SPERMATOZOAL ULTRASTRUCTURE OF THE TAXON ENCHYTRAEUS (ANNELIDA, OLIGOCHAETA) AND ITS SIGNIFICANCE FOR SPECIES DISCRIMINATION AND IDENTIFICATION

Abstract

Species of the oligochaete genus Enchytraeus are morphologically rather similar; they are difficult to distinguish or even inseparable on conventional light microscopic methods. In individuals from 19 populations of different origin and in some cases unknown species identity the ultrastructure of mature spermatozoa was investigated. Their morphometric and qualitative data were used as morphological taxonomic characters. With the exception of a genetically polymorphic field population of E. albidus, the material investigated came from genetically monomorphic laboratory inbred strains. The structure of the thread-like spermatozoa generally conforms with the sperm type known for the family. Numerical characters used were 1. length of the acrosome, 2. length of the corkscrew-like nucleus, 3. number of helical spires of the nuclear flange, 4. periodicity of the flange spires proximally and distally, 5. angle of the flange to the longitudinal axis of the nucleus, 6. length of the midpiece, 7. number of mitochondrial helical spires, and 8. length of the flagellum. Besides the shape of the nucleus, the complex structure of the acrosome provides further specific qualitative differences such as, length ratio of primary acrosome vesicle to the entire acrosome, extension of a subdistal electron-dense material, and absence or presence of specific structures underneath the primary acrosome vesicle. Eight populations show highly significant differences from each other and from the remaining populations on the basis of morphometric data alone. In addition, these populations - recognizable as different species on conventional methods - can be discriminated by qualitative differences of their spermatozoa. The spermatozoa of 10 other populations appear to be highly similar due to the length of their nuclei. Number of nuclear spires, length of acrosome, flange spire periodicity and especially structural details of the acrosome, however, separate these 10 populations into two significantly different groups, one with six species and the other with four. The group of four, which has also been found in parallel, independently conducted non-morphological investigations, are considered to belong to one species. Certain morphological data divide the group of six into two groups of three, which is also supported by non-morphological methods and crossbreeding experiments - both groups are distinct species. Statistical comparisons generally show no or only slight variability of morphometric data for sperm of one individual. The variability between individuals of the genetically monomorphic laboratory cultures is higher but in no case does it prevent species identification. The greatest intraspecific variation has been observed in the E. albidus field population: spermatozoa of individuals differing in the pattern of single enzymes deviate significantly in their morphometric data, and on the basis of these morphometric data alone they are inseparable from spermatozoa of certain other species. However, qualitative characters are not affected and species can easily be discriminated on the basis of acrosomal structures. The present example of a comparison of spermatozoa demonstrates the possibilities and importance of ultrastructural analysis as an advanced morphological method for solving taxonomical problems at the species level.