It's Not a Jelly: Siphonophores

A siphonophore.

Among the invertebrates, the phylum Cnidaria is one of the best known to the layperson.  This group includes everything from sea anemones to corals and jellies.  The cnidarians  are named for their cnidae; these are the stinging cells unique to their tissues.  Every cnidarian possesses cnidae, but the species vary considerably when it comes to the structure and number of these cells.  You can wander along the beaches of California and touch a giant green anemone or moon jelly without detecting a sting at all, yet one chance collision with a box jelly in Australian waters can send you to the hospital writhing in pain.

Originally, I thought I might shed some light on the true jellies (also called "jellyfish," though they are not fishes).  While the jellies do share a fairly basic body plan, they exhibit a truly extraordinary variety of size, shape and color.  However, the objective is to highlight groups that fly (or float) under our radar, ones that get little attention in mainstream media and even in our educational institutions.

The morphology of the by-the-wind sailor.

Physalia physalia washed ashore.  Watch your step!

One group of cnidarians, the siphonophores, deserves more attention.  The siphonophores often are mistaken for jellies.  It might not apply in every case, but jellies typically can be identified by their conspicuously bell-shaped medusa.  Siphonophores, on the other hand, do not exhibit this familiar shape.  In fact, siphonophores are not individual organisms but colonial cnidarians.  The "body" is created by a large number of individuals, referred to as zooids, that usually have highly specialized functions.  Different zooids of a given colony function in feeding and reproduction.  Some zooids, packed with cnidae, serve in defense.

Group of Velella velella, the by-the-wind-sailor.

Often, the colony is associated with a gas-filled float called a pneumatophore and specialized "swimming bells" called nectophores.  The floats help the colony to maintain its position at the surface, or elsewhere in the water column, and the nectophores help to propel the colony.  Many siphonophores lack structures for swimming and simply "go with the flow.

The blue button, Porpita porpita.

"Physalia physalis, the Portuguese Man-o-War, is probably the most famous of the siphonophores because it is relatively common and issues a painful sting and the marks to go with it.  Most people refer to the Portuguese Man-o-War as a jelly, but it is actually a siphonophore.  It has a large, highly distinctive float that permits it to remain at the surface and trail the colony through the water.  By doing so, the tentacles and zooids can capture small prey items such as small fishes and crustaceans.  It is an impressive sight, with an impressive sting that earns the colony respect on first contact.

Velella that have drifted ashore in masses.

Other small, surface-dwelling siphonophores include the blue button (Porpita porpita) and the by-the-wind sailor (Velella velella).  These drifters are at the mercy of the currents and winds.  On occasion, the by-the-wind sailor washes ashore in great numbers, where the individual colonies die.  The first time I happened upon dried masses of these on the beach, I felt more like I was walking through large drifts of dead leaves (see picture of a smaller aggregation).

Rhizophysa, 3 views of one colony.

Many of the species discovered to date are fragile deep-sea colonies.  Discoveries of new species of siphonophores are not uncommon.  Some species can be observed in relatively shallow waters, where divers might have a chance encounter.  These include species such as Rhizophysa, which can extend its feeding tentacles to increase its size from four or so inches to a couple of feet.  When it comes to size, we cannot escape mention of the species Praya dubia, the giant siphonophore.  Colonies of this midwater species can achieve lengths of 40 meters.  In other words, if this was a single organism, it would technically be the longest animal on the planet (longer than every dinosaur, except perhaps Diplodocus).

A bioluminescent deep-sea species,
found beyond 700 meters.

The steady work of expeditions using  manned or submersibles and remotely operated vehicles (ROVs) undoubtedly will reveal additional diversity in this group.  Studies of these strange creatures at the lab bench should provide some insights in the fields of developmental genetics and perhaps in related areas such as cell communication.

Now, siphonophores do not at all hold up well in captivity.  One will not likely see future aquarium displays of by-the-wind sailors and frail deep-sea species.  Still, with the increasing pool of high-quality images, I can imagine an awe-inspiring gallery with an accurate 130-foot silicon model Praya hanging above. 

Praya sp.

Apolemia lanosa, taken by ROV Tiburon at 1150 meters.