INVERTEBRATE ZOOLOGY - G. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

SECTION DIPLOMA-LAYERED ANIMALS (DIPLOBLASTICA), OR RADIATA

PHYLUM CNIDARIA, OR COELENTERATA

SUBPHYLUM MEDUSOZOA

CLASS SIPHONOPHORE (SIPHONOPHORA)

Exclusively marine, swimming polymorphic colonies comprising individuals of both polypoid and medusoid origin. Colony sizes range from 1 cm to 20 m. About 170 species are known, distributed in seas and oceans with high Water salinity. They occur from surface water layers down to depths of 5,000 m.

Structure. A siphonophore colony (Fig. 89) consists of a main stem bearing individuals that vary in Structure and function. At the apex lies an air sac, the pneumatophore, the floor of which is lined with Glandular Epithelium capable of secreting a gas composition similar to atmospheric air. At the upper pole of the pneumatophore, there is an aperture surrounded by a sphincter Muscle. Thanks to the air-filled pneumatophore, the colony floats near the water surface; during a storm, the gas is expelled from the pneumatophore, and the colony submerges to a greater depth. Below the pneumatophore lie swimming bells, or nectophores—undeveloped medusoid individuals devoid of tentacles and mouths. Other individuals of the colony are arranged on the stem in groups called cormidia. Each cormidium is covered above by a bract, which is a modified polyp. A cormidium includes: a feeding polyp (gastrozooid), an excretory polyp (palpon), and two sexual medusoids (gonophores). The gastrozooid has a large Mouth and a branched tentacle (Pepsin/tentilla), which in some species can reach 10 m in length. Using the tentilla, which bear batteries of stinging Cells, prey is captured and paralyzed. Food is digested in the gastrozoial gastral cavity and then distributed throughout the colony, as the gastral cavity of the colony stem communicates with the gastral cavities of individual polyps. Palpons are mouthless polyps with a short, unbranched tentacle. Gonophores resemble medusae attached to the colony by an umbrellar margin, or sometimes take the form of sacs with Gonads. A single colony bears both male and female gonophores.

Reproduction. During sexual reproduction, eggs formed after Fertilization in female gonophores develop into swimming larvae known as planulae. A planula transforms into a larva of more complex structure, on which colony individuals subsequently develop by budding.

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Fig. 89. Diagram of siphonophore structure (after Kholodkovsky):

1 - pneumatophore; 2 - swimming bell (nectophore); 3 - sexual individuals (gonophores); 4 - covering plate (bract);

5 - palpon; 6 - feeding polyp (gastrozooid); 7 - tentacle (pepsin); 8 - colony stem

Among siphonophores, the best known is the Portuguese man-o'-war (Physalia physalis, Fig. 90), a large species featuring a large pneumatophore (up to 30 cm) colored in blue, violet, and purple hues, and long tentacles (up to 2–3 m). The pneumatophore bears an S-shaped curved crest that allows the colony to sail at an angle to the wind and maneuver, alternately approaching and moving away from the shore. With its bright coloration, Physalia resembles ancient Portuguese galleons.

Fig. 90. Portuguese man-o'-war siphonophore (Physalia)

The swimming colonies of siphonophores and the aforementioned chondrophores belong to a distinct ecological group of aquatic organisms known as the pleuston (the assemblage of planktonic organisms inhabiting the surface microlayer of water). Specific communities of aquatic organisms form around them: some use them as a means of transportation, while others (various species of Mollusks and crustaceans) feed on their Tissues or seek refuge from predators within the zone of Influence of the colony.