Thursday, March 19, 2015

Party Animals

Starting around this time every year, thousands flock to Florida's beaches with only one thing on their minds. They come from all along the Atlantic and the Gulf in droves to spend a few days getting wild with others they've barely met. That's right it's time for Spring B....eginning of loggerhead sea turtle nesting season.

What did you think I was talking about?
Courtesy: Daytona Spring Break via Flickr

Well actually spring break and loggerhead (Caretta caretta) nesting season don't match up perfectly. Thank god, can you imagine what kind of stuff a bunch of drunk bros would get up to with hundreds of sea turtles? Only the very first nesters come in during April, and the breeding season continues through the summer. Around the world's sub-tropical regions loggerheads return to the beaches they were born at to pass on their genes. The Southeast coast of the US hosts more loggerhead sea turtles than most other breeding regions around the world. Florida is especially important to these turtles. Ninety percent of all loggerheads born in the US nest in Florida.

Spring breakers and sea turtles have other things in common besides an intense urge to mate. They both have many stepped journeys that take them to the beaches of the southeast. After birth, young turtles head out to the open ocean and usually spend their first 5-7 years out at sea. It may seem a little odd for baby turtles to head out to the open ocean since there aren't a lot of places to hide, but there are far fewer predators out there. Plus the Atlantic has a unique region called the Sargasso sea which is named for its abundant floating fields of Sargassum (pronounced: sarj-ass-um) algae. Sargassum is perfect for little turtles because it's about the same color as they are and has a lot of spatial complexity for them to hide in. In these floating prairies, young loggerheads can grow fat on a diet of just about anything. Unlike other toddlers they aren't particularly picky and chow down on jellies, squid, shrimp, crabs, clams (even Tridacna), and fish. As they grow loggerheads rely on increasingly hearty prey so, like a college student raiding your fridge when he/she comes home for the summer, the turtles return to more abundant coastal waters to feed.

"Mom! Mom! You need to run to Costco again!"
Courtesy: rosepetal236 via Flickr

But how do the turtles find their way? Human college students have the benefit of Google maps and Expedia, but loggerheads' flippers make it hard to type. Instead sea turtles use the magnetic field of the earth to navigate. To understand how this works we need to know a little bit about the magnetic field itself.

Beneath the planet's mantle, but above the very center is a region called the outer core. The outer core is made of liquid iron mixtures and it moves around in loops thanks to the heat put out by the inner core. The conductive nature of the iron ensures that as the mixture moves around it generates electrical current, which in turn creates a magnetic field. The planet's magnetic field varies in several ways across the surface of the earth. It has an angle relative to the center of the planet that becomes steeper the closer you get to the poles. The magnetic field also varies in its strength changing along seemingly more random lines (It's weakest around South America and strongest around Siberia and the ocean south of Australia). If you can somehow detect these different lines of magnetism (called isolines) you can use them to figure out where you are on the planet, just like you would use lines of latitude and longitude.

"Stop asking me to pull over, I know exactly where we are."
Courtesy: Wendell Reed via Flickr

Amazingly scientists have enough behavioral evidence to confidently say that turtles navigate by detecting the earth's magnetic field, but don't have enough evidence to figure out exactly how they do it. The problem is it's easy to do experiments and scan data to see where turtles are going, but it's really hard to find possibly microscopic structures that sense magnetism. Perhaps the most compelling way animals might detect isolines is by having evenly spaced crystals of magnetite in their bodies. Magnetite is an iron mineral that has magnetic north and south poles, just like man made magnets. If a series of magnetite crystals is laid out in a line then they'll push away from each other when they're turned perpendicular to a magnetic field, but pull towards one another if they're lined up with it. Magnetite has been found in salmon, several species of birds, and sea turtles, all of which are highly migratory. Scientists think these animals might be able to gauge how much the crystals are pulling or pushing on one another to figure out how isolines are oriented.

Orienting themselves by magnetic fields is so important to loggerheads that they familiarize themselves with the unique magnetic coordinates of their home beach as they hatch. Then as the turtles return, if the isolines have shifted which they often do, the turtles will dig nests away from their birth place.

