See with five arms

Seeing without eyes: a snake star sees with the body and apparently also uses its color pigments on the surface. A team of researchers led by Lauren Sumner-Rooney and Esther Ullrich-Lüter from the Berlin Museum of Natural History has clarified in a series of experiments how the species Ophiocoma wendtii is spatially oriented. The nocturnal animal, which lives in the Caribbean and the Gulf of Mexico, carries thousands of light receptors distributed over the body, as the researchers report in the journal “Current Biology”.

The snake star, the five Arms and a diameter of a little over 20 centimeters, takes on different colors during the day: During the day it is dark red to black, at night it has a beige stripe pattern. During the day, the animal hides under rocks from predators such as fish, while at night it raids.

Attracted to darkness

In experiments, the researchers checked what the orientation ability of the light-shy animal depends on. The snake stars, such as sea cucumbers, which belong to the echinoderms and whose eyesight is only rough, moved clearly towards dark stimuli – but only during the day. In their natural environment, such as coral reefs, they probably move towards crevices when they are discovered by predators. It is unclear how O. wendtii processes the sensory stimuli decentrally, because the animal has no central nervous system, but a nerve ring in the small central disc.

«We were surprised that the reactions shown during the day disappeared when we tested the animals at night even though the photosensitive cells still seemed to be active, ”explains Lauren Sumner-Rooney of Oxford University. The researchers then found the alleged explanation in elaborate experiments.

Accordingly, the dark pigment cells or chromatophores expand on the body surface during the day. They ensure that light hits the photoreceptors at an acute angle. On the other hand, if the chromatophores contract at night, the light falls in at a wider angle and the animals cannot make out the direction. Spatial vision is less important at night, explains Ullrich-Lüter, since most of the snake star's enemies are diurnal.

“Extraocular vision”

The finding could also explain why a close relative of O. wendtii, O. pumila , apparently cannot see: This snake star also carries photoreceptors, but has a pale color – similar to O. wendti i at night. In the experiments, he did not respond to dark stimuli. However, Ullrich-Lüter assumes that the photoreceptors also have a function in O. pumila – possibly for the detection of darkening. That would make perfect sense: the diurnal snake star will bury itself in the sand if it is in danger.

O. wendtii is only the second animal without eyes to have eyesight, the researchers write in “Current Biology”. The first animal is the purple sea urchin ( Strongylocentrus purpuratus ). There the photoreceptors are located in small indentations of the round lime skeleton and on the feet, as Ullrich-Lüter 2011 reported in the journal «PNAS». The distribution of the round 200 000 light sensors over the round body in combination with a decentralized nervous system allow the animals to Process information from 1500 light receptor groups.

Sea urchins and starfish are closely related, both of which belong to the echinoderms (Echinodermata) has existed for more than 500 million years. Ullrich-Lüter believes that extraocular vision had probably already developed at that time with common ancestors of the animals. “We strongly believe that.”

(W old Willems, dpa )

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