ABSTRACT
Obtaining all developmental stages from an ovigerous decapod female is common in the laboratory. This is a significant advance for larval taxonomic studies, morphological descriptions, systematics, phylogenetics and evolutionary theory. Yet for such studies reliable data must be founded on quality observations and interpretation of setotaxy using a modern high-powered microscope equipped with differential interference contrast. Incorrect setal counts are problematic, especially since first-stage zoeas of congeneric brachyuran species appear to have identical setotaxy. This similarity provides such a high degree of predictability within a taxon that setal differences (incongruence) in a group may suggest incorrect assignment of taxa. However, relationships based on differences and similarities are not necessarily founded on shared derived characters, and instead may be supported by symplesiomorphies. The methodology involved in larval phylogenetics is also problematic. For example, oligomerization is considered to be an evolutionary trend within Crustacea. Decapod larval development suggests that heterochronic processes may provide a dominant evolutionary mechanism influencing loss of characters. Although using an unordered transformation series in a phylogenetic analysis is acknowledged to generate the most parsimonious trees, such an assumption does not necessarily represent a linear evolutionary pathway towards gradual terminal delay of characters as postulated by heterochrony for decapod larvae. A mosaic of heterochronic processes provides a complex evolutionary mechanism influencing oligomerization (reduction and loss) within brachyuran zoeae. This is best captured in a phylogenetic analysis by using “irreversible-up” (terminal delay, not terminal addition) transformation series. Reconstruction of trees using this assumption about character evolution generates longer trees and frequently involves more evolutionary steps to compensate for homoplasy. Yet there is evidence to suggest that homoplasy is common within many brachyuran larval lineages. Nonetheless, larval phylogenetics does appear to have advantages since all decapod zoeal stages are adapted to a planktonic existence, and therefore setal patterns are subject to similar selection pressures. Morphological differences among larvae may provide additional phylogenetic information as compared to possibly convergent adult characters that are more the product of the interaction between genotype and environment.
