Anatomy of a tuna's internal compass

Southern bluefin tuna. (Image from http://www.thewe.cc)

NOTE: This post was originally published in August of 2009, it was one of the first few research papers that I wrote about on this site; it’s been receiving a spike in hits due to the recent announcement of a proposed ban on bluefin tuna fishing. This post does not talk about the conservation issues, but I wanted to revisit the fact that this species offers us a chance to learn more about wide-ranging marine species ability to circumnavigate unbelievable distances, and the fact that we still have many lessons to learn about the diversity of life on earth from rare and imperiled wildlife.

I was very intrigued by a research brief that recently ran in New Scientist magazine on the cause for daily deep-diving behavior in southern bluefin tuna. According to the un-bylined brief, these top-predator fish descend deeply like clock-work, penetrating the depths at almost exactly 30 minutes before sunrise and again 30 minutes after sunset. Apparently scientists differ over the reasons behind this behavior. Some say that the tuna make rapid descents to keep track of their prey. Others suggest diving rids the fish of pesky parasites. But a recent study interprets their behavior more complexly. A researcher from the University of Tasmania in Australia is lead author to a paper published in Behavioral Ecology and Sociobiology that concludes the tuna dive regularly to calibrate and fine-tune their internal compasses at the precise time of day when they can get the clearest reading possible.

Given my interest in learning more about magnetoreception and geo-mediated navigation in a spectrum of animals, I just had to dig deeper into this story. So I downloaded the paper and read it.

Jay Willis and his team of four additional researchers studied archival tag data of 21 juvenile bluefin tunas during a migratory period in the Great Australian Bight spanning 5,000 t0 16,000 kilometers. They examined the timing and profile of their daily “spike dives,” as well as characterizing the magnetic field in their environment.They used surgically implanted tags that store information on four minute intervals about light, depth, water temperature and internal body temperature; data was collected for individual tuna over time spans from 135 to 494 days.

The authors wrote:

Our analysis shows that spike dives are a distinct and habitual behavior for juvenile SBT with a precise association with dawn and dusk. The shape and timing in relationship to the physical environment led us to suggest a number of new hypotheses about the navigational system of juvenile tuna that we hope will lead to productive avenues for future research.

The team also analyzed a light-mediated organ present in the fish and known to be related to light reception, and to navigation in other vertebrates. It appears as a somewhat translucent area between the tuna’s large eyes that allows light to penetrate through a cartilaginous tube to a half-spherical-shaped lens-like organ called a “pineal gland” which lays just in front of their brain. The team dissected the heads of 20 bluefin tuna in the field to examine this organ, and they analyzed an MRI scan of a related albacore tuna too. They found that light bounces through the interior tube but does not reach surrounding tissues external to the tube. And they found that light waves are best aligned to fully penetrate the tube and wash the pineal gland in light when the fish is ascending at a 35-degree angle from vertical.

Characterizing the dive profile, the researchers found that the tuna’s dawn dive was faster on the ascent, and that the dusk dive was faster on the descent. Graphed, the mean dawn and dusk dive profiles mirrored each other. When the dusk profile was inverted, the shapes were similar. The dusk descent rates tended to be faster though, maximizing at 605 meters in 4 minutes, whereas the maximum dawn ascent rate was 386 meters in four minutes. (The different diving and ascending rates that varied between individuals was found to be correlated to the tuna’s size size, i.e. diving rates were constrained by an individual’s overall size.) They found that the maximum ascent rates and dawn and the maximum descent rates at dusk corresponded very nearly to 30 minutes before dawn and 30 minutes after dusk, which is when the sun elevation is -6 degrees in both cases.

Seventy-five percent of the time, the researchers were unable to distinguish the dawn and dusk spike dive profiles when data were selected for characterization at random. But they were able to distinguish the dawn-dusk spike dives from all other dives, when data were selected at random.

Drawing on previous research seeking to dissect how animals create geomagnetic navigational maps, the researchers posit that the tuna’s light-receptive pineal gland is used during the spike dives as one component that makes their internal compasses tick. The authors wrote:

Spike diving occurs close to dawn and dusk and may thus be associated with the acquisition of light-based cues for orientation or navigation; spike diving also involves time spent at considerable depth in darkness and may be used to acquire light-independent navigational information. Together, these cues could then provide the basis for map information and/or be used to calibrate multiple compasses, both of which are critical for long-distance migrants.

The authors suggest that the shallowest portions of the spike dive correspond with the times of day when the fish would get the clearest reading of “celestial polarization patterns,” which are apparently best viewed just below the water’s surface and are known to aid other animals compasses. The authors suggest that the tuna’s diving behavior allows them to both obtain cues from polarized light near the surface, and perhaps obtain a vertical column of other environmental factors (such as light-independent factors at depth, or map information) that combine to help them create and calibrate their internal navigational compasses.

The researchers acknowledge a lack of information about specific magnetic reception in tuna, though they point out that yellow fin tuna have been documented to respond to magnetic stimuli. They suggest as one possibility that blue fin tuna may be gathering information at depths related to creating or maintaining a geomagnetic map, which their light cues then help them to navigate. But they suggest alternate possibilities that the blue fin tuna navigate via olfactory or acoustic maps, electric fields, or “temperature stratification and associated gradients at depth.” They also acknowledge that if the spike dives are not related to navigation, they may be related to hunting and keeping track of prey which also migrate up and down the water column in patterns similar to the spike dives of tuna.

But it is clear from the emphasis of the paper that the author’s believe there is a strong connection between the diving behavior (mapping), the light-receptive pineal gland (compass), and the bluefin tuna’s navigation. The adult southern bluefin tunas migrate thousands of miles but return to within a few kilometers of their spawning sites to breed. So it follows that they would be strongly selected for any behavior or physical navigation mechanisms that assist them in this journey to breed, and that the best navigators pass on their genetic disposition to the next generation of southern blue fin tuna.

I, for one, hope this team follows up with more research into 1.) how these amazing tuna create their geographical or geomagnetic maps, and 2.) experimentally testing whether or not they are in fact using light mediated by their pineal glands as an internal compass.

NOTES:
{1) Based upon the original research of Jay Willis and his team.
Willis, J., Phillips, J., Muheim, R., Diego-Rasilla, F., & Hobday, A. (2009). Spike dives of juvenile southern bluefin tuna (Thunnus maccoyii): a navigational role? Behavioral Ecology and Sociobiology, 64 (1), 57-68 DOI: 10.1007/s00265-009-0818-2

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