Siphonophora are very polymorph members of the cnidarian Superclass Hydrozoa. The fully developed siphonophore is organised as a colony, made up of highly modified medusae and polyps. Polymorphism is in this class far much further carried out than in other hydrozoan classes, the colony is a real superorganism. Furthermore, in contrast to most other hydrozoans, Siphonophora have no alternation of generations with a free living medusa and an attached polypoid stage, but both developmental stages are attached to the stem of the free floating colony. About 150 siphonophore species are currently known; they are strictly marine and rather stenohaline and appear only rarely in inshore waters or seas where salinities are below 33-44 ä. Siphonophora are holoplanktonic, except for the Families Physaliidae (pleuston) and Rhodaliidae (benthos). They occur in all worlds’ oceans from the surface to the deep-sea and are most common in warmer waters. A few species tend to be more neritic than oceanic, or regularly occur in coastal waters, entered with ocean currents or strong winds.
Regarding the North Sea, Siphonophora are unlikely to occur in the central and southern part of the area. Being of oceanic origin, they are not really adapted to the conditions of shallow, eurytherm and variable saline waters. However, some species could enter the North Sea from the South. In particular Agalma elegans and Physophora hydrostatica have been reported as exceptional species for this part of the area (Totton and Fraser, 1957a, 1957b). The Portuguese man-of-war (Physalia physalis ) that floats on the sea surface (pleuston), has a worldwide distribution in tropical and subtropical waters. It regularly strands on the southwest coasts of Great Britain, but seem not enter the Southern North Sea through the Dover Strait. However, there is a record of a rare stranding after a westerly gale on the coast of Belgium (Lameere, 1912, in Wilson, 1947). This siphonophore species is not taken into account for our area. Also in the Northern North Sea, Siphonophora are not indigenous; however, they enter there with North Atlantic waters that are introduced in the area by the continental slope current flowing along its eastern and northern boundaries.
Siphonophora, as coelenterates, consist of two cell layers: the outer ectoderm and the inner endoderm. Both layers are separated by the mesogloea, a more or less thickened amorphous layer.
To keep afloat and help to maintain vertical position, siphonophores have a gas-filled float (pneumatophore) at the very top, or swimming bells (nectophore) in the upper part of the colony, or both. The pneumatophore is thought to be developed directly from the larval stage and probably is a highly modified polyp. The gas content in floats can be regulated in order to keep the colony at particular depth. The nectophore is an asexual medusoid individual, with many of the structures common to free-swimming medusae, although mouth, tentacles and sense organs are lost. The swimming bell, with its musculated wall, functions for the jet propulsion, and also give buoyancy to the colony due to a large amount of mesogloea in the bells.
There are three kinds of polypoid zooids: (1) the digestive zooids, or gastrozooid, located on the siphosome, with a mouth and usually with a single branching tentacle, (2) the protective and prey-catching zooids, or dactylozooid, they are slender and tentacle-like, without a mouth and full with nematocysts, and finally (3) the reproductive zooids, or gonozooid, producing sexual medusoids: gonophores that are retained to the colony and form either eggs or sperm (they resemble gastrozooids, but have no mouth).
Division of the Class Siphonophora
The class is divided in three orders, based on the structure of the colony. Between brackets is the number of species in our area:
¥ Order Physonectae (15), with a small pneumatophore and a long chain of nectophores, followed by a long train of cormidia.
¥ Order Calycophorae (4), includes colonies with swimming bells (nectophore) but no float (pneumatophore). They have a long tubular stem extending from the swimming bell, from which various types of zooids are budded in groups that are called cormidia. The cormidium consists of a shield-like bract, a gastrozooid and one or more gonophores that may function as a swimming bell. Cormidium break loose from the parent colony and start an independent life. At which time they are called eudoxid.
¥ Order Cystonectae (1?), with large pneumatophore and no nectophores (e.g. the surface dwelling (sub)tropical Physalia, Portuguese man-of-war. However, members of this order are not likely to be found in the North Sea).
The pneumatophore or float is a usually small invaginated apical structure consisting of a cavity into which gas is excreted by the underlying pneumadenia or gas-gland. The pneumadenia is usually very small, but in the neustonic Physalia (Order Cystonectae) and the benthic Family Rhodaliidae (Order Physonectae), it is greatly enlarged.
Most siphonophores have a long stem onto which the various appendages are attached, grouped in such a way that two zones can be recognised: (1) the nectosome, situated at the aboral end of the colony (absent in the Cystonectae), and below (2) the siphosome. The nectosome bears – with one exception - only asexual medusoid nectophores (swimming bells). The siphosome usually makes up the longest part of the colony and consists of polypoids and medusoids attached to the stem.
