If the Eyes are the Window to the Soul, this Fish has a Sunroof

Things are lookin’ up

Common Name: Barreleye Fish

A.K.A.: Macropinna microstoma  (and related species)

Vital Stats:

  • Size: 15cm (6″) long
  • Depth: 600-800m (2000′-2600′) below sea level
  • Discovered: 1939
  • First Photographed: 2008

Found: Subarctic and Temperate regions of the North Pacific

It Does What?!

As you have likely already noticed, fish don’t have necks. At least not in the sense that they are able to look upward. So for a bottom-dweller lurking about in the cold depths of the ocean, being able to see that tasty bit of food floating by above is something of a problem. Some species get around this issue by floating vertically in the water so their whole bodies are pointing upwards. Simple enough. But in the spirit of meeting every challenge with an impossibly bizarre solution, nature has also produced a fish with eyes directly on the top of its head. After all, why re-orient the entire fish when you can just shift a couple of parts?

Those things on the front that look like eye sockets?
That would be its nose.

But the strangeness of the Barreleye Fish goes a little further than that. These aren’t just normal fish eyes in an unusual location. This species’ main prey are jellyfish and their relatives, which frequently come equipped with stingers that could damage the eyes of most predators. So rather than a normal spherical eye perched on top of its head, Macropinna has a tubular structure with the lens buried deep within its head (the dark green areas in the images). Overlying the tubular eyes is a tough, fluid-filled, transparent shield which the fish can look through. That’s right, it looks through the top of its own head. This way, stings from jellyfish will never damage the delicate ocular tissue.

What’s more, the fish’s unique tubular eyes are supremely adapted for the dark depths of the ocean. They allow unusually accurate depth perception (due to a large overlap of the two visual fields) and enhanced light gathering compared the spheroid eyes. In an environment up to 2600 feet (800m) down, where little daylight penetrates and everything appears in monochrome, these adaptations enable the barreleye to distinguish even faint shadows and silhouettes moving above it, and to precisely gauge how far up they are.

The Barreleye Fish, failing to look at the camera.

Researchers had long been puzzled as to how the barreleye eats, since, with its eyes on top of its head, its visual field didn’t include the area around its mouth. The species has been known since 1939, but only as small mangled bodies caught up in deep-sea fishing nets (adults are only about six inches long). In each case, the transparent casing of the fish’s head had been destroyed by the nets and the rapid changes in pressure as the nets were pulled up, making its anatomy difficult to study. In 2008, however, scientists from the Monterey Bay Aquarium Research Institute sent remote operated vehicles with cameras down to try, for the very first time, to snap some photos of these oddballs in action. What they learned was that, when it spots prey, the barreleye can actually rotate its entire tubular eye downward, like moving the telescope in an observatory. This way, it can turn and look at its target straight on as it pursues. Most of the time, though, the fish was seen to use its large, flat fins to hold itself nearly motionless, looking up through its personal sunroof, just waiting for some unlucky jellyfish to float on by.

Says Who?

  • Robison & Reisenbichler (2008) Copeia 4: 780-784.
  • Monterey Bay Aquarium Research Institute

All images taken by the Monterey Bay Aquarium Research Institute (MBARI)

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A Shellfish Goes to the Dark Side (Sacculina carcini)

The crab barnacle, hitchin’ a ride.
(Image by Hans Hillewaert)

Common Name: Crab Barnacle, or the charmingly descriptive Dutch term “krabbenzakje,” meaning “crab bag”

A.K.A.: Sacculina carcini (and other Sacculina species)

Found: In the coastal waters of Europe and North Africa

It Does What?!

Most barnacles, those almost quaint crusts seen decorating old piers and ships, live their lives by cementing themselves to a hard underwater surface and using their arm-like limbs to pull passing bits of food into their mouths all day. Not so for the crab barnacle, who decided that all that arm-waving was for chumps and set about evolving into the ultimate free-loader.

Normal, hardworking barnacles, for the sake of comparison…
(Image by Michael Maggs)

In its immature larval form, Sacculina has a similar body plan to other barnacles and is able to swim about freely; however, rather than finding a surface to settle down on, it finds itself a crab. Typically, this will be a green crab, species Carcinus maenas. The female barnacle (more on the males later) crawls along the surface of the crab’s shell until she comes to a joint – a chink in the armour – where she turns into a sort of hypodermic needle, injecting herself into the crab and leaving her limbs and shell behind. Now nothing more than a tiny slug-like mass, she makes her way to the crab’s abdomen and proceeds to grow rootlike tendrils throughout her host’s body, drawing nutrients directly from the bloodstream.

