On the phylogenetic relationships of Bathynellacea
The different views on phylogenetic relationships among crustacean specialists are clearly set out by Serban (1972), where he says that the caparace, which is lacking in the Syncarida, is very well developed in the Eucarida, and is present in intermediate stages of development in the Hoplocarida and Peracarida. Consequently, Serban defined Bathynellacea as: “... un malacostracé à structure paradoxale, une forme archaïque évadée de la classique évolution caridoïde et stylisée de manière singulière.” (pp. 114)1. The Bathynellacea would be one member of the only lineage that has escaped the most important evolutionary trend in Crustacea: the cephalization of the first thoracic segments. Serban justified the necessity of creating a superorder for the Bathynellacea because of this special and singular “bathynelloid” structural “Bauplän”. As a consequence, Serban dismissed the idea of considering the Bathynellacea as a degenerate member of the Syncarida or a structurally simplified syncarid. The corresponding question of whether or not the Bathynellacea are primitive Malacostraca or simplified descendants of this group was an unsolved question in 1972.
Later, Schminke (1981) proposed the so-called zoea theory, which made the origin of the Bathynellacea once again a matter of discussion. Schram (1981) approached the problem from the point of view that the “caridoid” structural basis is the primitive pattern for all Malacostraca and on this basis suggested some relationships, later modified by Hessler (1983). They considered that the differences between Bathynellacea and Anaspidacea could be due to the fact that the former is adapted to interstitial life while the Anaspidacea is epigean. However, this view has been put into doubt since the discovery of new syncarids adapted to interstitial life, the family Stygocaridae in the Order Anaspidacea. However, Schram & Hof (1998) in fact, under some conditions, found Bathynellacea sorting separately from Anaspidacea. It might appear that, the peculiar body plan of the Bathynellacea, very different from the typical Syncarida, cannot be explained easily as an ecological specialization. The Stygocaridae is a clear example of truly primitive syncaridans which, after conquering the subterranean environment, have maintained their peculiar morphology.
As pointed out by Serban (1972), Bathynellacea has a “bathynelloïde” structure (p.114) that is, a mixture of archaic simple carioid Malacostraca characters and a set of specific characters that differentiate them from Malacostraca and Syncarida. In this sense, it is interesting to quote Brooks (1962) (pp.240): “The Bathynellacea with their eight free thoracic somites and furcal lobes definitely have ancient heritage. They are probably descendants of the archaic ancestors of all the Syncarida”. We could add that they share with true Syncarida the following characters:
· absence of caparace (all other malacostraceans have one, although sometimes reduced);
· presence of 6 abdominal segments (number reduced in other malacostraceans);
· first thoracic segment always free (in all other malacostraceans it is welded to the cephalic tagma), and
· thoracopods always with epipodites (none or reduced in the other malacostraceans).
However, bathynelids are different from syncarids in the following:
· the protopod of the thoracopods retain its archaic shape;
· the endopod always has less than 5 segments (5 is the typical number in the carioid facies) and the exopod (1 to 4 segments) it is never reduced to a flagelum (which is typical in syncarids);
· the vast majority of bathynelid species lack pleopods and those that do have them, have 1 or 2 at most and always reduced (sometimes to a simple seta);
· bathynelids lack statocysts on the antenna I;
· the males lack a petasma and the thoracopod 8 is transformed into a proper ejaculatory organ, this transformation being unique within the crustacean groups.
Additionally, the development of Bathynellacea is completely different from the syncarids as Serban says (1972) (p.122): “...par ses nombreuses particularités, l’ontogénie de Bathynella s’inscrit parmi les principales caractéristiques qui éloignent les Bathynellacea du super-ordre des Syncarida”. The new form that comes out of the egg has fewer thoracopods (1 to 4) than the adult form, but in syncarids the young animal is identical with the adult (see too Schmincke, 1981).
The most recent works on the phylogeny of Crustacea based on morphological characters (Lange & Schram, 1999; Schram & Hof, 1998) suggest that Bathynellacea are an ancestor of the Malacostraca, including the Anaspidacea. However in their classification Bathynellacea is an order of Syncarida within the subclass Eumalacostraca. When fossil forms are included in the analysis, the Syncarida (Bathynellacea, Anaspidacea and Palaeocaridacea) remains a paraphyletic group and ancestor of the Malacostraca (Schram & Hof, 1998; Wills, 1997). The Hoplocarida appears within the Malacostraca, related sometimes to the Eucarida (Wills, 1997), and at others to the Peracarida or the Syncarida (Schram & Hof, 1998) (Fig. 2). Taking into account that the hoplocarids show several apomorphic characters, this result could be due to the lack of good apomorphies to characterize the Eumalacostraca. Many characters that define the Eumalacostraca could be homoplasies, e.g., the caparace has developed and/or reduced several times during the history of the Malacostraca and other Crustacea groups. Schram & Hof (1998) concluded that the majority of Crustacean subgroups are very clearly defined, but relationships between them are still very obscure. Consequently, it is hoped that the molecular data could help to clear up these relationships.
Fig. 2. Crustacean phylogeny inferred by morphology. Slightly modified from Lange & Schram, 1999.
Our results, which provide for the first time a sequence of a bathynellid mitochondrial 16S rDNA fragment, support a basal position of the Bathynellacea and a clear distinction from the Superorder Syncarida. Even if the mitochondrial fragment here analysed did not provide a great support for the deeper branches, a Shimodaira-Hasegawa test indicated highly significant difference between the tree topology here proposed and another where the Bathynellacea specimens were forced to appear as sister species of the Syncarida. Our results renew the old controversies on the systematic position of Bathynellacea within Malacostraca. It may be that Serban_s (1972) proposal of a new superorder “Podophallocarida” for the Bathynellacea, outside the Syncarida, is worth reconsidering. However, our results point to keeping Podophallocarida within the Eumalacostraca, and there is no need to build a new group Eonomostraca, as Serban suggested. More molecular data are urgently needed to settle these important questions on crustacean relationships.