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Behold! The magnificent Melibe genus of sea slugs… These transparent nudibranch mollusks are active carnivores which trap fast-moving, free swimming prey with a powerful weapon—their head. I don’t mean this figuratively: their transparent heads are expanding nets which shoot open and engulf small animals like copepods, shrimp, hydrozoa, and tiny fish fry. Their lethal hoods are surrounded by a mane of sensory tentacles, which make the slugs superficially resemble jellyfish and Venus fly traps).
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Hooded nudibranchs of various species live in tropical and temperate waters around the world (I was unable to ascertain whether they lived in the Arctic or not…maybe because they don’t or maybe because we don’t yet know). They tend to be diminutive animals measuring under 10 cms (4 inches) long which live hidden among seaweeds and kelps. The creatures are hermaphrodites and emit a sweet smell when removed from the water. In case they were not sufficiently bizarre for you, they escape predators like crabs, fish, and cephalopods by shedding their cerate (the lateral outgrowths protruding from the slug’s body).
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I have failed to do this slug justice (because WordPress has disabled video posting), but here is a link which shows the disquieting predatory head-expansion. I can hardly think of a creature more alien in appearance or manners, and yet they are quite appealing. The amazing Eliza Heery thought so too, and dressed as one for Halloween. What a world…
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The glass squid, Cranchia scabra

The glass squid, Cranchia scabra

The Cranchiidae are a family of squid commonly known as “glass squid” which live in oceans around the world. The squid are of no interest to commercial fisheries (yet) and a great deal about this family remains completely unknown. Most of the 60 known species of cranchiidae are small and inconspicuous–indeed the majority are transparent and thus nearly invisible. However the largest known mollusk, the colossal squid (Mesonychoteuthis hamiltoni), is part of the family, (so it might be wise not to antagonize them on the playground).

Glass Squid (Taonius belone) off Hawaii. Photograph by R. Young.

Glass Squid (Taonius belone) off Hawaii. Photograph by R. Young.

Glass squid are notable for having stubby swollen-looking bodies and short arms except for one long pair of hunting tentacles. The majority of glass squid have bioluminescent organs which they use to hunt, to communicate and to disguise the faint shadows cast by their transparent bodies (predators of the deep can see even the faintest shadows cast by the dim light from the surface). The cranchiid squids themselves sport a variety of interesting and complex eyes which range from giant circular eyes to stalked eyes to telescoping eyes.   This little gallery shows how delicate, diverse, and beautiful (and how utterly alien) these squid can be.

Banded Piglet Squid (Helicocranchia pfefferi) photo by keyofv

Banded Piglet Squid (Helicocranchia pfefferi) photo by keyofv

Bathothauma lyromma (note eyes on stalks!)

Bathothauma lyromma (note eyes on stalks!)

A drawing of the piglet squid (Helicocranchia pfefferi)

A drawing of the piglet squid (Helicocranchia pfefferi)

Sandalops melancholicus by Chun Carl

Sandalops melancholicus by Chun Carl

Teuthowenia megalops

Teuthowenia megalops

Cranchia scabra

Cranchia scabra

Juvenile cranchiid squid are part of the plankton and live near the surface where they hunt microscopic prey while trying to avoid thousands of sorts of predators. As they mature, they change shape and descend to deeper waters—indeed some species become practically benthic and can be found more than 2 kilometeres under the ocean. Glass squid move up and down the water column by means of a fluid filled chamber which contains an ammonia solution (which maybe explains why they are not on the human menu yet).

