These crustaceans are small organisms ranging in size from 1-3 mm as adults (see Schram, 1986). They live in the spaces between sediment grains and apparently graze on bacterial, algal, and fungal films. Their tiny size and limited number of trunk limbs insures limited powers of dispersal; I suspect that under normal circumstances a bathynellacean lives and dies probably within 5-10 mm of where it hatched. The group was first recognized and described in the late 1800s (Bathynella natans Vejdovsky, 1882), but it was not until numerous species entered the literature, due largely to the efforts of Eugene Serban, Wolfram Noodt, and Kurt Schminke, that certain biogeographic patterns became evident. Schminke (1974) was the first to draw attention to these patterns. While most genera and species of bathynellaceans are highly endemic, a few genera have extended ranges, e.g., Cteniobathynella occurs in Brazil and central Africa, Notobathynella is found in Australia and New Zealand, and Chilibathynella and Atopobathynella occur in Australia and in Chile. A general consideration of what was known at the time resulted in Schminke (1974) postulating a dispersal of the parabathynellids from a hypothetical center of origin in East Asia along three pathways: 1) to Europe, 2) to Africa and on to eastern South America, and 3) to Southeast Asia, then to Australia/New Zealand, and on to western South America. Schram (1977) examined the distribution of fossil malacostracans in the Paleozoic and postulated a different scenario, viz., that the group as a whole appears to have taken origin on the island continent of Laurentia and dispersed through out the world with the formation of Pangaea in the Late Paleozoic. In Schminke’s analysis, several areas of the world were noteworthy because of their apparent lack of bathynellaceans (India, South Africa, and – most peculiar of all – all of North America); but these areas in the intervening years have now all been found to have bathynellaceans, so the group as a whole is ubiquitous.
A genus of special interest, however, is Hexabathynella, because, in an order noted for local and regional endemism, this genus is the only bathynellacean taxon that occurs worldwide. Camacho (2003) analyzed the group, which at that time contained 18 species, and her cladogram (Fig. 1) revealed two interesting patterns.
Fig. 1. Relationships of 18 species of Hexabathynella modified from Camacho (2003).
* indicate the 5 species from brackish-marine habitats.
• indicate the necessary 7 independent invasions of terrestrial habitats if Hexabathynella originated in a marine habitat.
The first pattern relates to the affinities of the sub-clades. The Hexabathynella of the Balkans are nested amongst the southern hemisphere clades, while those of Western Europe exhibit a sister status to the North American species. While the Balkans often yield interesting and peculiar taxa in many groups, this supposed Gondwana connection is noteworthy. The second pattern relates to hypotheses about the evolution of the genus, and by extension perhaps the whole order. The basal-most species on her cladogram, H. paranaensis from Brazil, is one of five brackish to marine species in the genus. Because of this, and on theoretical grounds, Camacho reasoned that Hexabathynella most likely took origin in the marine realm and then radiated onto the continents.
This pattern conformed to what she termed the Thallasoid Theory (Boutin and Coineau, 1990), which is actually a later version of the Regression Theory of Stock (1980). Under the terms of this hypothesis, forms that are marine in origin became stranded in fresh water when sea level regressed. The opposing hypothesis is the so-called Limnicoid Theory (Schminke, 1981), which assumes that the primary origin occurs in fresh water, with subsequent invasions into marginal brackish and marine habitats. As far as bathynellaceans are concerned, the widespread manifestation of freshwater habitats in the order as a whole might argue for this latter theory.
Nevertheless, these opposing hypotheses can be tested on the cladogram. If a marine origin of Hexabathynella occurred, then the tree in Fig. 1 tells us that there had to have been 7 independent invasions of fresh water from the marine/brackish water realm. If the origin of Hexabathynella took place in fresh water, then only 5 independent invasions of salt water would be needed, one for each of the 5 species. Thus, the limnicoid explanation appears more parsimonious.
Moreover, for any cladistic analysis the relationships on the tree itself can be tested with the identification of new species and their incorporation into the analysis. Cho and Schminke (2006) did that when they added four new species of Hexabathynella into their cladistic analysis of the genus. A somewhat different set of relationships emerged from their work (Fig. 2).
Fig. 2. Relationships of 22 species of Hexabathynella modified from Cho and Schminke (2006).
* indicate the 5 species from brackish-marine habitats.
• indicate the necessary 8 independent invasions of terrestrial habitats if Hexabathynella originated in a marine habitat.
With regard to the patterns seen in the 2006 tree, two observations can again be made. First, all the European and North American taxa now occur together in a series of sister clades, whereas the Gondwana clades cluster together nearer the base of the cladogram, a more logical historical patterning that suggests a possible origin of Hexabathynella in the southern land masses with subsequent dispersal into the northern hemisphere in Paleozoic times. Second, we can again test hypotheses of marine versus freshwater origins. If Hexabathynella has a marine origin, then the Cho/Schminke tree reveals that at least 8 invasions of freshwater had to have occurred. If Hexabathynella has a freshwater origin, the same 5 invasions of salt water occurred as seen on the Camacho tree. Therefore, the Cho/Schminke cladogram implies an even more parsimonious likelihood that Hexabathynella arose in fresh water.
We see in the above simple examples that classic, area cladistic, analysis combined with a careful attention to determining parsimony can pay dividends, enabling us to solve this particular biogeographic problem and sort out the conflicting hypotheses about the evolution of this group. Naturally, the arguments could be strengthened if we had a comprehensive cladistic analysis of the entire array of Bathynellacea. That, however, is not available at this time. Moreover, such an analysis could then allow a deeper exploration of paleobiogeography of the bathynellaceans as a whole.