WORMS BURROW BY CRACKING

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Churning up sediments and redistributing nutrients and organic matter, burrowing marine worms are sometimes described as ecosystem engineers. But how do these creatures move through their sticky environment? Kelly Dorgan from the University of California Berkeley explains that

Filming bull sperm swimming in circles at 250 frames s -1 through a phase contrast microscope, Riedel-Kruse accurately measured the position of the beating flagellum and sperm head within less than 1 mm. 'Of course this cannot be done accurately by hand,' explains Friedrich, so Riedel-Kruse developed accurate computer algorithms to determine the position of the flagellum.
With 1 million datapoints for the flagellum's position in hand, it was time for Friedrich to begin testing the theory. Knowing the position of the flagellum and its instantaneous speed, Friedrich plugged these values into Gray and Hancock's theory to calculate the way in which the sperm head wiggled as it moved along its circular path. Amazingly, Friedrich's calculations agreed remarkably well with the movies: his calculated sperm movements were identical to the sperm's movements in the movies.
Having used the theory to calculate the sperm's trajectory, Friedrich was also able to calculate a key parameter in Gray and Hancock's equations called the drag anisotropy ratio. According to Friedrich, whenever we move we push against something solid, like the earth, but sperm swimming in fluids have nothing solid to push against, so they have to rely on an imbalance between the friction forces that the flagellum experiences as it moves sideways and forward. The ratio of the two friction coefficients is the drag anisotropy factor and it must be greater than 1 for the sperm to move forward. Comparing his calculations with the movies of swimming sperm, Friedrich found that the perpendicular friction coefficient is 80% higher than the parallel friction coefficient, giving him a drag anisotropy factor of 1.8. Using this value, he was then able to calculate the radius of curvature of the sperm's circular path and found that it agreed perfectly with the movies. Gray and Hancock's theory had stood the test of time remarkably well.
Friedrich says, 'It is exciting that we can use physics to better understand biological questions. Of course sometimes physicists are ignorant of the nice details of a study and there can be clashes between the two cultures, but it is well worth the joint efforts of biologists and physicists to find out how biological phenomena work.'

STRANGE SHAPE AFFECTS SAND DOLLAR LARVAE SWIMMING
Most youngsters look a lot like their parents. They have the same number of arms and legs in roughly the same arrangement -but not sand dollar larvae. While their parents are disc shaped bottom dwellers, the shuttlecock shaped larvae bob around freely in the ocean. 'They are plankton that drift with the currents,' explains Tansy Clay from the University of Washington. Over the course of their development the larvae grow additional arms. Starting off with four, they develop another two and finally a total of eight before eventually settling on the seabed. And sand dollars are not the only larvae to take this strange shape. The larvae of several unrelated species are also shaped like shuttlecocks. 'So what is the purpose of I In ns si id de e J JE EB B Inside JEB highlights the key developments in The Journal of Experimental Biology. Written by science journalists, the short reports give the inside view of the science in JEB.

THE JOURNAL OF EXPERIMENTAL BIOLOGY
Inside JEB ii this crazy shape and why would different organisms independently evolve it?' puzzles Clay. Intrigued by the larvae's bizarre appearance, Clay and her supervisor, Daniel Grünbaum, decided to find out how the different larval stages swim. Knowing that their coastal water habitats are turbulent, the duo decided to focus on how the larvae fare in the flows in turbulent eddies (p. 1281).
According to Clay, previous attempts to understand how plankton move in turbulent eddies had represented them as simple spheres and ellipsoids. The calculations suggested that rounded plankton tilt in vertically circulating water so that they begin travelling horizontally toward downwelling water and are sucked down. Would the shuttlechock-shaped larvae suffer the same fate or would their strange morphology direct them into up-welling water?
The duo took a two-pronged attack: running computer simulations of the movements of all three larval life stages; and filming real larvae bobbing about in a tank. Basing the computer simulations on model larvae built from cylinders, Clay calculated how all three-life stages moved and found that they were directed into up-welling flows in mild turbulence found in a calm estuary. But as the turbulence increased, Clay's calculations suggested that the four-and eight-armed larvae tilted and swam horizontally until they became trapped in downward plumes of water, while the intermediate six-armed larvae were tilted and drawn toward up-welling flows.
Clay admits that she was surprised that the differences in the larvae's shapes had such a significant effect on their calculated movements. 'I expected some differences but not that extreme,' she says.
Turning to live larvae bobbing about in tanks, Clay generated vertical water flows in the middle of the tank by warming one side of the chamber and cooling the opposite side while she filmed the larvae's movements in the central portion. Tracking the fluid movements with algal cells, Clay could see that the four-and eight-armed larvae behaved as her simulations had predicted, travelling horizontally until they were engulfed in a plume of down-welling water. But when she looked at the six-armed larvae, they were doing something completely different: they didn't move horizontally like the simulations. 'It seems that they were resisting horizontal movement towards down-welling water,' says Clay.
So the larvae's strange shapes do change the ways in which they swim and could allow them to move selectively to different locations within the water column at different stages of development.

