In the limit between being
individual and colonial.
In spite of the danger, has always fascinated me this animal group (phylum), the Cnidarians , formerly known as Coelenterata. The common feature of all is a type of stinging cell that does not exist in any other group. However, the variety of shapes, colors and lifestyles of this group is impressive. In fact ranges from hydra pools (we studied in school and have rarely seen since), the reef-building corals, anemones and jellyfish scare us both on the beach. And some animals that are not known, nor the general public or by scientists.
But what strikes me is neither the danger nor the extraordinary beauty of one of its components, but many of them forced me to wonder what is the concept of the individual in the animal kingdom.
Junior or Portuguese caravel, Phisalia phisalis , photographed on a beach in Mozambique.
This photo shows the small size of the specimen. However,
the tentacles of this animal can move 30 meters in length.
Possibly it is a very young colony, since only a polyp is seen hunter.
An anemone is clearly an isolated individual, that it grows much comes to divide and produce two new copies, which are separate and independent life. But a branch or gorgonian coral, which are produced by budding new polyps that remain together for life to a common base, "are still individuals but never be isolated? Science tells us says yes. Among other things because they all perform the same function in the colony. This brings us to the concept of colonial animal, which in some cases all members of the colony are the same, while in others there are majors and some are involved in prey capture, digestion and others, for example, others only reproduction. In one of these colonies an isolated individual can not live if it has the support of other specialists. What is then the individual?
In the photos above and below, these jellyfish-like animals are they individuals or colonies? Well, not really jellyfish are colonies of polyps that have a common portion. In the two species that I put in pictures, even of two different orders, the Portuguese man up and down Velella agree that the colony of polyps hanging from a raft.
The animal (or animals) in the picture below belongs (or fall) to the species Velella Velella, a disk less than 10 cm in diameter that floats on the sea surface with a fold at the top shaped candle and a small number of polyps that hang from its lower
Velella Velella also found stranded on a beach in Mozambique. 
The drawing is a schematic of a cut made in their medium and in it we see the communal area (consisting of the disc and the wing) and polyps units: In the center is a large polyp, called gastrozoide, which is responsible for digesting food (small fish and other critters) in the outer edges of a polyp transformed into mere tentacles hunters (dactilozoides) loaded with stinging cells, and among them, three rows of polyps players (gonozoides), which is dedicated to producing children by budding. polyps are actually doing the function organ, which would coincide with the digestive system, arms, and the reproductive system in a more developed animal.
With those children who commented that are produced by budding comes another thing that I love of these animals: two generations to look completely different animals. Because those suckers are not new Velella similar to those of photography and drawing, no, that would be easy. What is occurring in this small jellyfish polyps players are very simple and nothing like this animal (or animal colony). These medusillas not float, are able to swim and are the sexual generation, with gonads that produce eggs and sperm. Their fertilization it produces an egg from which a larva that eventually produce a new colony floating, not without several body changes.
With those children who commented that are produced by budding comes another thing that I love of these animals: two generations to look completely different animals. Because those suckers are not new Velella similar to those of photography and drawing, no, that would be easy. What is occurring in this small jellyfish polyps players are very simple and nothing like this animal (or animal colony). These medusillas not float, are able to swim and are the sexual generation, with gonads that produce eggs and sperm. Their fertilization it produces an egg from which a larva that eventually produce a new colony floating, not without several body changes.
The Velella are from time to time on English beaches and are not at all dangerous, but the Portuguese caravels are the "jellyfish" more dangerous out there. The bite of a large specimen with many tentacles several meters long, can kill a person. In our coasts, both Atlantic and Cantabrian and Mediterranean, have been enough examples in recent years. Then I link two notes of prey, but any search will give us many more, as well as information about its danger, the effects and treatment of bites.
- Basque Journal in the Cantabrian Sea.
A final curious aspect of these species is their ability to be carried by the wind, but either way, but thanks to the orientation of its body floating or sailing, as in a boat can heel over to different directions and not necessarily in the same wind that marks the sea current.
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