Abstract

Palm, H.W. 1995. Systematic investigations of trypanorhynch cestodes from Atlantic fishes. Berichte aus dem Institut für Meereskunde Kiel, No. 275, 238 pp.

Trypanorhynch cestodes are characterized by the possession of a scolex bearing 4 evertible hooked tentacles and 2 or 4 suckers, by which the cestodes move through and attach to the digestive tract of their elasmobranch final hosts. In their life cycles, these cestodes use invertebrates as first intermediate hosts, plus teleosts and some invertebrates as second and parathenic hosts. The presence of the metacestode stage in the musculature of fishes can decrease the commercial value of affected stocks world wide. The purpose of the present study was to develop a new classification of trypanorhynch cestodes as a basis for a natural system. This was neccessary because of weaknesses in existing systems that cause identification of many of the approximately 200-250 valid species to be difficult. Currently, the armature of the tentacles in the metacestodes and adults is used as the major character in all the superfamiles: Homeacanthoidea, Heteracanthoidea, Otobothrioidea and Poecilacanthoidea. This system has grouped some families together that share similar tentacle arrangements but have few other morphological similarities in common.

Light- and scanning electron microscopy were used to assess the morphology, the tentacles and the surface of the scolices of 29 trypanorhynchs from the Atlantic Ocean and 2 species from the Pacific Ocean from all four superfamilies. Fresh material was collected from teleosts and elasmobranchs along the coasts of Brazil, Nigeria and the southern Gulf of Mexico coast of the USA; preserved material was obtained from collections in Brazil, England, Russia and USA. Species descriptions mostly followed existing standards; however, the orientation of tentacle surfaces and numbering of hooks were reinterpreted. Arising from this was the development of a hook formula which summarizes the characteristic armature of a trypanorhynch species. The hook formula as well as the SEM micrographs were fundamental in the development of an identification key for the described Atlantic species. For 17 of these species the first SEM documentation is presented. At present, this study is the most comprehensive compilation of trypanorhynch surface ultrastructure, and it has demonstrated again the potential of microtriche morphology as a systematic character.

The following features are reported for the first time: 1. absence of microtriches from Pterobothrium heteracanthum; 2. circular fields of microtriche-free surface on the apex of the scolex of Poecilancistrum caryophyllum; 3. regularly arranged clumps of cilia-like microtriches on the bothridia of Patellobothrium quinquecatenatum; and 4. surface hooks on the bothridia of Tentaculariidae. For the first time, the reduction of tentacular hooks was demonstrated. The two species Pterobothrium acanthotruncatum Escalante & Carvajal, 1984 and Otobothrium crenacolle Linton, 1890 are proposed as junior subjective synonyms of P. heteracanthum Diesing, 1850 and O. cysticum Mayer, 1842, respectively. Four new species and two new genera are reported: Grillotia kovalevae sp. nov, distinguished by having 2 bothridia without ciliated pits and a metabasal armature with 7 calary and 2 intercalary hooks, and Pseudogrillotia zerbiae sp. nov., distinguished by the presence of 2 bothridia without ciliated pits and a metabasal armature with 7 calary hooks and a band of hooks on the external surface. The new genus Poeciloacanthum with the species Poeciloacanthum oweni gen. et sp. nov. is characterized by having ciliated pits on the borders of 2 bothridia and a single chainette on the external tentacle surface. The other new genus Pseudolacistorhynchus with the species Pseudolacistorhynchus noodti gen. et sp. nov. has 2 bothridia without ciliated pits and a single chainette on the external surface, completing the rows of 5 calary and 3 intercalary hooks. An additional result of the study of the tentacular armature was the evident relationship between the hook shape, hook arrangement and length of the tentacle with the location of the parasite in the digestive tract of the final host.

The present investigation shows that the tentacular armature is only useful to describe superfamilies in an empirical system of trypanorhynchs. It is unlikely to reflect their phylogeny because the present day armatures, as in the poeciloacanthous species, are believed to be analogous developments within the trypanorhynchs. On the basis of new evidence, a new classification containing a phylogenetic interpretation is presented using the following characters each of which could be divided into one original and one modern state. Ciliated pits on the bothridia and praebulbular organs are used to distinguish the superfamilies Eutetrarhynchoidea nov., Otobothrioidea Campbell & Beveridge, 1994 sensu nov. and Tentacularioidea nov.. The following characteristics were chosen to represent the families: without / with blastocyst, 4 / 2 bothridia, and complete rows of hooks (homeoacanthous or heteroacanthous typica) / rows of hooks partly reduced (heteroacanthous atypica or poeciloacanthous). Of the 19 families previously accepted by Campbell & Beveridge (1994), 10 were retained (Eutetrarhynchidae, Gilquiniidae, Gymnorhynchidae, Hepatoxylidae, Lacistorhynchidae, Mixodigmatidae, Otobothriidae, Paranybeliniidae, Shirleyrhynchidae, Tentaculariidae, all sensu nov.); one family is reinstated (Aporhynchidae) and a new one is erected (Pseudotobothriidae fam. nov.). Advantages of the new classification of trypanorhynch cestodes are 1. it is closer to a natural system than those currently existing as it does not use the hook arrangement on the tentacles to distinguish superfamilies; 2. existing genera are easily reassigned to the families and superfamilies; 3. the identification of the examined species is simplified; 4. criteria for erection of families and superfamilies are clearly defined.