Coelenterates ("hollow gut") are predators that feed on other fish, crustaceans, and mollusks. They are
radially symmetric animals of simple structure (95% water) and exist in two predominant life-forms, either as
sedentary, asexual polyps (hydroids) or as free-swimming and sexual medusae. They are the lowest form of
life organized into different layers. Generally the polyps are saclike creatures attached to the substrate at the
caudal (aboral) end, with a single orifice or mouth at the upper end surrounded by stinging tentacles
(dactylozooids). This form predominates in the hydrozoans and anthozoans. The medusa is a bell-shaped
creature, with a floating gelatinous umbrella from which hang an elongated tubular mouth and marginal
nematocyst-bearing tentacles. This form predominates in the scyphozoans and is also found in the
hydrozoans.
The intracytoplasmic stinging organelles, or nematocysts (cnidocytes), are located on the outer
epithelial surfaces of the tentacles or near the mouth and are triggered by contact with the victim's body
surface. The nematocyst is contained within an outer capsule called the cnidoblast, to which is attached a
single pointed "trigger" or cnidocil. The nematocyst (3 to 10
µm in diameter) is filled with fluid and contains
a hollow, sharply pointed, coiled or folded "thread" tubule (nema). This tubule may attain lengths of 200 to
850
µm and is sufficiently hardy to penetrate a surgical glove. In the undischarged state, the toxin is located in
the folds and invaginations of the tubule's membrane. The tubule is lined with hollow barbs, which help it
penetrate and anchor into the victim. In the undischarged state, the barbs occupy the lumen of the twisted and
folded tubule. When the cnidocil is stimulated, either by physical contact or by a chemoreceptor mechanism,
it causes the opening of a trap door (operculum) in the cnidoblast, and the venom-bearing tubule is everted
within 3 microseconds. This exocytosis has been hypothesized to occur because of osmotic swelling of the
capsular matrix caused by influx of water (leading to a hydrostatic pressure of up to 150 atm), release of
intrinsic tensile forces (up to 375 MPa on the inner capsule wall), or deformation of the wall-induced internal
pressure. The sharp tip of the thread tube enters the victim's skin and envenomation occurs as toxin is
translocated by hydrostatic forces from the surface of the everted and extended tubule through the now
helically-arranged and extended hollow barbs. It has been estimated that the velocity of ejection attains 2
m/sec, which corresponds to an acceleration of 40,000 g, with an estimated skin striking force of 2 to 5 PSI. A
human encounter with a large Portuguese man-of-war could conceivably trigger the release of several million
stinging cells. The thread penetrates the epidermis and upper dermis, where the viscous venom diffuses into
the general circulation. The agitated victim moves about and assists the venom's distribution by the muscle-
pump mechanism. Based on mouse studies, it appears that the rapid death of a victim is related to discharge of
venom directly into capillaries, as opposed to that which must diffuse into the bloodstream from the dermis.
In the case of the Indo-Pacific box-jellyfish Chironex fleckeri, which may carry up to 59 tentacles
bearing millions of nematocysts, it is the cigar-shaped microbasic p-mastigophores that are most important in
human envenomation. The capsule of the structure holds a hollow coiled tube and granular matrix. The thread
tube has a thick butt end that is attached to the operculum. The tube contains three rows of helically arranged
spines. When the nematocyst fires into the human victim, the tube everts through the opercular end of the
nematocyst, with the butt anchoring first to keep the nematocyst adherent to the victim. The thread then everts
through the hollow butt and uncoils, presenting the spines and accompanying toxins to the living tissue. While
the major toxic fractions appear to be present in the nematocysts, there appears to be toxic material present in
tentacles denuded of such organelles. The largest nematocysts of C. fleckeri can penetrate human skin to a
depth of 0.9 mm.
Coelenterate venoms are viscous mixtures of proteins, carbohydrates, and other nonproteinaceous
components. To date, they have been difficult to fractionate. Although they are heat labile in vitro, this does
not seem to apply in the clinical setting. The primary difficulties encountered in jellyfish venom purification
are the lack of stability and the tendency of active toxins to adhere to each other and to support matrices.
Cytolytic toxins have been characterized from Physalia physalis, Rhizostoma pulmo, Chironex fleckeri, and
Carybdea marsupialis. Many jellyfish and marine animal venoms generate autonomic neurotoxicity. This
may be a result of their ability to affect ion transport (sodium and calcium in particular), induce channels or
pores in nerve and muscle cell membranes, alter membrane configurations, and release mediators of
inflammation. Coelenterate venoms can target the myocardium, Purkinje fibers, A-V node, and aortic ring, as
well as injure the hepatic P-450 enzyme family.
Clinical Aspects
