Critical to the function of multicellular organisms is the specialization of cells, cooperation between cells, and a body plan to sustain integration. Yet the term multicellular organism applies to the order Siphonophorae in more ways than one. Siphonophores can be further defined as a colonial organism and perhaps the most exemplary models. Characteristic of siphonophores are multicellular components known collectively as zooids. The classification of siphonophores as both colonial and as individuals makes them one of nature’s most impressive morphological anomalies.
But what are the benefits of being a colonial organism? The answer is simple: colonialism in nature is an advantageous systematic division of labor within the colony. For example, resources are used more efficiently when active components only exert energy on a specialized task. Claims have even been made that siphonophores have the highest division of labor between zooids and most precise organization of all colonial animals.
The specialized components of siphonophores are zooids. In fact, the zooids of siphonophores are so specialized that they can be restricted to only one function, requiring obligatory cooperation and tight integration. The zooids of a siphonophore are so interdependent, allowing siphonophores to be regarded as colonial organism despite the components.
Siphonophorae is an order of carnivorous organisms which belong to the phylum Cnidaria. Each zooid of the colony is considered either a modified medusa, similar to a free-swimming jellyfish, or a polyp. Both are reminiscent of free-living cnidarians. A variety of zooids exists, functionally specialized and arranged in specific patterns. They vary in distribution across the ocean and different depths as well.
The body plan of a siphonophore is used to further classify a species into distinct suborders. There are three that belong to Siphonophorae: Physonectae, Cystonectae, and Calycophorae. To categorize an organism into a suborder, they must possess at least two out of three classifying segments. Listed respectively from the anterior end of the organism to the posterior end, the segments include a pneumatophore used for buoyancy, the nectosome responsible for locomotion, and the siphosome used for prey capture and additional zooid activity. The pneumatophore and zooids of each segment are attached along a central stalk called the stem . The only members of Siphonophorae composed of all three segments are the physonects. The body plans can be easily visualized in Fig.1 which is a representation of suborders and corresponding body plans.
Despite differences in the body plans, zooids retain four defining characteristics: First, they arise by budding from single protozooid. Second, it is crucial for zooids to distribute nutrients throughout the entire organism; therefore, colonies must be strongly integrated to allow the direct exchange of metabolites to occur. Third, the behavior exhibited by zooids in colonies is controlled by nerves, conducting epithelia, or both.
The final defining characteristic is one which must always be kept in mind. The term zooid has powerful evolutionary connotations. Zooids will retain individuality, despite how specialized they are, or inferior to the developing siphonophore .
Ironically, the pneumatophore of a siphonophore is not actually a zooid, and functions as a float in some cases, but is more likely to function as a sensory structure. Larger pneumatophores hold the colony upright, assisting with other zooid bearing segments which may be long and heavy.
Moving backwards, the specialized medusoid zooids of the nectosome are encountered. These are the nectophores. Nectophores are dedicated solely to the locomotion of the colony. There are no reproductive or feeding structures present. A further examination of nectophore physiology reveals that nectophore zooids grow quickly after budding. Nerves run from the stem to the nectophores, coordinating the swimming contractions.
Finally, at the posterior end of the nectosome begins the siphosome, which bears remaining specialized zooids. The siphosomal growth zone marks is the anterior portion of the siphosome and origin of the siphosomal stem, becoming longer as the colony grows. Pro-buds emerge in the siphosomal growth zone and the attached zooids are organized in a specific repeating pattern. When a cormidia is first developing it is referred to as the pro-bud. a visualization is found in Fig. 2.
The gastrozooids are zooids that specialize in feeding. Efficient prey capture and consumption is achieved by dividing gastrozooids into two regions: the oral hypostome and the aboral basigaster. The hypostome extends furthest with a folded mouth to consume large prey. The basigaster attaches the gastrozooid to the siphosomal stem and has a thick layer of tissue where the maturation of nematocysts occurs.
The tentacle possessed by each gastrozooid is also associated with the basigaster and attaches to the bottom. The tentacle is not a zooid but cooperates with gastrozooids to obtain food. Extending from the tentacle are strings of tentilla tipped with nematocyst batteries. A battery contains a cnidoband filled with nematocysts and an elastic ligament.
Discharge of a battery occurs when the terminal filament of the tentilla is pulled and the spiny penetrative threads of each nematocyst are released into the prey. As the prey struggles it is continuously pierced until the tentacle has contracted enough for consumption by the associated gastrozooid.
In addition to the gastrozooids, are palpons. They are attached to the siphosomal stem, possessing a single palpacle, similar to a tentacle. At the base of the palpon is a region for developing nematocysts which develop in stages like those of gastrozooid. No sensory cells near the tip mean it cannot function as a sensory zooid but it is also thought that they participate as digestion modules despite an inability to feed.
Siphonophores also have a few tricks to lure prey toward themselves. Aggressive mimicry is exhibited when the nematocyst batteries of the siphonophore act as bait by looking similar to copepods or fish larvae. Bioluminescence in marine environments is rare but siphonophores belonging to the genus Erenna have been observed demonstrating the flickering of tentilla. These events may have appeared as prey behavior to small predators which would attract them toward the lures.
Once the ingestion and digestion zooid functions are completed, nutrients must then be transported to the other regions of the colony, sometimes over distances of centimeters or meters. Digested food matter in the stem canal flushing up and down the stem by rhythmic movements of the gastrozooids and palpons . Cilia assist the process of nutrient distribution and valves at the bases of the zooids allow the exchange of fluids with the stem.
Aside from the digestive zooids of the siphosome are the sexual zooids called gonodendra. Each is directly attached to the stem and bears multiple gonophores thought to be greatly reduced medusae. The gonophores of male gonodendrons contain large populations of beginner cells. Female gonodendrons are connected to gonophores which each contain a single oocyte. The protozooid eventually buds off of the gonophores. The stem elongates and buds from two blastogenic zones, otherwise known as the nectosomal and siphosomal growth zones in Fig. 2 begin to mature.
From ornate and colorful to transparent and shapeless, the organization and complexity of siphonophores cannot be paralleled. The homology between free swimming organisms and the zooids establishes that there must clearly be an advantage to such extreme integration. The specialization and coordination between zooids to form a single colonial organism is profound and will always challenge the term ‘individual’ which is certainly worth time for contemplation.