I must make an apology. Last Wet Beast Wednesday I declared that I was balancing out the vertebrate/invertebrate balance of this series. However, I realized that all the invertebrates I've covered have been arthropods. This is a grave misrepresentation of invertebrate diversity and I must make amends. Thus, this week we're returning to the no bone zone and talking about siphonophores.
(Image: a collage of different siphonophore species, sourced from Wikipedia)
Siphonophores are cnidarians: soft-bodies, radially symmetrical organisms that also include jellyfish, anemones, and corals. They are definitely amongst the weirdest of cnidarians. Most of them look like either a jellyfish or an anemone, but siphonophores run a whole gamut of shapes: from the jellyfish-like Portugese man-o-war to the vaguely comb jelly-like Praya dubia, to whatever the heck this thing is
It looks like a bunch of glass vases on a stick (image: Marrus orthocanna)
Their appearance is the least weird thing abut them. Siphonophores are colonial organisms. Each individual animal is composed of multiple smaller organisms called zooids. All zooids share the same DNA, but are specialized to perform different functions as determined by their morphology. Because each type of zooid is specialized to perform a single function, they are dependent on each other to survive and cannot exist alone. Cnidarians exist in two forms, which many will switch between during their life cycle: sessile polyps and mobile medusas. Siphonophores do this differently, with each zooid type being either a medusa of a polyp. Biologists have described multiple types of zooids found across many species of siphonophore. These include nectophores (used for propulsion), gastrozooids (used for digestion), palpons (used to regulate gastrovascular fluids), gonophores (used in reproduction), and pneumatophores (gas-filled floats only found in some species).
(image: a diagram of different zooid types. Source)
There are 175 known species of siphonophore. The majority of species are pelagic, remaining in open water their entires lives, though a few reside on the sea floor. While they do have some ability to direct their movements, like jellyfish, they are often pulled along by currents. The majority of species live in the deep sea and deep-sea species are often larger than shallow-sea species. Like their jellyfish and anemone relatives, siphonophores are predators who use tentacles to capture prey. Each tentacle is covered with stinging cells called nematocysts that fire venom-coated barbs into organisms that touch them. The prey is then pulled into gastrozooids and digested. Most species are capable of bioluminescence, which is likely used for defense. Some species also develop bioluminescent lures used to attract prey. Some siphonophore can get extremely large, with the species Praya dubia reaching u to 50 meters (160 ft), making it longer than a blue whale and possibly the longest animal in the world depending on how you measure the bootlace worm.
(image: a Praya dubia catching fish)
Siphonophores reproduce with varying methods, some of which are poorly-understood. A new colony usually begins form a single fertilized egg. This egg hatched into a single protozooid that then produces other zooids via budding. In many species, the protozooid will form a central stalk from which groups of other zooids will bud. Other species use polyps that can be ejected into the water carrying eggs and sperm which they use to fertilize themselves. In any case, special zooids called gonophores are used to make the gametes. Different species are either dioecious (each colony has either male or female gonophores) or monoecious (each colony contains both male and female gonophores.
(Image: a diagram of a siphonophore life cycle. Source)
Cnidarians are believed to have existed since the precambrian, though as soft-bodied animals fossilize more rarely, the exact origin of siphonophores is not known. Some scientists speculate that the colonial nature of siphonophores may represent an early stage in the development of true organ systems. If this is correct, the development would go something along the lines of colonies of single-celled organisms -> colonies of single-celled organisms with differentiated functions -> siphonophore-like colonial organisms -> individual organisms with differentiated organ systems.
This one's called a "long, stringy, stingy thingy". No really, look it up (image: Apolemia uvaria)
The most famous siphonophore and the first one described is Physalia physalis, the Portugese man-o-war or bluebottle. It is a neuston, an organism that lives at the boundary between water and air. Its most notable feature is the enlarged float filled with carbon monoxide that keeps it floating at the surface of the water. The float functions as a sail, letting it travel thousands of miles. Stinging tentacles trail below it to collect prey. The sting of a bluebottle is very painful to humans and can even be lethal in rare circumstances. Many a beach trip has been ruined by a man-o-war sting. Its morphology and development is different enough from other siphonophores that I may dedicate a whole post to it in the future.
(image: a Portugese man-o-war)
