The Bizarre World of Pelagic Invertebrates

I have come to view the oceans as an immense bowl of salty soup. Billions of creatures ride its currents, some visible to the naked eye, but many so tiny they can only be seen under a microscope. Some creatures will eventually settle down and live their adult lives far from where they started out. Others will spend their entire lives offshore. The latter — in particular invertebrates that spend their lives adrift in the ocean — is what this article is about. Many are bizarre-looking creatures not commonly seen by scuba divers.
Two of the pelagic invertebrates discussed here, Thetys vagina and Pyrosoma atlanticum, do not have common names. They are members of the phylum Chordata (as are mammals) and subphylum Tunicata. Of all the marine animals in the world, tunicates are vetebrates’ closest relatives. Both have a notochord (a cartilaginous skeletal rod) when they are embryos. The notochord is lost as most tunicates grow older but it segments and turns into a backbone in mammals and other vertebrates.
Tunicates are among the most common marine invertebrates and are named for their cellulose body covering, called a tunic or test. While tunicates don’t have eyes (some have eyespots), ears (though there is an otolith to sense gravity or movement) or a nose, they do have a mouth, heart, stomach, intestines and gonads.
Besides the phylum Chordata and subphylum Tunicata, Thetys vagina is a member of the order Salpida. It is the largest known species of salp and the only one of its genus. It forms double-row chains of three-quarters of an inch long clones that are sequential hermaphrodites and start life as females. It is also found individually. The solitary animals are larger and can be nearly 13 inches long. The test is firm, thick and spiny. It doesn’t collapse when out of the water. T. vagina lives in the Atlantic, Pacific and Indian Oceans and is found from the surface to depths of nearly 600 feet. It has occasionally been found in what The Living Bay calls, “inconceivably large numbers.” T. vagina eats marine plankton, including dinoflagellates, silicoflagellates, diatoms and tintinnids, as well as copepods.
According to The New Oxford American Dictionary, botany and zoology, vagina means, “any sheathlike structure, esp. a sheath formed around a stem by the base of a leaf.”
Usually found in very deep water (down to 2,400+ feet), masses of Pyrosoma atlanticum, also a member of the phylum Chordata and subphylum Tunicata, are occasionally found on the surface. Those shown here, however, were found 50 feet down on the bottom of the ocean off Santa Barbara Island. Many of them were deteriorating and were probably either already dead or dying. Though this colonial animal can be 23 inches long, I would estimate those I photographed were less than half that size.
Out of the water, this rigid, tube-shaped creature retains its shape. One end is closed, the other is open. The outer surface is shiny and covered with little bumps called processes. The tube is comprised of zooids (individual animals) that can be one-third inch long and pale pink, yellowish or bluish. Cilia push water through the gill slits by expanding and contracting, allowing plankton and other food particles to be caught in mucus filters as the colony moves through the water. P. atlanticum is bioluminescent and capable of producing a bright blue-green light.
The string jellyfish, Apolemia uvaria, is a member of the fascinating phylum Cnidaria (the “C” is silent), which means “nettle” in Greek. Cnidarians have two forms, polyps and medusae. The latter (i.e., jellyfish) are unattached, upside down versions of polyps. Phylum members also include corals, sea pens, gorgonians and hydroids. All of them are carnivores and all have nematocysts, used for defense and catching food. 
The string jellyfish is also a member of the class Hydrozoa and the order Siphonophorae. It is easy to see how Apolemia uvaria got its common name. It is not easy to see the features that make it a jellyfish. I would love to have a look at this creature under a microscope.
There were quite a few string jellies in the water off Santa Barbara Island during September 2001. They ranged in length from a few inches to several feet. I had seen them before and heard they could sting so I tried to avoid them. Somehow, however, I surfaced with part of a string on the top of my second stage, next to my lips. The sting didn’t become incredibly painful until I was out of the water. Someone told me to submerge my swollen lip in seawater. This did alleviate the pain and swelling and I was very happy to make the next dive.
A. uvaria has several common names, including barbed wire jellyfish, which seems very apropos. It is found midwater in oceans worldwide. A string of these colonial animals can be more than nine feet long and two inches in diameter. The central string contains groups of pink and white tentacles. When it stretches out to catch plankton, it resembles a drift net. Unfortunately, one string caught me! Although jellies have a reputation as stingers, the string jelly is the only creature discussed here that does so.
The comb jelly, Beroe forskalii, is a member of the Ctenophore phylum, the members of which live in oceans worldwide. The phylum name comes from the Greek for “comb” and “carry.” These are the largest animals that use cilia to swim. Adults of the various species can be nearly six inches long. They resemble cnidarians in that their bodies are a mass of jelly. There is one layer of cells on the outside and another inside. In ctenophores, these layers are two cells deep, those of cnidarians are only one cell deep. Although neither ctenophores or cnidarians have brains they do have a decentralized nerve net.
This jelly has eight rows of comb-like plates, which diffract light and produce a “shimmering, rainbow effect,” according to the Monterey Bay Aquarium web site. They are found in most oceans of the world. The innocent appearing B. forskalii, which resembles a tiny flashing neon sign that swims, is a voracious predator, able to swallow creatures (usually other jellies) as big or slightly bigger than it is. Before it makes a meal out of one of its cousins, it inflates itself and expands its stomach, becoming ball-shaped. Then it opens the very large mouth at one end and uses rows of microscopic but sharp and flexible “teeth” (actually cilia) to grasp its prey. It can swallow several jellies, one after another. I’ve seen a video of this and the comb jelly looks like a transparent, disembodied head as it chows down in mid water. You can clearly see everything it has eaten inside it. It is truly astonishing.
Thanks to Genny and Shane Anderson as well as Steve Haddock of Monterey Bay Aquarium Research Institute for their help in the preparation of this article. 
Pelagic  Invertebrate Stats
Thetys vagina
Phylum: Chordata
Subphylum: Tunicata
Class: Thaliacea
Order: Salpida
Family: Salpidae
Pyrosoma atlanticum
Phylum: Chordata
Subphylum: Tunicata
Class: Thaliacea
Order: Pyrosomatida
Family: Pyrosomatidae
Apolemia uvaria
Phylum: Cnidaria
Class: Hydrozoa
Order: Siphonophorae
Family: Apolemiidae
Beroe forskalii
Phylum: Ctenophora
Class: Nuda
Order: Beroida
Family: Beroidae 
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