Among the more important predators of midwater habitats in Monterey Bay and elsewhere are the members of the Subclass Siphonophorae. Whether siphonophores are single individuals or colonies of well-integrated polymorphic hydroid and medusoid individuals is a matter of debate among specialists. Some species are truly impressive organisms, forming delicate chains in excess of 100 feet long. Many others are far more diminutive and inconspicuous. All siphonophores are predators and typically spread a veil of nematocyst-laden tentacles for capturing unsuspecting prey. Some of the “individuals” along the length of the stem provide tentacles for defense (dactylozooids) and food capture. Depending on the type of siphonophore, others may function as swimming bells (nectophores), aid flotation (pneumatophore), provide additional defense (bracts), digest prey (gastrozooids), or serve for reproductive functions (gonozooids). The central stem to which all these units are attached is hollow. The gastrovascular cavity passes throughout the stem, and extends into the float (if present), swimming bells, tentacles, bracts, gastrozooids and gonozooids.
A bewildering array of about 150 species inhabit the oceans. Many present difficulties in identification, even for experts. Particularly for large-chain species that typically live in midwater habitats, often all that is seen near the surface are pieces of the chain. Plankton tows and other collection methods tend to break these delicate species into unidentifiable bits. For this reason, the density of siphonophores in midwater habitats has probably been grossly underestimated since they are not properly counted. Explorations using submersible and remotely operated vehicles are revealing a far greater ecological importance than previously realized. Some of the smaller calycophoran siphonophores can survive some net trauma, but are difficult to identify even when intact. Siphonophores are not common visitors to surface waters of the West Coast. The species presented here represent the species that will most likely be observed by a casual observer, either while boating or diving.
The siphonophore that most people have heard of is the notorious Portuguese man-of-war, Physalia (Order Cystonecta). The cystonects possess a gas-filled float known as the pneumatophore that keeps them at the surface with tentacles trailing below. They lack nectophores and gelatinous bracts. Physalia only ventures as far north as southern California, which is fortunate for swimmers and divers farther north since it packs a powerful sting. It possesses a large number of gastrozooids that lack tentacles, with large dactylozooids endowed with long potent tentacles taking up the slack. The tentacles can reach lengths of 20 meters or more and easily dispatch prey such as fish. Beneath the brilliant blue gas float are clusters of smaller dactylozooids and gonozooids.
Siphonophores in the Order Physonecta typically form long chains with an apical pneumatophore followed by a group of muscular swimming bells (nectophores). Posterior to this are feeding and reproductive segments known as cormidia that form the bulk of the chain. Each cormidium typically consists of a gelatinous bract (for buoyancy or protection), a gastrozooid with a long branched tentacle, defensive dactylozooids (one or more), each with an unbranched tentacle, and male and female gonozooids. Frequently all that you will see near the surface of species such as Apolemia is a portion of the chain that has broken away. Even broken chains, however, must be treated with respect since the stinging capability is retained.
Siphonophores that lack an apical pneumatophore but possess at least 1 relatively large swimming bell are grouped in the Order Calycophora. The apex of the stem is hidden by the posterior swimming bell. Calycophorans have repeating cormidia that bud from an anterior growing zone (just below the last swimming bell), with the last cormidium being the oldest. Each cormidium possesses a bract, a gastrozooid, and gonozooids (one sex only) which can also function as swimming bells. Unlike physonect siphonophores however, the cormidia lack dactylozooids. Cormidia can detach and then form free-swimming reproductive structures known as eudoxids. Whereas certain calycophoran species may form chains that are in excess of 100 feet long, many are tiny, inconspicuous active swimmers with a rocket-shaped swimming bell.
The fertilized eggs of siphonophores develop directly into planulae. Depending on the type of siphonophore, the larva may then form what resembles a swimming bell, pneumatophore or gastrozooid. Typically the larval swimming bell is replaced by an adult version. Development into a complete siphonophore is initiated by a budding process that forms thesteadily elongating and differentiating chain.