Month: February 2017

Cockeyed

27 February 2017 / wvanstrien

Strawberry squid looks upwards with a bulging eye

Light is scarce in the mesopelagic region of the deep sea, which asks for special adaptations of the eyes of the animals that live there. Squids of the family Histioteuthidae addressed this challenge by developing two different eyes, Katie Thomas and colleagues report.

If symmetry is a characteristic of beauty, then adult deep sea squids of the family Histioteuthidae are really ugly, because in addition to a normal right eye, they have a protruding left eye which is twice as large and usually yellow coloured. They are cockeyed. And, while not very nice, this is functional, as Katie Thomas and colleagues report.

As early as 1975 Richard Young proposed an idea of why these squids, which hunt prey like fish, shrimp and smaller squid, possess dimorphic eyes.

The squids live at a depth of several hundred meters in the oceans where it is dark apart from dim, downwelling sunlight. How do the animals manage to find their food in this nearly complete darkness? When prey animals are swimming above the squids, they may perceive their contrasting silhouette against the almost dark background, provided that their eyes are very sensitive to light. Below, they can only detect prey that produces bright flashes of light, as many deep sea animal species do for various reasons. To be able to localise such prey, the squids need eyes that produce images with high spatial resolution.

Video recordings

The enlarged left eye of cockeyed squids, Young stated, is light sensitive and more apt to detect silhouettes upwards, whereas the small right eye produces images of higher resolution which enable the squids to localise bioluminescent prey below. But as the animals live at great depths, he was not able to access and observe them to determine whether they actually turn their bulging left eye upwards.

Nowadays, this is possible. For 25 years now, the Monterey Bay Aquarium Research Institute (California) has been sending remotely operated underwater vehicles into depth to make video recordings. Thomas used the video footage to observe the strawberry squid Histioteuthis heteropsis and Stigmatoteuthis dofleini and to find out how these squids behave.

She ascertained that adult cockeyed squids almost always oriented the head downwards in an oblique body position, with the ten arms stretched straight ahead. And, as expected, the animals twist their heads so that the large left eye is directed upwards and the small right eye slighty downwards. So, what Young had supposed proved to be right: the animals have two different eyes that are adapted to two different sources of light, dim downwelling sunlight from above and light flashes in the dark below.

Yellow

Then, why has the left eye a yellow colour in most of these squids?

Many prey prevent detection by predators that approach from below by producing a ventral glow that matches the weak downwelling sunlight, so their silhouette is camouflaged against the background (counter illumination). A predator’s yellow eye filters out ultraviolet light, and this probably results in different colours of the ventrally emitted light of prey and the background light, breaking the camouflage and rendering the prey visible.

Willy van Strien

Photo: Young strawberry squid Histioteuthis heteropsis (not in its normal swimming posture) © Katie Thomas

View the strawberry squid Histioteuthis heteropsis on video

Sources:
Thomas, K.N., B.H. Robison & S. Johnsen, 2017. Two eyes for two purposes: in situ evidence for asymmetric vision in the cockeyed squids Histioteuthis heteropsis and Stigmatoteuthis dofleini. Phil. Trans. R. Soc. B 372: 20160069. Doi: 10.1098/rstb.2016.0069
Young, R.E., 1975. Function of the dimorphic eyes in the midwater squid Histioteuthis dofleini. Pacific Science 29: 211-218.

Glow in the dark

16 February 2017 / wvanstrien

Flashlight fish turns headlights on to catch prey

It is difficult to find food in the dark. But the splitfin flashlight fish Anomalops katoptron has no problem: it turns its headlights on when it hunts on zooplankton, as Jens Hellinger and colleagues report.

Only in complete darkness, the splitfin flashlight fish Anomalops katoptron will leave its hiding place. During daytime the fish, which lives in shallow coral reefs in the Pacific, resides in cavities and cracks in the reef where it is invisible to its predators, thanks to its dark colour. But in dark moonless nights it ventures to the open water to forage in a school of conspecifics. The diet consists of swimming zooplankton, prey that is difficult to find in the dark.

But Anomalops katoptron has a light organ under each eye that emits blue light, Jens Hellinger and colleagues point out. The light is produced by symbiotic bacteria that live densely packed within these organs. The bacteria have got a safe place to live in, in exchange for producing light.

Blinking

The bacteria glow continuously, but the fish can turn his lights off by rotating them, exposing their dark backsides instead of the transparent sides. During the day, the lights are almost always off, otherwise the fish would be visible in spite of its dark colour. Occasionally, he blinks.

When the splitfin flashlight fish is active, at night, he blinks more often, Hellinger observed when he studied a number of fish in a tank in the laboratory, and the lights shine about half of the time. And if the fish detects prey, it has its lights on almost continuously.

Many animal species exist that emit light, particularly in the sea, and their luminescence has several functions. Most luminescent species emit light to chase off or embarrass predators. Anglerfish lure prey: their dorsal fin is modified to a ‘fishing rod’ with a luminous bulb that attracts little creatures. And still others lure or recognize partners by flashing patterns; male ostracods, for instance, perform a spectacular light show to attract females, much like fireflies do on land.

Until now, it was not clear where the splitfin flashlight fish Anomalops katoptron uses its light for. It now turns out that it is mainly to detect prey in the dark.

Photo: California Academy of Sciences (via Flickr. Creative Commons CC BY-NC-ND 2.0)

Source:
Hellinger, J., P. Jägers, M. Donner, F. Sutt, M.D. Mark, B. Senen, R. Tollrian & S. Herlitze, 2017. The flashlight fish Anomalops katoptron uses bioluminescent light to detect prey in the dark. PLoS ONE 12: e0170489. Doi:10.1371/journal.pone.0170489

The trick of a snake

8 February 2017 / wvanstrien

Puff adder sticks out its tongue to lure a frog

A South African frog that perceives and approaches a tasty worm may be deceived. The worm may turn out to be the tongue of a snake, as Xavier Glaudas and Graham Alexander write, and if it is, the frog is in trouble.

The venomous puff adder (Bitis arietans), which lives in South Africa, hunts its prey by lying in ambush. Mostly nocturnal, camouflaged and hidden in the vegetation, it waits unobtrusively until a victim comes close, and then it strikes. But the striking range is only ten centimetres, so a prey often will stay out of reach.

Luring prey

But the snake uses a trick, Xavier Glaudas and Graham Alexander noticed when they reviewed a large amount of video recordings they had made of puff adders in ambush in the field. A puff adder often lures a prey by extending and moving the black tongue, the two points spread. The tongue then looks like a squirming worm and apparently, a frog is easily deceived. It hops closer to inspect the snack, and as soon as it comes within striking range, the snake will try to seize it. The frog that thought to find a meal is eaten himself.

Is the puff adder really mimicking a worm by extending the tongue to lure prey? According to the researchers, it does. The snake only extends its tongue if there is a frog or a toad close by, they argue; it doesn’t upon perceiving the presence of other prey, such as a mouse that doesn’t eat worms. Also, snakes use their tongues to sample odours, but chemosensory tongue flicks only take half a second while ‘lingual luring’ bouts take much more time.

The puff adders also wave their tails, and according to Glauda and Alexander that behaviour is also performed to lure prey. But they don’t have any recordings to show this, because their camera had been focussed on the heads of the animals.