| Summary: | Thesis (M.A.)--Boston University === The four-spined stickleback shows a variation of 30-33 vertebrae, 9-14 dorsal fin rays, 7-ll anal fin rays, and 2-6 dorsal spines. No correlation exists between any of the characters except dorsal and anal rays, which are strongly correlated.
Vertebrae show a sligltt tendency to decrease in number from north to south, while dorsal and anal rays and dorsal spines vary irregularly throughout the range of the species, with no significant difference between populations at either end of the range. Individual differences, however, are sufficient to divide the species into subspecies and races, using the criteria of modern taxonomists. But the presence of clines, plus the fact, that between any two populations, no matter how divergent, intergrading populations can be found throughout the range of the species, suggests that the variation encountered may be environmental and not genetic.
Numerous field and experimental studies, reported in the literature, have revealed many different and often contrasting correlations between environmental factors and meristic characters. Taning (1952) showed that each meristic character in Salmo trutta has its own phenocritical period, and was able to produce a range in means of 3.2 vertebrae in offspring of the same parents by varying the temperature during the phenocritical period. The phenocritical period for anal rays was found to overlap slightly with that of the vertebrae, while dorsal and pectoral rays are determined much later. Taning found that vertebrae, and in part anal rays, are mainly determined genotypically, while dorsal and pectoral rays are determined phenotypically.
Heuts (1947) showed that there exists in the European stickleback, Gasterosteus aculeatus, the phenomenon of two adaptive peaks. Natural selection favors those individuals with low mean plate numbers in waters of low salinity, and those with high mean plate numbers at high salinities. Heuts (1949) found that fin ray numbers in both types are similarly modifiable by temperature within their own respective habitats.
The findings of Taning and Heuts may help to explain the pattern of variation in Apeltes quadracus. The variation encountered in mean vertebral numbers may be due to the temperature lability of a single genotype, indicating that no intraspecific categories should be recognized. Since dorsal and anal rays are correlated with each other but not with vertebrae or spines, they probably develop at about the same time in ontogeny and may be highly modifiable by temperature.
As spines show no correlation with the other characters considered, a different explanation for their variation is offered. Cox (1923) found a distinct correlation between spine number and salinity in Apeltes quadracus in the Maritime Provinces. I have found a similar, though imperfect correlation in several localities, with four-spined individuals more abundant at high salinities, and five-spined individuals predominant at low salinities. A selective mechanism similar to the one described by Heuts is believed responsible, with many-spined individuals being selected for at low salinities, and fewer-spined individuals being favored at high salinities.
The explanations offered here are conjectural, and alternatives could be given. Only further study, especially experimental work, may fully explain the variation in Apeltes quadracus. [TRUNCATED]
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