Morphometric and Biochemical Differences Between Sympatric Populations of the Clathria “Spicata” Species Complex (Demospongiae: Poecilosclerida: Microcionidae) from Northern Australia
Morphological systematics of Recent Porifera are relatively difficult in comparison with other invertebrate phyla. On the one hand, morphological differences between supposedly distantly related higher taxa may appear to be minute or trivial, which is apparently a product of a long but conservative evolutionary history which was not produced by any radical morphological changes (Wiedenmayer 1977), but has produced an extremely diverse biochemical constitution (Bergquist 1978). Conversely, species may show a high degree of intraspecific morphological variability (e.g., Pansini 1982). In some cases that variability may be correlated directly with measurable differences in environmental factors (e.g., Palumbi 1984, 1986; Thompson and Murphy 1989), whereas in other cases species appear to be much more morphologically conservative across wide geographical ranges (e.g., Hooper 1987). Our knowledge of the effects of various environmental factors on intraspecific variability in sponges is still very limited (e.g., review in Simpson 1984), and it is not presently possible to construct models which may predict the behaviour of species (and their characters) under environmental changes (e.g., Simpson 1978; Palumbi 1984). These uncertainties have allowed authors to make quite different interpretations of the systematic importance of morphometric characters and levels of taxonomic divergence within the classification (cf. Hallmann 1912; Wilson 1921; Hooper 1991b). The present study examines a case in which only cryptic morphometric characters were able to consistently differentiate between two sympatric sibling species of Clathria, but species were otherwise clearly differentiated into two chemical populations. Methodology, descriptions of taxa and more complete results are presented elsewhere (Hooper et al. 1990).
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