Belonella belone

Belonella belone

Vitreledonella richardi (photo by Underseahunter Group)

Vitreledonella richardi (photo by Underseahunter Group)

Today we have a special (but largely visual) treat: the pelagic octopus Vitrelladonella richardi. This cephalopod is “transparent, gelatinous, and almost colorless.”  Since they are not only transparent but also live in the deeper part of pelagic zone of the ocean (the portion which is near to neither the top nor the bottom) they are rarely seen and little is known about them.  The females are ovoviviparous and broods her eggs within her body.  Both genders are strongly bioluminescent and use light for hunting, communicating, and hiding (by mimicking the faint light from the surface they can become even more invisible).   Even if we don’t know a huge amount about these octopuses, we are privileged to have some amazing photographs of them, thanks to the new generation of submersibles and submersible drones, which are exploring the pelagic regions of the ocean.  Look at how exquisite and alien these octopuses appear!  It’s hard to believe we share the planet with such strange animals…

Vitreledonella richardi (photo by Underseahunter Group)

Vitreledonella richardi (photo by Underseahunter Group)

The world is a strange place filled with astonishing and bizarre animals. Among the strangest creatures are those which are transparent*—animals which barely seem to be there because the tissues that make up their bodies are permeable to light.  There are transparent catfish, transparent insects, transparent crustaceans, and even transparent frogs (to say nothing of cnidarians, the majority of which are transparent!).  Today’s post however concerns transparent mollusks.  In addition to having transparent bodies some of these incredible invertebrates have transparent shells, can invert their bodies, or can glow.  Check them out:

The Glass Squid (Teuthowenia pellucida)

The Glass Squid (Teuthowenia pellucida)

The Glass Squid (Teuthowenia pellucida) also known as the googly-eyed glass squid lives throughout the oceans of the southern hemisphere.  The creature is about 200 millimeters (8 inches) long and has light organs on its eyes.  Although transparent it has a bluish cast and it possesses the ability to roll into a ball or to inflate itself.  These tricks do not always work and the little squid is frequently eaten by weird deep-sea fish and sharks.

Zospeum tholussum

Zospeum tholussum

Recently discovered in a huge cave system in Croatia,  Zospeum tholussum, is a small delicate snail with a transparent shell.

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This pterotracheid heteropod mollusk is a member of the Carinaria genus.  It lives in the open ocean and flaps through the water scooping up plankton in a modified snail foot. Just as its snail foot has changed into a swimming/harvesting organ, the mollusk’s shell has shrunk into near nonexistence.

0corolla (calceola)Above is an unknown pteropod (possibly of the genus Corolla), which was photographed near Oceanographer Canyon off the coast of Massachusetts.

Vitrelladonella Richardi

Vitrelladonella Richardi

Vitrelladonella Richardi is a deepwater pelagic octopus found in tropical and subtropical waters around the world.  The little octopus only measures 80cm (2.6ft) in length.  Like many of these almost invisible mollusks very little is known about how it lives, but it is certainly beautiful in a very alien and otherworldly way.

*The author of this opinion piece is opaque.  His opinion may not represent the larger community of organisms.

The Glass Catfish, Kryptopterus bicirrhis (An exceptional photo by Holger Knudsen)

The most transparent known vertebrates are the Asian glass catfish of the genus Kryptopterus.  The two most popular species are Kryptopterus minor, the ghost catfish, and Kryptopterus bicirrhis, commonly known as the glass catfish, which is a mainstay in the tropical aquarium.

Asian glass catfishes live in slow turbid streams throughout Thailand, Malaysia and Indonesia.  They are schooling predators who feed on tiny arthropods, worms, and minnow fry. Camaraderie is important for the little fish—outside of a school, individuals rarely survive (even in an aquarium where all of their other needs are met and they lack predators).  Their remarkable transparency serves as camouflage, hiding them from predator and prey alike.  Growing to a maximum of four inches the catfish live for up to eight years.

A School of Glass Catfish

Asian glass catfish are scaleless and lack pigment, however the cellular dynamics of their transparent tissues are still not fully understood.  In living specimens, the animal’s skeleton is quite visible and its internal organs can be seen with a silver sack.  A viewer with a powerful magnifying glass can watch the fish’s heartbeat and determine the contents of its stomach.  When the catfish dies so does its transparency–after death they turn an opaque white.

The catfish is commercially important for the aquarium trade.  It seems possible that exporters in South East Asia have devised a way to breed the fish en masse, but, if so, it is wholly unknown in the west. The Phantom Glass Catfish is also a major ingredient of some of the salty fish sauces used in Malaysian and Indonesian cooking.

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