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Nudibranchs are gastropod mollusks which live in the oceans worldwide from the polar regions to the tropics. The slugs live in virtually all depths and various species range from the shallow intertidal surf to depths of more than over 700m. Although the majority of nudibranchs are benthic creatures which crawl along the seafloor, some prefer other lives and float upside down under the oceans surface or swim in the water column.
Nudibranchs lose their vestigial shell during a larval phase. To protect themselves they rely on toxins or unpleasant tasting chemicals which are advertised with extremely vivid colors. In order to enliven the gray winter months, here is a little parade of lovely nudibranchs. Enjoy!
Biologists estimate that there are approximately 8.8 million species of eukaryotes (animals with complex cell structure) currently alive on Earth. So far, humankind has only cataloged 1.9 million species and entire biomes remain largely unknown to us.
To illustrate this point, here is a photograph of a completely unknown genus of nudibranch mollusk photographed 1 mile beneath the surface of the ocean near Davidson seamount (which is an extinct underwater volcano just off the coast of Monterey). I wish I could tell you more about the strange mollusk, but this photograph, taken from a robotic deep sea submersible in 2002 is pretty much all that humankind knows about this species. The mission photographed a huge number of other gelatinous creatures in the middle depths of the ocean, and in fact caused scientists to rethink the importance of such animals in the oceanic ecosystem. The Monterey Bay Aquarium Research Institute (MBARI) worked on the mission with the National Oceanic and Atmospheric Administration. Their website explains the robotic study by paraphrasing Bruce Robinson, an ecologist who pioneered the use of robot submersibles:
One of the most important discoveries has been the realization that gelatinous animals are important as grazers and predators that comprise a large percentage of the open ocean animal biomass. Robison estimates that gelatinous animals make up about 40 percent of the biomass in the deep sea water column.
Nudibranch mollusks are largely thought of as colorful predators of the tropical reef, so it is a big deal if they (together with other floating mollusks, cnidarians, and siphonophores) constitute such a substantial percentage of the biomass of the largest portion of the ocean. As an unscientific postscript I think the delicate translucent nudibranch is very beautiful with its alien and ghostlike (and, yes, gelatinous) features.
The Portuguese man o’ war (Physalia physalis) is not a jellyfish, in fact it is not a discreet animal at all, but instead a siphonophore—a colonial medusoid made up of specialized animal polyps working together as an organism. These siphonophores have stinging tentacles which typically measure 10 metres (30 ft) in length but can be up to 50 metres (165 ft) long. Being stung by a man o’ war does not typically cause death, but sailors and mariners who have survived the experience assert that it taught them a new definition of agony.
But the fearsome man o’ war is not the subject of this post. Instead we are concentrating on the animal which feeds on the man ‘o war (as well as other siphonophores which drift in the great blue expanses of the open ocean). One is inclined to imagine that men o’ war are eaten only by armored giants with impervious skins and great shearing beaks (and indeed the world’s largest turtles, the loggerheads, are the main predators of siphonophores), however another much less likely predator is out there in the open ocean gnawing away at the mighty stinging colonies. Glaucus atlanticus, the blue sea slug, is a tiny shell-free mollusk which lives in the open ocean. The little nudibranch only grows up to 3 cm in length but it hunts and eats a variety of large hydrozoans, pelagic mollusks, and siphonophores (including the man o’ war).
Although not quite as gaudy as its lovely cousins from tropical coral reefs, Glaucus atlanticus is a pretty animal of pale grey, silver, and deep blue with delicate blue appendages radiating out from its six appendages. The little mollusks live in temperate and tropical oceans worldwide. They float at the top of the water thanks to a swallowed air bubble stored in a special sack in their gastric cavity. Because of this flotation aid, the slug is able to cling upside down to the surface tension of the waves. Since it is entirely immune to the venomous nematocysts of the man o’ war, the sea slug can store some of the man o’ wars venom for its own use. The tendrils at the edge of Glaucus atlanticus’ body can produce an extremely potent sting (so it is best to leave the tiny creatures alone, if you happen to somehow come across them).
Each and every Glaucus atlanticus is a hermaphrodite with a complete set of sex organs for both genders. Incapable of mating with themselves they ventrally (and thoroughly) embrace another blue sea slug during breeding, and both parties then produce strings of eggs. The hatchling nudibranchs have a shell during their larval stages, but this vestige quickly disappears as they mature into hunters of the open ocean.
Nudibranchs are among my favorite animals to look at. These tropical marine mollusks feature extraordinary colors and fantastical shapes which would make the most flamboyant nineteen eighties rock star weep with envy. One of the largest and most powerful nudibranchs is also one of the most beautiful. Hexabranchus sanguineus lives thoughout the tropical waters of the Indo-Pacific Ocean and can be found from the Red Sea to Hawaii. The creature’s common English name is the Spanish Dancer because, when it swims free, it undulates its bright red paradodia in the manner of a flamenco dancer.
Although the Spanish Dancer is surprisingly quick and agile when it uses this means of locomotion, it has an auxiliary method for getting around and can also be found crawling in a much more traditional slug-like manner. The creature grows to be 40 centimeters or larger and has several distinctive color patterns ranging from bright red to bright yellow to pale pink (or sometimes various combinations of these colors).
The Spanish dancer can afford to be extravagantly colorful because it contains toxic chemicals inside its body (again one is drawn to comparisons with 1980’s musical entertainers). Predators therefore avoid the creature as it proceeds about the reef feeding on various sponges and bryozoans. Spanish Dancers are hermaphrodites. Although each Spanish dancer possesses the reproductive organs of both genders, it is very rare for an individual to fertilize itself. When they do mate, the parent carefully deposits a large pink rosette of eggs which is almost as distinctive and lovely as the adult.
The Spanish dancer is sometimes inhabited by one or more Emperor Shrimps. These little arthropods do not help their mollusk host, but neither do they harm it (a commensal relationship). Chameleon-like the little shrimp can adapt to the extraordinary coloring of their vivid hosts.