If that wasn’t disturbing enough, consider Sacculina’s mode of reproduction. In addition to its internal root system, the parasite forms an external sac (hence the nickname ‘crab bag’) where the female crab normally keeps her fertilized eggs. This is where the male barnacle comes into play. Upon finding a crab already infected by a female, the male will do the same needle trick, injecting himself into the external sac and living for the rest of his life as a parasite inside the female’s body. Fertilization takes place and the sac is soon full of microscopic Sacculina larvae.

In case you needed a closer look.

Since the barnacle infection has rendered the host sterile, and because crabs aren’t very bright, the crab will now care for this sac of larvae as if they were her own young. But what if the infected crab was male, you ask? No problem. The parasite is able to interfere with his hormones to such an extent that, in addition to changing his body shape to that of a female, he now actually behaves like, and even carries out the mating gestures of, a female crab.

Now, this may not seem so bad from the point of view of the crab; I mean, it doesn’t know it’s carrying around evil changeling spawn, right? But it’s a bit worse than that. Wanting to keep all the available energy for its own use, the parasite prevents the crab from moulting its shell or re-growing lost claws, as crabs normally do. This leads to a variety of secondary infections which, coupled with malnutrition, leads to the premature death of the crab. But nature isn’t without a sense of fair play… research has now found that Sacculina sometimes succumbs to viruses and yeast naturally present in the crab’s body, via infection of its rootlets. Take that, bloodsucking barnacle!

Says Who?

  • Powell & Rowley (2008) Diseases of Aquatic Organisms 80: 75-79.
  • Zimmer (2000) “Do parasites rule the world?” Discover Magazine (August issue).
  • Russell et al. (2000) Journal of the Marine Biological Association of the U.K. 80: 373-374.
  • Mouritsen & Jensen (2006) Marine Biology Research 2: 270-275.
  • Goddard et al. (2005) Biological Invasions 7: 895-912.

The Curious Case of Turritopsis nutricula

Turritopsis nutricula

Common Name: The Immortal Jellyfish

A.K.A.: Turritopsis nutricula

Found: Tropical and temperate oceans around the world

It Does What?!

Ever been under a lot of stress and found yourself longing for the simplicity of childhood? What if, by force of will, you could actually turn back into your childhood self? And once you’d re-grown up, you could do it again. And again, and again… Welcome to the unusual lifestyle of Turritopsis nutricula, the so-called immortal jellyfish.

Jellyfish, also known as medusae (singular: medusa), are the mature life stage of Phylum Cnidaria, Subphylum Medusozoa. They start off as a bottom-dwelling structure that looks a lot like a series of plants connected by stolons (like strawberry plants… translucent, underwater strawberry plants). These “pseudo-plants” are called polyps, and when they mature, they bud and release many tiny medusae into the ocean, like a plant releasing pollen.

The polyp stage of Turritopsis nutricula

In most species, these medusae go off and live the jellyfish version of the good life- swimming, eating plankton, releasing sperm or eggs to be fertilized and form polyps for the next generation, and finally dying at the ripe old age of anywhere from a few hours to six months, depending on the species. Not so for the Immortal Jellyfish.

Reaching a size of only 4.5mm across, when Turritopsis nutricula becomes stressed, whether due to aging or a change in its environment, it can begin a process called transdifferentiation. First, its tentacles (80 to 90 of them in adults!) shorten and are re-absorbed into the body. The medusa becomes unable to swim and settles onto the bottom. It there transforms into a blob-like mass of cells and, within two or three days, forms a new polyp. In about a month, new jellyfish are ready to be released.

In theory, T. nutricula can pull this trick any number of times, which would effectively make it immortal. However, as scientists point out, these little guys frequently die from disease or predation before they can regenerate (Whovians, insert your own Doctor Who joke here), keeping the population under control. Not entirely under control, though, apparently- one researcher describes the spread of T. nutricula through the world’s oceans as a large-scale, “silent invasion.”  Beware the Immortal Jellyfish.

Says Who?

  • Miglietta & Lessios (2009) Biological Invasions 11: 825-834
  • Piraino et al. (2004) Canadian Journal of Zoology 82: 1748-1754

[Thanks to The Marine Biology Image Database for the use of these images: Migotto AE, Vellutini BC (eds). 2011. Cifonauta: marine biology image database. Available at http://cifonauta.cebimar.usp.br/ ]