Karl-H. TOMASCHKO. Roland GUCKLER
Universität Ulm, Abteilung Aligemeine Zoologie, 89069 Ulm, Germany
The secretion of defensive chemicals in marine arthropods was described recently in the pycnogonid Pycoogonum litorale, a primitive marine chelicerate (1). In response to mechanical disturbance, P. litorale secrets eight ecdysteroids at a total level of 10-3 M. The ecdysteroids are secreted by epidermal glands which are distributed in large number (up to 30000 per animal) on the entire body surface.
The secreted ecdysteroids act as chemical defense against decapod crustaceans. The common shore crab Carcinus maenas is a very abundant predator in the pycnogonids habitat. Nevertheless, this crab is repelled by P. litorale. It perceives the secreted ecdysteroids by structure-specific membrane associated ecdysteroid receptors at the anterio-dorsal oesophagus. Overcritical concentrations (> 10-5 M) lead to an immediate reflex-like dilatation of the anterior oesophagus. This is part of the normal rejection behaviour and enables the crab to rinse the oesophagus by ventilating it with its mouth parts. This reaction is interpreted as protection mechanism against the uptake of overcritical amounts of ecdysteroids that might interfere with the hormonal equilibrium of the crabs.
Similar rejection behaviour was observed in other decapod crustaceans as well, e.g. in Homarus americanus, Cancer pagurus, in the freshwater crayfish Astacus astacus and even in a land hermit crab Coenobita spec..
The results demonstrate that ecdysteroids in arthropods are not only used as molting hormones, but also as interspecific llelochemicals. Their simultaneous use as hormones and allomones requires specific adaptations as to their synthesis, storage and controlled secretion in pycnogonids. Furthermore, it presupposes the existence of membrane associated ecdysteroid receptors at the mouth parts of the predatory crustaceans. Both specialisations may be the result of a synevolution between ecdysteroid producing organisms and ecdysteroid sensitive arthropods.
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