Discussion

From the results of MacMunn (1885) and the present experiments it seems that in some actinians the respiratory requirements of the animal are provided by pigments found mainly in its symbiotic algae.

The phaeophytin of band D in I. olivacea is a chlorophyll pigment, and symbiotic algal cells are present in large numbers in epithelial cells.

Green glycerol extracts of fresh material of Isactinia have absorption curves with peaks at 211 millimicrons in the U.V. region and 397 millimicrons in the visible range. These values however do not correspond with the characteristic peaks of the two respiratory pigments actiniochrome and actiniohaematin known to be specific to sea-anemones. In glycerol, actiniochrome has peaks at 563 and 595 millimicrons (centre at 579) and 458 and 477 millimicrons. Actiniohaematin has the characteristic absorption of a, b₁, c, of cytochrome. These cannot be recognized as present in Isactinia olivacea. The absence of actiniohaematin is compensated for by other respiratory pigments.

The results also show the presence of flavins or lyochromes. These compounds are important in biological oxidation-reduction systems. Flavins are universal in plants and animals (especially riboflavin), and promote cell respiration, growth and fat absorption. In animals, flavins are derived either from symbiotic algae or from food. As Fox (1953) suggested, this class of biochrome plays an indispensable role in the basal metabolism of animals, thus explaining their presence in I. olivacea.

Band A' and A" were mentioned above as possible carotenoid compounds. Carotenoids have been found in various coelenterates by several previous workers. Therefore their presence in I. olivacea is not surprising. Extensive work on the pigments of Actinia equina by Abeloos-Parize (1926) and Fabre and Lederer (1934) demonstrated the presence of actinioerythrin, a- and b-carotenes and violerythrin. Anemonia sulcata was found to have carotenes (Elmhirst and Sharpe, 1919) mainly sulcatoxanthin. Several carotenoids were found in Metridium senile by Fox and Pantin (1941). They found ectodermal cells contained red fat droplets evenly distributed throughout the cell or nearer the cell's free surface. In I. olivacea sections do not show conspicuous fat droplets similarly placed. The presence of carotenoids in I. olivacea is here attributed to the presence of symbiotic algae. Goodwin (1962) emphasized that all carotenoids in animals are of dietary origin. He stated further that "animals", especially invertebrates, do not possess the ability to oxidize tertiary carotenoids to produce pigments which are often characteristic of the species".

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It was suggested by Cheesman, Lee and Zagalsky (1967) that carotenoids function either in the stabilization of protein and carotenoid, or as protective coloration; they also suggested that carotenoids play an active role in electron transport and enzymatic activity.

In the case of J. olivacea, there is no evidence of carotenoids playing an important part in the stabilization of protein and carotenoid compounds. However, further research may isolate and identify carotenoid compounds similar to that of "metridene" or "astaxanthin". This is not impossible because band A' which is not yet finally identified, could be one of these carotenoid compounds.

There is the further possibility that in I. olivacea carotenoids are used for protection against harmful effects of radiation. The anemones are found in well-illuminated rock pools, and in the laboratory they tend to move towards the light. They therefore need a layer of pigments which could form a shield and filter out harmful irradiations. With regard to the survival value of colour, I. olivacea may be classified as a predator. The close similarity of its colour to that of surrounding sea-weeds makes its presence less conspicuous to the crustaceans, worms and other small animals constituting its food. The density of the carotenoid pigments is important in this respect, because green and brown varieties of this species differ by the concentration of the two bands A' and A". For example, the green variety found in rock pools where olive-green sea-weeds are predominant, show only very faint traces of these pigments on the chromatogram. The brown variety however, has very concentrated bands of these pigments and they confer on the animal the intense brown colour that makes it virtually indistinguishable from its immediate environment.

Bands E, E' and F, although not positively identified, do give some indication of their importance in being light sensitive because their absorption curves show peaks in the ultra-violet region and no peaks in the visible range. They absorb in the wave length from 210 to 267 millimicrons. These pigments could be of vital importance to I. olivacea because the animal lives in well illuminated rock pools and it therefore needs some device for filtering the ultra-violet wavelength. As Herring (1965) stated, "the ultra-violet wavelengths are generally considered as the most biologically harmful. ..." It seems likely therefore that these pigments either protect the animal itself from excessive sunlight, or provide a filter for the protection of the symbiotic algal cells within the tissues of the animal.

The results obtained in this paper are very similar to those obtained by Strain, Manning and Hardin (1944). They made a thorough analysis of pigments of algal zooxanthellae which inhabit the tissues of the sea anemone Cibrina xanthogammica. An extract of pigments yielded, on chromatographic separation, no fewer than a dozen pigments, including chlorophyll a, a', and c, phaeophytin a-carotene and several newly described xanthophylls.