of the island with a few new scratches on my arms and legs, and begin to search the undersides of twigs for pupal cases. As I scan for suspicious irregularities on the branches from a distance I notice a bump on one nearby branch. I walk over to get a closer look and, sure enough, I have found one of my targets. The oblong case is 2cm in length and is rounded, with the exception of the flattened trap door at the top, which allows the adult moth to emerge. I carefully remove the case from the branch with some difficulty, as the adhesive silk that the caterpillars use to anchor themselves to the branches is extremely tough and strong. Into a bag the case goes, and later I will dissect it and try to reconstruct the history of invasion that has taken place.


Pupae

I am concerned with these pupae because it is possible that the population dynamics of certain mangrove herbivores are affected to an equal or greater extent by invertebrate predation, compared to bird predation. Pupal dissections have revealed that the pupae are often attacked by parasitoids and ants, so invertebrate predation is likely to be important for this species. My goal is to quantify the relative effects of different predatory taxa on this focal herbivore, and to test whether the extreme patchiness of its spatial and temporal distribution results from the effects of one or all of its natural enemies. Predation on this species may be important for mangrove productivity, as the caterpillars can remove large amounts of biomass during periodic outbreaks.

Motoring away from the mangrove island for the last time on this expedition, I look back at the edge of the mangrove forest. I wonder to myself what I will find waiting for me in the bird-exclusion frames the next time I return, and I imagine all of the interactions between individual predators and prey that I will never see in person, but that will be collectively captured in statistical patterns in my data. I feel happy that I will be able to return to this fascinating place soon, and that, one day, the bug-bites, scratches, stinging sunscreen-tainted eyes, and sunburns that I acquired as part of my work in the forest will have been suffered for a worthwhile purpose: helping communities better understand and preserve beautiful and beneficial Caribbean mangrove ecosystems.

-Alexander

Guest post by Dr. Daniel Gruner, University of Maryland, from the Belize Expedition

This week, along with graduate student Alexander Forde, I found myself working in a new system: the tropical mangrove cays off the coast of Belize. Ordinarily I study the ecology of a range of organisms (plants, nematodes, insects, birds ...) on dry land. Mangrove forests differ from most forests in that the trees thrive with their toes constantly wet, usually in salt water that is toxic to most land plants. So, Alex and I would need to adapt to perpetually submerged toes--and legs, and most everything else--to study this ecosystem.

Probably derived from the roots "mangle" and "grove," this name is apt. These forests are a tangle of aerial roots and branches, peat and mud that make them challenging to navigate. Hip waders would be hopelessly hot and restrictive, so we simply splashed into the swamp wearing only surf booties on our feet, our typical terrestrial field clothes and gear and plenty of sunscreen. In the murky waters it can be difficult to judge the depth of the muddy bottom with each step. Many times we found ourselves sinking to the waist (or deeper!) before climbing out the other side. At first tentative, we quickly overcame any hesitation to perfect a shuffling tap dance, first testing the solidity of the substrate with a toe tap before plunging forward. We were soon making rapid progress in our explorations through this fascinating habitat.

Peter Gawne, Belize Expedition

Every moment of every day on this island seems to be a race against the clock. We rise with the sun and quickly get to work. While the days are long, they are not without limit and there is much work to do before the sun goes down.

Time in the field is an extremely limited resource for scientists. The daylight hours are spent in parts unknown. Samples must be gathered. Measurements must be taken. All of the data that is going to sustain a scientist's work is to be gathered in the field. Later there will be time, albeit limited, to crunch numbers, analyze samples and work toward the eventual goal of publication (as mentioned in post #2).

When working underwater the issue of time is compounded. Beneath the surface, time is measured in minutes rather than hours. There is a constant struggle between air--supply, decompression limits and the work that must be done. Each failed attempt to capture a fish eats precious minutes that will never be recovered.