Low abnormality rate shows the tentacular armature to be a reliable morphological character in trypanorhynch cestode taxonomy

A total of 4193 specimens with everted tentacles of the trypanorhynch cestode Mixonybelinia lepturi obtained from Gempylus serpens and Trichiurus lepturus were examined in order to identify abnormalities in hook shape and armature pattern. The abnormality rate of the tentacular hooks and armature was 0.09%, ranging from changed hook direction and slight changes of single hook shapes to completely altered hooks and armature patterns. In a single case, the armature pattern could not be assigned to any existing trypanorhynch family. The observed malformations were classified in 5 categories and 9 types, a categorization also to be used as a standard for other trypanorhynchs. The low abnormality rate demonstrates high reliability of tentacular hooks and armature patterns as taxonomic characters in trypanorhynch systematics, confirming that trypanorhynch larvae have identical tentacular armatures compared with their adults. Considering the low rate of abnormality, the role of environmental factors in causing the unsymmetrical changes on originally symmetrical tentacles is probably weak. Therefore, a genetic origin seems to be more probable. Such phenotypic armature variations may cause transfer of worms to new attachment sites within the same host or into a new host species, where new tentacular armatures with increased fitness are eventually formed by natural selection under the model of micro-allopatric speciation. The finding of an abnormal tentacle that unites hollow and solid hooks together with completely altered hook shapes and armature patterns is good evidence in demonstrating the ancient origin of different hook shapes and the evolutionary changes of armature patterns.