The Cnidaria are both heterotrophic and autotrophic regarding
carbon. Although the presence of zooxanthellae in animal tissues accounts
for up to 150% of the energy necessary for corals, they remain predators
by their behaviour. It has been shown that the complex mechanisms used by
corals to capture plankton, apply to detritus as well as dissolved organic
matter. Heterotrophic nutrition uses tentacles, mucus, ciliary cells and
especially cnidocytes. The cnidocytes on the tentacles capture and paralyse
the prey, and then the tentacles convey it to the mouth. The prey and various
organic particles can be trapped at the cœnosarc level, where they
become stuck in the mucus. Due to the combined movements of the cilia and
the tentacles, the aggregates are transported towards the mouth and then
to the cœlenteron to be digested. Sometimes the mesenteric filaments
can be ejected for external digestion. Corals are also capable of directly
absorbing dissolved organic matter as well as organic matter in suspension.
Cnidocyte: stinging cell
When capturing their prey, the Cnidaria use a type of cell which is characteristic
of the whole group, the cnidocyte. This is a sensorial secreting cell,
30 types of which have been described in the Cnidaria phylum.
This highly specialised cell is used to capture prey and for the protection
of the organism. It consists of a capsule enclosing a stinging filament,
the cnidocyst, wrapped round an axe turned inside like the finger of a
glove, swimming in a liquid and generally toxic matrix which is injected
into the eventual prey by the action of the cnidocyst. The cell has a
cnidocil orientated towards the exterior, whose mechanical stimulation
sets off an electric signal provoking the explosive exocytosis of the
cnidocyst. In Hydra, this exocytosis mechanism lasts less than 3 milliseconds.
Inside the capsule, the original osmotic pressure is of 1,5 107 Pa, due
to the presence of inorganic cations (K+,
Mg2+ ou Ca2+)
and of some rare polyions. The cnidocysts only function once and cannot
regenerate. The cells are replaced by new cells derived from the differentiation
of pluripotent cells, the interstitial cells.
Mucus: adaptation characteristics
of a benthic organism
In corals, mucus is used in such different processes as nutrition, protection
against sedimentation and desiccation when emerged during low tide. The
mucus is a hydrophilic substance made up of glycolipid and glycoprotein
complexes, characterized by a carbon-phosphorus liaison resisting hydrolysis,
which makes it very different from that of other invertebrates and vertebrates.
At the surface of the colony, the mucus is colonized by a microflora, and
also traps suspended matter. After sloughing, it is then rapidly degraded
and recycled by bacteria and zooplankton. The mucus production is accentuated
at times of stress provoked for example by a hyper-sedimentation. The mucus
traps the particles and is then eliminated by ciliary movement.
