Species Variation and Diversity

Polyclad flatworms resemble a taxon with an incredible diversity in tropical waters (Newman & Cannon, 1994; see section: Taxonomy). For quite a long time, the colour pattern of the splendidly coloured worms alone has been considered sufficient for their classification (Hyman, 1954, 1959). However, it has been shown that for identification of many species this is not enough for a reliable identification (Faubel, 1983, 1984). Field studies of Newman & Cannon (1994) revealed that very similar and almost identical colour pattern occur in different genera (Pseudoceros Pseudobiceros; Pseudoceros Pseudoceros) and even in different families (Pseudocerotidae Euryleptidae) making a closer examination of subtle differences between species necessary. Comparative anatomy taking in account a suite of characters such as eye number and eye arrangement, form of pseudotentacles, pharynx, and especially analysis of the fine structure of the reproductive system has been proven to be an essential tool for species diagnosis of turbellarians (Newman & Cannon, 1994). Since the serial reconstruction of the male and female reproductive structures is difficult and makes special lab equipment necessary, it is reserved for specialists. Recently, molecular data (DNA sequencing) have been used to tell apart similar polyclad species (see section: Phylogeny). Without the employment of such tools, classification of polyclad flatworms can be erroneous in some cases. Specimen with similar colour pattern may resemble distinct species but may also belong to the same species featuring a genetically fixed colour and pattern variability (modification). On the other hand, specimen with an identical colouring may be members of different genera or even families. Therefore, if two polyclad specimen with similar colour pattern are to be compared, several possible scenarios are conceivable.