Siphonophorae
Introduction

Siphonophores are complex, highly polymorphic hydrozoan cnidarians, whose "colonies" are formed by many medusoid and polypoid "individuals" that function physiologically as a single entity. They occur in a bewildering variety of shapes and sizes, from about 1 mm to several tens of metres in length. However, because of their fragility they are difficult to study both taxonomically and ecologically. The most famous, and first to be described, siphonophore is the Portuguese Man O'War, Physalia physalis (Linné, 1758), that floats at the surface of the ocean, with its stinging tentacles hanging down many metres into the water below. However, this species is exceptional, and almost all other siphonophore species are permanently planktonic, exclusively marine animals. The only other exception is a small group, of the family Rhodaliidae, that have become benthic attaching themselves to the sea-bed like tethered air-balloons.

There are 3 basic types (orders) of siphonophores that are classified according to whether an apical, gas-filled float is present (Cystonectae and Physonectae) or absent (Calycophorae). Those with a float are sub-divided on the basis of the presence (Physonectae) or absence (Cystonectae) of a group of swimming bells immediately below the float. About two-thirds of the ca. 150-160 currently recognised species belong to the Order Calycophorae, and these predominate in samples collected by nets. However, from limited collections with submersibles, about two-thirds of the species and three-quarters of the specimens are physonects. This is probably because most calycophoran species are small and difficult to see, while many physonects are large and brightly coloured, and their spectacular appearance quickly draws one's attention to them. Many of the species collected by submersible have proved to be new to science. Thus, even though net collections have already demonstrated the importance of siphonophores in the marine ecosystem, the true significance of these animals can only be appreciated fully by combining the data from such collections with in situ observations.

Siphonophores are present throughout the World's oceans, and throughout the water column down to a depth of at least 4500 m, although individual species may have restricted latitudinal or depth ranges. The majority are truly oceanic, but a few species are neritic, occurring in shallow inshore waters where the salinity is not greatly reduced. In addition, in regions of high salinity, such as the Red Sea, species diversity can be greatly reduced.

Unlike most other hydrozoan coelenterates, siphonophores do not show an alternation of generations between an attached, asexual polyp stage and a free-swimming, sexual medusa. Instead, modified versions of both stages are found together, attached to the stem of the free-floating animal. Totton (1965) considered the fully grown siphonophore to be an enlarged, larval nurse carrier, which he called a paedophore. This does not itself become sexually mature, but buds off the adult sexual medusoids, which may be released, along with other stem components, to lead a separate life. Individual animals of most species appear to produce medusoids of both sexes, but not necessarily simultaneously. However, for a few species, such as the Portuguese Man O'War, each animal produces medusoids of only one sex. Fertilisation is external and development proceeds rapidly over a period of 2 to 3 weeks. A larval nectophore and/or bracts are usually developed first, and these can be morphologically different from those of the adult animal.

Little is known about the life span of an individual siphonophore, but it is presumed that some of the smaller species live for only a few months, while some of the larger ones may survive for 10 or more years. The abundance of the shorter-lived species thus varies seasonally, and the period of abundance varies from species to species. However, as Mackie et al. (1987) discuss, one cannot always assume that these variations in abundance are a true reflection of seasonal change, as temporal changes in their horizontal or vertical distribution may not have been observed.

Much of the biology of siphonophores has been reviewed by Mackie et al. (1987). They are important predators and often occur in such numbers that they form the dominant group of carnivores (Pugh, 1984). They feed primarily on small crustaceans that they ensnare by discharging their nematocysts. Some species, however, feed on soft-bodied animals, such as small fish, which they capture by injecting poison from penetrant nematocysts. Most of the nematocysts are grouped on the animal's tentacles, which can extend to considerable lengths, but other structures also may possess them.

Unlike many jellyfish, which actively pull or push their tentacles through the water, siphonophores are passive feeders, setting tentacular nets that often form complex patterns. Biggs (1977) discussed some of the methods that siphonophores use to capture their prey. He noted that the fishing cycle consisted of two phases: a fishing period, when the tentacles are spread out and the animal waits for a prey item to swim into its net; and a swimming phase, when the tentacles are retracted and at the end of which the fishing net is reset. The whole process is highly coordinated and the disposition of the tentacles is by no means a tangled mess. The exact fishing posture depends on several factors, including the animal's ability to remain neutrally buoyant, and the degree to which the stem can be extended. Some merely use an extended "long-line" posture, with the tentacles hanging down from the buoyant stem. Species using this posture tend to spend long periods fishing, and to capture larger prey items. Their "sit and wait" strategy, which expends little energy, relies on the greater swimming speed of the larger prey to increase the chances of an encounter. The active swimmers usually set complex nets, and some use a "veronica" movement that results in the tentacles being spread out from the stem to form a helix of 2 or 3 turns (Mackie and Boag, 1963).

The three-dimensional disposition of the tentacles allows a greater volume of water to be searched for prey. Madin (1988) has calculated that the encounter volume, within which the tentacles are disposed, may be 0.5 mö3, and probably is even greater for some of the large physonect species. Others "squid-jig", periodically contracting and relaxing an individual tentacle, or its side branches, to enhance prey capture. In addition, several species have evolved tentacular structures that appear to mimic other organisms in order to lure in their prey (Purcell, 1980; Pugh, 1989). Although many siphonophore species are generalist feeders, taking any prey item, within a certain size range, that they encounter, some appear to be highly selective and feed only on certain taxa (Purcell, 1981). The siphonophore species that have adopted these various feeding strategies can have different patterns of geographical and vertical distribution. This has lead Pugh (1986, 1991) to speculate that these different distributions should reflect that of their preferred prey items.

The role of siphonophores in marine food chains is unclear, but they do not appear to be a dead end branch. Despite their gelatinous nature, they are preyed upon by a variety of other organisms, particularly other gelatinous carnivores such as medusae, ctenophores and pelagic molluscs. Several species of fish also feed on them, as do turtles. In addition, siphonophores have associations with a variety of organisms, of which hyperiid amphipods probably are the most important. The nature of the association can vary from simple phoresis, where the other animal simply is carried about, to total parasitism, as with some amphipods whose juvenile stages develop and gradually consume parts of the siphonophore (see Harbison et al., 1977). Many of these amphipod associations are species specific, as is also that of the nudibranch, Cephalopyge trematoides, which feeds exclusively on Nanomia bijuga (Sentz-Braconnot and Carré, 1966). Fish also are known to associated with siphonophores. The best known example is the association between the Man O’War fish, Nomeus gronovii, and the Portuguese Man O'War (see Totton, 1960). In situ observations also have shown associations of fish with siphonophores, including Caristius sp. and a cystonect species (Janssen et al., 1989). Various parasites have been observed on and within siphonophores (see Totton, 1965), but there is little detailed information.