Extra arms destabilise sea urchin larvae

Abstract

The ocean is a big place, especially for minute larvae searching for a permanent home to settle in. ‘Many marine organisms have planktonic early stages and rely on them for dispersal’, explains Karen Chan from the Hong Kong University of Science and Technology, adding that the minute pioneers have to navigate the constantly fluctuating water column while interpreting their surroundings until they find their final resting place. ‘If we can figure out how various cues are used by larvae, it would help us better understand their abundance and distribution’, says Chan, who was a postdoc at the Woods Hole Oceanographic Institution in the USA when she decided to find out how larvae of the Atlantic purple sea urchin (Arbacia punctulata) cope with water turbulence during these early stages of development.Describing how the shuttlecock-shaped larvae acquire additional pairs of arms over the course of their development, Chan explains that the limbs are covered in beating hairs, which propel them through the water. Collecting larvae as they were produced by two pairs of sea urchins, Chan, Jeanette Wheeler and Lauren Mullineaux then monitored the larvae's development until they acquired four arms (8 days) and then six arms (23 days), although Chan admits that it was a challenge to rear sufficient larvae to fill their tank, which was over 100 l in volume. Then, the team generated turbulence in the water by oscillating a pair of vertical grids up and down to simulate calm nearshore waters and open ocean conditions while filming the larvae as they swam in the gently churning water. ‘We had to complete all the trials for one larval stage in as little time as possible, to minimise any between-trial differences in growth’, says Chan, who admits that running the experiments was exhausting. And, she also recalls that tracking the orientation of the larvae's arms, with Erik Anderson's help, was challenging. ‘We explored various automated algorithms and continue to refine these methods with Erik, but we realised at the time that the human eye was one of the most efficient pattern-recognition machines to identify a complex 3-dimensional shape, when observing a 2-dimensional slice through it’, she explains.Interpreting the larvae's tumbling motions, the team discovered that all of the animals swam harder as the turbulence increased. But when she focused on their vertical motion, Chan noticed that the 4-armed larvae swam vertically most strongly in the still water, while the older 6-armed larvae swam upward hardest in the gently swirling nearshore-like water. The team also analysed the larvae's orientation in the eddying water and observed that the larger 6-armed larvae tumbled more in the turbulent conditions than their smaller 4-armed younger siblings, which surprised Chan. ‘We initially expected the opposite’, she says, adding that she had expected the larger larvae to be more stable because they have a heavier skeleton, which she thought would act as ballast to stabilise their position.Chan suggests that the reduction in stability as the larvae acquire more arms could trigger them to swim harder than the younger 4-armed larvae, resulting in more of the 6-armed larvae migrating into shallower water. However, it is unclear how the sea urchin larvae sense the movements in the water that trigger their swimming actions. ‘They lack the sensory organs for rotation and acceleration that other larvae and plankton possess’, she says, adding that she is keen to identify the sensory mechanisms that help the larvae to respond as they wash around in ocean currents.