LOGGERHEADS STAY AT SURFACE TO SOAK UP SUN
Sandra Hochscheid is fascinated by loggerhead turtles. 'I was always interested in what they do under the surface,' says Hochscheid from the Stazione Zoologica Anton Dohrn, Italy. But when Hochscheid began hearing reports that the champion divers had been spotted on the surface on calm days, she was intrigued. Having spent most of the last decade tracking loggerhead diving behaviour with satellite technology, Hochscheid had never noticed the turtles taking extended breaks at the surface. 'They only need a few minutes to recharge their oxygen supplies,' explains Hochscheid, so why were the reptiles taking such long surface breaks? Curious to find out whether the anecdotes were true and, if so, why, Hochscheid and her Italian colleague Flegra Bentivegna, Abdulmaula Hamza from the Libyan Environmental General Authority and Graeme Hays from Swansea University, UK, decided to track loggerhead's diving behaviour to see if they could find any evidence of the surfacing behaviour (p. 1328).
First the team had to find turtles that could be tagged with a sophisticated data logger that would send information back when the animals surfaced. Closely involved with a rehabilitation programme that returns injured loggerheads to the Mediterranean, the team had access to eight juvenile turtles from the Stazione Zoologica's Rescue Centre, as well as two turtles that had been caught in a net off the coast of Libya. Attaching the tracking devices to the tops of the animals' shells, the team released them near the sites where they had been rescued, and waited for the data to start coming in.
Over a period of more than a year, the dataloggers contacted their satellites, which calculated the turtles' locations on the surface, and then transmitted: how deep and long they had dived; the temperature of the water surrounding them; and the length of time they spent at the surface. 'It was a really huge data set,' remembers Hochscheid, but as she was analysing the data she realised that the anecdotes were true. The turtles did occasionally spend extended periods at the surface, ranging from tens of minutes up to a marathon 17 h on one occasion, with more than 80% of the visits occurring during daylight.
Looking closer to see if the behaviour was related to the turtle's diving activity, Hochscheid noticed that the majority of stays at the surface occurred around noon, when the sun was at its highest, and that some of the turtles were diving much deeper than had been expected. 'A few of the larger turtles were diving off the continental shelf and were going into very deep waters experiencing a 10°C temperature difference between surface and depth,' says Hochscheid, who suspects that these animals remain at the surface to warm up after their cold deep dive and also to top up their vitamin D levels. But this couldn't explain the night time surfacing events.
Scrutinising the duration of the dives that preceded the long nocturnal surface breaks, Hochscheid realised that some of the turtles were remaining submerged well after their oxygen stores must have run out. Knowing that turtles can extend the length of a dive by switching to anaerobic metabolism after exhausting their oxygen supply, Hochscheid suspects that the turtles may be clearing lactic acid -which accumulates during anaerobic metabolism -from their muscles during their long stopovers.
So loggerhead turtles seem to extend surface breaks for at least two reasons: to soak up the sun and get warm by day and to recover from anaerobic exercise at night.