When the colony is fully developed, it is considered to be in the polygastric stage, because it contains many gastrozooids (feeding and digestive polyps). Totton (1965) considers the fully grown siphonophore as a larval nurse carrier, a paedophore, from which medusoid gonopohores are budded-off.
Regarding the orientation of the various parts of the siphonophoran colony, the ventral side is the axial side — towards the stem— and the dorsal side is the abaxial side — farthest from the stem. The apical or anterior side is to the aboral end, i.e. the furthest from the oldest gastrozooid.
Siphonophores are very fragile and often fall apart upon sampling. What usually remains for study are the nectophores, therefore these are taxonomically most important and their characters are generally used for identification. The key to the Siphonophora of the North Sea starts at Page 15: Siphonophora.
Below, the important morphological details are stipulated.
— The nectophores of the Physonectae:
(1) The arrangement of the lateral ridges,
(2) The course of the radial canals on the nectosac. By contractions of the muscles in the wall of the nectosac, water is pressed out through the ostium (basal opening), by which jet propulsion the colony moves through the water,
(3) The shape of the apical wings (or apico-lateral processes),
(4) The relative size of the thrust block — the region that lies against the nectosomal stem. The nectophore of the Calycophorae is a simple rounded structure. There is a gutter-like furrow on the ventral side, the hydroecium. Along the dorsal surface of the hydroecium runs the somatocyst
— The nectophores of the Calycophorae are streamlined, usually there are only one or two of them present in the colony. Calicophoran siphonophores are rapid swimmers. The first (in development), apical nectophore is called the anterior one; the second nectophore, if present, is the posterior nectophore. Important morphological characters of, in particular the anterior nectophore are:
(1) The pattern and number of longitudinal ridges — and in the case of Lensia, the eventual presence of a transverse velar ridge;
(2) The eventual presence of teeth around the ostium;
(3) The shape of the somatocyst;
(4) The shape of the basal lamella (mouth plate) — a ventral process below the ostium;
(5) The angle at which the basal facet of the mouth plate is inclined with respect to the main axis of the nectophore.
— The siphosome of the Physonectae
The siphosome is generally much longer than the nectosome, and consists of a variety of structures (polypoid as well as medusoid in origin) with different functions. Generally, these are grouped together to form a cormidium. Each cormidium consists of a single gastrozooid with an attached tentacle, that is, a long contractible tube often with numerous tentilla (side branches), provided with nematocysts. The nematocysts are used to capture prey. Cormidia of Physonectae may also have a number of palpons — presumed to be reduced gastrozooids and carrying eventually a small tentacle or palpacle. Furthermore, cormidia also have bracts — absent in Cystonectae (e.g. Physalis, not in our area of interest), leaf-like structures in Physonectae, and more complexly organised in the Calycophorae, where they are part of the eudoxid (sexual stage). Bracts serve as protection but may also contribute to the buoyancy when provided with a considerable volume of mesogloea.
— Eudoxid stage. In the Calycophorae a cormidium can become detached, it consists then of a single bract, a gastrozooid and a sexual gonophore. Eudoxids are identified mainly on the general shape of the bract and the pattern of the canals therein. This might be, however, difficult. The eudoxid bracts of the Family Diphyidae is conical or helmet-shaped. The basal process is called the neck shield; the canals are reduced to a simple swelling, the phyllocyst. The eudoxid bracts of the Family Prayidae can display a complex arrangement of canals with, however, a basic pattern of six canals: two hydroecial, two longitudinal, one dorsal, and one ventral canal. The longitudinal canals might be reduced to spur canals; where the main canals come together, there may be a central organ or median swelling present.
The peadophore (sensu Totton, 1965), when fully developed, produces medusoid gonophores that are either male or female and, when finally adult, produce sperm or eggs respectively. As far as known, only the cystonect Physalia physalis is dioecious, i.e. an animal carries either male or female gonozooids. All other siphonophore species are monoecious, i.e. carry both male and female gonozooids in one animal. The adult gonophores may become free-swimming medusae releasing the gametes (as in the Physonectae), or remain attached to the (free swimming) eudoxid, as is the case in the Calycophora. External fertilisation gives rise to a planuloid-actinula larva.
[Adapted from Kirkpatrick and Pugh, 1984; Totton, 1965]
The following siphonophore species are included in this volume: