Studies in the life history and taxonomy of the genus Enteromorpha (1966)
AuthorsRawlence, David J.show all
The genus Enteromorpha is usually placed in the family Ulvaceae (Bliding, 1963; Taylor, 1937, 1960; Chapman, 1956). This family includes those green algae with flat or tubular thalli, and cells with one or two lateral chromatophores and a single pyrenoid. The asexual spore-producing and sexual gamete-producing generations are morphologically identical (Bliding, 1963 P.41). Enteromorpha includes the branched or unbranched monostromatic members of the Ulvaceae which are hollow and tubular. A variety of life histories have been described for the Genus. These include monomorphic diplontic - Enteromorpha intestinalis var asexualis (Bliding, 1963), Enteromorpha biflagellata (Bliding, 1944); Monomorphic diplohaplontic - several species including Enteromorpha ramulosa (Hartmann, 1929), Enteromorpha intestinalis (Kylin, 1930 a.; Eliding, 1948 a.); Monomorphic haplontic Enteromorpha stipitata P. Dangeard var linzoides nov. var. (Bliding, 1960). No dimorphic diplohaplontic life histories have as yet been described for any member of the Ulvaceae. There have, however, been indications of their existence in some genera. Pocock (1961) investigated this possibility in Letterstedtia. Chapman (1956) discussed the possible alternation of a branched and unbranched generation in Enteromorpha australiensis unsupported to date by experimental evidence. The main object of this thesis was to study any local Enteromorpha population which appeared to have a dimorphic diplohaplontic life history. The writer, found evidence of such a life history in an Enteromorpha population in the Motunau River, North Canterbury. This thesis details a series of observations and experiments with the object of confirming this observation. During preliminary experimental work, however, several additional problems emerged. These required investigation before the original objective could become meaningful. One of those problems was the classification of the populations studied. Early attempts to classify the Motunau River population proved difficult. The characters usually considered to be of greatest taxonomic importance were found to vary considerably within the population. As a result a dichotomous key would not place a plant unambiguously into a single species. It was felt that a greater knowledge of the variation in the diagnostic characters was necessary before any species could be accurately identified. In order to elucidate the systematic position as well as the life history of the Motunau Enteromorpha population it was necessary to establish cultures. Fertile plants had to be collected and zooid release effected in the laboratory. Extreme difficulty was experienced in both these respects. It was difficult to find any fertile plants and even more so to effect zooid release. Emphasis was therefore placed upon periodicity observations and the conditions influencing zooid release in the laboratory and natural environment. Once the desired cultures were established, the extreme variability of orthodox taxonomic criteria led to the search for others, and the possibility of using embryology as a taxonomic criterion was investigated. Here additional problems emerged. Plants of the same age were observed to have widely different embryo form. This situation was found in a number of populations. The objectives of this thesis therefore became to study (1) the natural variation of selected taxonomic characters, (2) zooid release in the natural environment and in the laboratory, (3) the variation in embryology of several populations, and (4) to determine the type of life history possessed by an Enteromorpha population growing in the Motunau River, North Canterbury.