Effects of discharging nematocysts when an eolid nudibranch feeds on a hydroid

The exposure to discharging nematocysts of the eolid nudibranch Cratena peregrina feeding on polyps of the hydroid Eudendrium racemosum was examined. The smaller size-class of microbasic eurytele mastigophor nematocysts is located on the tentacles of the hydroid polyps, while the larger size-class of holotrichous isorhiza nematocysts is located on long cnidophores and basal rings of some polyps. Discharging nematocysts of the tentacles do not appear to have damaging effects on the dorsal appendages of the snail, the cerata. In contrast, contact of a cnidophore with a ceras elicits massive discharge of nematocysts, which results in adherence, membranolysis and local destruction of the epidermis within seconds. The differential locations and effects indicate that the small tentacle nematocysts serve for capturing prey, while the large cnidophore nematocysts serve for the hydroid's defence. Both small and large, discharged and intact nematocysts were found in the pharynx and stomach of the snail after feeding. Thus, the alimentary tract appears to be efficiently protected against discharging nematocysts. Small and large intact nematocysts, which were capable of discharge, were also found in the cnidosacs.

Retinoids and pattern formation in a hydroid

The retinoids (retinol, retinal, retinoic acid) cause alterations in the pattern of limb elements in vertebrates (Summerbell & Harvey, 1983). As shown here, retinoids also influence pattern specification in hydroid polyps (Hydractinia echinata) in a way suggesting interference with the generation and transmission of signals responsible for the dimension and spacing of structures. A pulse-type application of low doses (e.g. retinoic acid 10-6 to 10-10 M, 4 h) causes metamorphosing primary polyps to develop more tentacles but fewer stolons per unit circumference, to shorten the length of the hydranth while the stolon elongates, and to bud secondary hydranths at high frequency 2–3 days after treatment (Fig. 3). Dose-response curves display optimum peaks. It is argued that the increase in budding rate is due to a reduction of the range of spacing signals emitted by the primary hydranth. In regenerating hydranths, low doses (10−10 to 10−9M) improve the rate of head formation, whilst medium doses (10−8 to 10−6M) result in more tentacles being regenerated. However, prolonged treatment with high doses (10−6 to 10−5 M) causes the animals to reduce all head structures and to transform eventually into stolons, in contravention of the rule of distal transformation that they normally obey (Fig. 8). The effects of the retinoids are counteracted by a putative morphogen, the endogenous inhibitor isolated from Hydra by Berking (1977). The Hydra-derived ‘head-activator’ displayed no stimulating effect on the number of tentacles and buds formed.

Colonial Responses of Hydroid Polyps

1. Conduction of excitation in response to local mechanical or electrical stimulation has been studied in various hydroid species. 2. There are systems in the coenosarc of the stems and stolons of all species which conduct excitation at rates of from 1 to 3.5 cm./sec. 3. Two physiological types of conducting systems have been found. (a) Through-conducting systems, showing: all-or-none response, sharp threshold, reproducibility. (b) Local systems, showing: spread of response dependent upon stimulus strength, no sharp threshold, responses which decline with increasing distance from the stimulated point, variability. 4. Colonial co-ordination is better developed in those species whose colonies are structurally better developed. It is effected in most species by either (a) or (b). Both types are present together in Hydractinia echinata.