Despite their incredible abilities loggerheads are an endangered species. Their biggest threat is coastal development and negative interactions with humans since they prefer the same beaches we do. So for anyone out there planning a trip to Florida this spring break; remember that loggerhead turtles are there to have a good time too. If we treat each other with respect then humans and turtles can all have as much fun as this kid:


Brothers, J. Roger, & Lohmann, Kenneth J., "Evidence for Geomagnetic Imprinting and Magnetic Navigation in the Natal Homing of Sea Turtles", Current Biology, Vol 25 Issue 3, 2015, DOI 10.1016/j.cub.2014.12.035. Accessed via:

Lohmann, Kenneth J. & Johnson, Sonke, "The Neurobiology of Magnetoreception in Vertebrate Animals", Trends in Neuroscience, No. 23, 2000. Accessed via:

The Earth's Magnetic Field, University of North Carolina's Oceanweb. Accessed via:

"Caretta caretta, Loggerhead Sea Turtle"
The Encyclopedia of Life
Accessed via:

Saturday, March 7, 2015

A Clam for Ken Kesey

"That's a CLAM!?" This reaction to a particular animal is almost guaranteed when people look at them for the first time. Check it out.

Oh man, who just leaves a tie-dye shirt laying around on a coral reef?
Courtesy: Nick Hobgood via Flickr

Okay first off, that looks nothing like what belongs in my chowder. Second, this animal just goes to prove that the 60's got to everyone. So where does counter-culture clam come from and why's it so psychedelic? Editor's Note: This post is greatly improved when accompanied by Strawberry Alarm Clock's: Incense and Peppermints, or your own favorite psychedelic rock song.

Well what you're looking at is a Tridacna (pronounced: Tri-dack-na) clam. They're more commonly called giant clams, but I don't really like that name because not all of them are giant. In fact one species, Tridacna maxima, has the common name "small giant clam"; that's just silly so were gonna stick with their scientific name for this post. Anyway Tridacna don't look much like their relatives, but all the pieces are still there. They have heavy rippled shells like their cousins the cockles, and the part that you see sticking out is their mantle (the body of a mollusk) and their siphons (The channels clams use to feed and breathe). These combined anatomical parts are what make up the "meat" of the clam that we eat.

Plankton filled water flows into the hole on the right, and strained 
water flows out of the tube on the left.
Courtesy edgeplot via Flickr

Tridacna clams live on coral reefs in the tropical Western Pacific. Once they settle out from the plankton they spend their entire lives in that single spot. They don't dig into the ground, but instead let it all hang out. Truly these bivalves (animals with two shells) have gotten the hippy lifestyle down pat.

In fact it turns out that Tridacna's trippy colors and patterns are essential to its survival. The reason Tridacna don't dig into the sand is that they're part of a self sustaining commune. Just like the stony corals around them; these mollusks host symbiotic algae under their skin. In order for the algae to photosynthesize they need to be exposed to the sun. The hinge of the clam's shell is heavier than the opening, so it can tilt face up and spend the daylight hours with its skin spread out in the light. The algae get protection from consumers, and the clam gets nutrients without having to feed. Having a backup way to get your food is especially helpful on coral reefs because the water around them is usually lacking in plankton. Completely clear water is great for snorklers' ability to see, but not so great for filter feeders' ability to eat.

"Come on baby light my zooxanthellae" -Jim Molluskson
Courtesy: Eric Johnson via the NOAA Photo Library

Now as anyone who's forgotten sun screen on a tropical vacation can attest, the sun at the equator is incredibly strong. Solar radiation in the middle of the day is so intense that photosynthesis can actually decrease as the clam's symbiotic algae try to protect themselves from sun burn. But this is a collective man, and the clam does its part to help the algae function efficiently.

The algal cells under the clam's skin are arranged in stacked towers, which is confusing because that means the cells on top shade out those below them. To counter this Tridacna have their own cool cells called iridiocytes (pronounced: ear-id-ee-oh-sites) which bend light in different directions. The iridiocytes reflect yellow and green light (which aren't used by the algae) away from the stacks, and reflect blue and red light (which are useful) towards them. Essentially the iridiocytes screen out the best light for the algae, soften its intensity, and evenly distribute it across the stacks of cells.

The combination of colorful algae and reflected light come together to create the mind expanding visual experience of looking at a Tridacna's skin. Cameras can't really capture how magnificently colored these animals are, so I really encourage you all to take a trip to your local aquarium and see them for yourselves. But here's another picture to tide you over 'til then.

  Couretsy: Nick Hobgood via Flickr


Holt et al., "Photsymbiotic giant clams are transformers of solar flux", Journal of the Royal Society Interface, Oct. 2014, DOI 10.1098/rsif.2014.0678 Accessed via:

Soo, Pamela & Todd, Peter A., "The behaviour of giant clams (Bivalvia: Cardiidae: Tridacninae)", Marine Biology, 2014, DOI 10.1007/s00227-014-2545-0 Accessed via: