A typical starfish has five-sided symmetry. With no clear head, the starfish can move in any direction, led by any one of its five arms. If you were feeling particularly cruel, you could fold one up in five different ways, so each half fitted exactly on top of the other. We humans, like many other animals, have only two-sided symmetry. We’re ‘bilateral’ – our right half mirrors our left, and we have an obvious head.
These two body plans might look radically different, but looks can be deceiving. Chengcheng Ji and Liang Wu from the China Agricultural University have found that starfish have hidden bilateral tendencies, which reveal themselves under times of stress.
Purple sea urchins look like beautiful pincushions. They have no obvious eyes among their purple spines, but they can still respond to light. If you shine a spotlight on one, it will sidle off to somewhere darker. Clearly, the purple sea urchin can see, and over the past few years, scientists have worked out how: its entire body is an eye.
For decades, scientists knew that sea urchins can respond to light, even though they don’t have anything that looks remotely like an eye. The mystery deepened in 2006, when the full genome of the purple sea urchin was published. To everyone’s surprise, its 23,000 genes included several that are associated with eyes. The urchin has its own version of the master gene Pax6, which governs the development of animal eyes from humans to flies. It also has six genes for light-sensitive proteins called opsins.
While these genes are usually switched on in the developing eye, Maria Arnone found that the sea urchin’s versions are strongly activated in its feet. Sea urchins have hundreds of “tube feet”, small cylinders that sway around amid the spines. They can use the feet to move around, to manipulate food, and apparently to see.
Alexandrium is part of the sea’s collection of plankton. It’s a single-celled creature but it can create colonies by amassing together in long chains. At their most extreme, these colonies can form large swarms to produce harmful red tides.
As chains, Alexandrium swims and grows faster, but it is vulnerable to predators such as copepods – small relatives of crabs or shrimp. Erik Selander from theTechnical University of Denmark found that when the chains detect the chemical traces of copepods, they break apart. By turning back into single cells, they make themselves harder to find. They also swim at a slower pace to avoid creating telltale movements in the water. When threatened by predators, these plankton enter stealth mode.
A complete ban on fishing can save coral reef communities in more ways than one. A few weeks ago, I blogged about a study which found that the coral trout, a victim of severe overfishing, was bouncing back in the small regions of the Great Barrier Reef where fishing has been totally forbidden. It certainly makes sense that fish will rebound when fishing ceases, but a new study reveals that the bans have had more indirect benefits – they have protected the corals from a predatory starfish.
The crown-of-thorns starfish (Acanthaster planci) is a voracious hunter of corals and a massive problem for reef conservationists. It’s bad practice for any science writer to anthropomorphise an animal, but the crown-of-thorns really does look incredibly, well, evil. Its arms (and it can have as many as 20) are covered in sharp, venomous spines. As it crawls over the reef, it digests the underlying coral by extruding its stomach out through its underside.
From time to time, their numbers swell into plagues of thousands that leave behind the dead, white skeletons of corals in their wake. These outbreaks eventually die off as the starfish eat themselves out of food supplies, but not before seeding downstream reefs with their tiny larvae that drift along the southern currents. During their peak, they destroy far more coral than other disturbances such as bleaching events or hurricanes.
Now, Hugh Sweatman at the Australian Institute of Marine Science has found that these outbreaks are much less frequent in the “no-take marine reserves”, where fishing is absolutely forbidden. Every year between 1994 and 2004, Sweatman carried out a census of starfish numbers in up to 137 areas across the Great Barrier Reef’s massive length.
Many animals have cunning ways of hiding from predators. But the larva of the sand dollar takes that to an extreme – it avoids being spotted by splitting itself into two identical clones.
Sand dollars are members of a group of animals called echinoderms, that include sea urchins and starfish. An adult sand dollar (Dendraster excentricus) is a flat, round disc that lives a sedate life on the sea floor. Its larva, also known as a pluteus, is very different, a small, six-armed creature that floats freely among the ocean’s plankton.
A pluteus can’t swim quickly, so there is no escape for one if it is attacked by a hungry fish. Instead, Dawn Vaughan and Richard Strathmann from the University of Washington discovered that the pluteus relies on not being spotted in the first place.
They exposed 4-day-old larvae to water which contained mucus from the skin of a potential predator – the Dover sole. Within 24 hours, every single larva that was exposed to the mucus has grown a small bud that eventually detached and developed into a second larva, genetically identical to its parent and smaller in size. In contrast, larvae that were exposed to untouched seawater stayed undivided.