http://science.sciencemag.org/content/sci/171/3973/757.full.pdf

Allelochemics: Chemical Interactions between Species

A Unified Approach to Marine Plant-Herbivore Interactions. I. Populations and Communities

components to unify several aspects of the individual, population, and community level effects of herbivores on marine algae. Here we extend these considerations to biogeographic patterns of marine community structure by assessing the importance of latitudinal variation in algae-herbivore interactions. Our treatment of population- and community-level phenomena drew heavily on a measure of the expectation of herbivore damage for a given plant growing at a specific place. This function has three components [notations from (84)]: (a) the probability that the plant will be encountered by herbivores, P(En); (b) the conditional probability that the herbivores will eat at least a part of the plant given that they encounter it, P(EaEn); and (c) the expected decline in fitness of the plant individual (relative to undamaged plants) given that it is encountered and at least partly consumed. These three terms are useful in defining the two general types of plant escapes from herbivores (94): noncoexistence escapes, where P(En) is near zero; and coexistence escapes, where encountered plants are either not consumed [P(EaEn) is small] or are consumed at little cost to plant fitness (84). Since each component of the expectation of herbivore damage is a function of plant, herbivore, and environmental characteristics, the frequency and pattern of these escapes from herbivory can vary (a) among algae of different shapes, sizes, and chemical compositions; (b) among herbivore regimes with different densities, diversities, or species compositions; and (c) among areas because of interactions among environmental, plant, and herbivore characteristics. Here we examine evidence for predictable variation between geographic regions in these features of algae-herbivore interactions.

https://www.jstor.org/stable/pdfplus/2097064.pdf

A unified approach to marine plant-herbivore interactions. ii. biogeography

The cover of foliose algae is sparse to non-existent above a low-level algal zone on many shores in N.S.W., except in rock-pools. Above this algal zone, encrusting algae, mostly Hildenbrandia prototypus, occupy most of the primary substratum on sheltered shores. Experimental manipulations at midtidal levels were used to test hypotheses about the effects of grazing by molluses and of physical factors during low tide on this pattern of algal community structure. Fences and cages were used to exclude grazers: molluscs grazed under roofs and in open areas. Cages and roofs provided shade, and decreased the harshness of the environment during low tide: fences and open areas had the normal environmental regime. In the absence of grazers, rapid colonization of Ulva and slower colonization by other foliose algae occurred in all experimental areas. The rate of colonization by Ulva sporelings was initially retarded on existing encrusting algae, but after a few months, cover of Ulva equalled that on cleared rock. Most species of algae only grew to maturity inside cages, and remained as a turf of sporelings inside fences. No foliose algae grew to a visible size in open, grazed areas. Grazing thus prevents the establishment of foliose algae above their normal upper limit on the shore, but the effects of physical factors during low tide prevent the growth of algae which become established when grazers are removed. Physical factors thus limit the abundance of foliose algae at mid-tidal levels. The recolonization of cleared areas by Hildenbrandia was not affected by the presence of a turf of sporelings, nor by the shade cast by roofs, but was retarded in cages where mature algae formed a canopy. Even under such a canopy, Hildenbrandia eventually covered as much primary substratum as in open, grazed areas. This encrusting alga is able to escape from the effects of grazing by having a tough thallus, and by its vegetative growth which allows individual plants to cover a lot of substratum, and by the tendency for new individuals to start growing from small cracks and pits in the rock, which are apparently inaccessible to the grazers. Mature foliose algae are removed from the substratum by waves, and many individual plants died during periods of hot weather. Sporelings in a turf were eliminated, after experimental fences were removed, by the combined effects of macroalgal grazers, which invaded the areas, and microalgal grarers which ate the turt from the edges inwards. The results obtained here are discussed with respect to other studies on limits to distribution of intertidal macroalgae, and the role of grazing in the diversity and structure of intertidal algal communities. Some problems of these experimental treatments are also discussed.

The effects of grazing by gastropods and physical factors on the upper limits of distribution of intertidal macroalgae.

Coelenterates (cnidaria and ctenophores) are well recognized as predators in food webs of marine ecosystems but are less often considered as prey. This is partly because they are digested very rapidly. In studies based on predator stomach contents the measured masses of different prey organisms are rarely scaled by their relative rates of digestion. Predators that are frozen and thawed, or for which whole stomachs are placed in preservatives, may have already lost much of their coelenterate content when they are examined. There is also a tendency to assume that gelatinous organisms, with their high water and salt content relative to organic content, are poor food. However, given the high rates of digestion (and presumably of assimilation) coelenterates may provide sources of energy comparable to better recognized prey such as arthropods. It is already becoming well documented that a number of cnidaria and ctenophores as well as fish utilize gelatinous organisms as prey. Data is accumulating more slowly on predation by a wide range of other carnivores such as molluscs, arthropods, reptiles and birds.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0025315405011458

Predation on pelagic coelenterates: a review

Nudibranchs Glaucus and Glaucilla store and utilize for their own defense the nematocysts of the venomous siphonophore Physalia.

https://science.sciencemag.org/content/sci/166/3912/1532.full.pdf

Physalia nematocysts: utilized by mollusks for defense.

A fifty-three metre inter-tidal transect on a headland near Sydney, New South Wales, has been studied. Its profile, fauna and flora are illustrated and described.

Herbivore-Algal relationships on a coastal rock platform (Cape Banks, N.S.W.).

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Physalia nematocysts: utilized by mollusks for defense.

Herbivore-Algal relationships on a coastal rock platform (Cape Banks, N.S.W.).