Two new ascothoracidan species in the family Ascothoracidae Grygier, 1987, Ascothorax rybakovi sp. nov. and Cardiosaccus pedri gen. et sp. nov., are described from the Kuril-Kamchatka Trench at abyssal depths of 5150–5700 m. They are among the deepest known ascothoracidans. They live as mesoparasites in permanent cysts within the bursae of the brittle stars Ophiacantha pacifica Lütken and Mortensen, 1899 and Amphiophiura pacifica Litvinova, 1971, respectively. This is the first study of new forms of Ascothoracidae in which SEM has been used in the descriptions, including the lattice organs and the early nauplius. Cardiosaccus differs from Ascothorax in displaying the following apomorphic traits: (i) absence of thoracopod 1; (ii) a smaller fifth antennular segment that is completely fused with the claw; (iii) absence of small setae at the base of this claw; and (iv) an extremely large ventral protrusion of the fourth abdominal segment. Material of Parascothorax synagogoides Wagin, 1964 from near the type locality, studied with SEM, could not be distinguished from Ascothorax by any reliable morphological criteria. Cyst structure and the manner of cyst formation cannot be used, because although cyst is the result of the parasites activity, this is a feature of the host, not something inherent to the parasite. Moreover, the inferred settlement of the larvae of P. synagogoides on the outer edge of the bursal slit is best regarded as plesiomorphic behavior, reflecting how the Ascothoracidae became bursal mesoparasites in the first place. We therefore regard Parascothorax as invalid and refer P. synagogoides to Ascothorax. Within the family Ascothoracidae as now constituted, Ascothorax is cosmopolitan and speciose and includes species that exclusively infect a single host species, whereas Cardiosaccus is monotypic and restricted to the northwest Pacific. Ascothorax rybakovi sp. nov. is the third species of this genus known to harbor hyperparasitic cryptoniscid isopods.
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The authors would like to thank Prof. Angelika Brandt and Prof. Marina Malyutina for inviting ASP to take part in the KURAMBIO II cruise and to Prof. Pedro Martinez Arbizu for providing assistance and help in his lab on board. We are grateful to Anastassya Maiorova for permission to use her photographs of the living animals. We are indebted to Prof. Mark J. Grygier for correction of our MS. We are obliged to Dr. Sabine Stöhr for consultations on hosts’ distribution. We would like to thank Dr. Lena Menzel for editorial revision.
For ASP and GAK, this study was financially supported by the Russian Foundation for Basic Research (grants 17-54-52006 MNT_a, 18-04-00624 А, and 15-29-02447 ofi_m). SEM and TEM studies, and drafting of the paper were supported by the Russian Scientific Foundation, grant 14-50-00029.
Conflict of interest
The authors declare that they have no conflict of interest.
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed by the authors.
Sampling and field studies
All necessary permits for sampling and observational field studies have been obtained by the authors from the competent authorities and are mentioned in the acknowledgements, if applicable.
Cardiosaccus pedri sp. nov.
Ascothorax rybakovi sp. nov.
Communicated by A. Brandt
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Kolbasov, G.A., Petrunina, A.S. The family Ascothoracidae Grygier, 1987, a review with descriptions of new abyssal taxa parasitizing ophiuroids and remarks on the invalidity of the genus Parascothorax Wagin, 1964 (Crustacea: Thecostraca: Ascothoracida). Mar Biodiv 49, 1417–1447 (2019). https://doi.org/10.1007/s12526-018-0921-6
- Cysticolous parasites of echinoderms
- New species
- Key to species
- Lattice organs