Three bathypelagic scolecitrichids new to the North Pacific, with comments on diagnostic features of Scolecitrichidae and Tharybidae and on the positions of Heteramalla, Rythabis and Parkius (Copepoda: Calanoida)

Abstract

Additional descriptions and illustrations of the females of Heteramalla sarsi Roe, 1975, Amallothrix aspinosa Roe, 1975 and Landrumius gigas (A. Scott, 1909), collected in the central and northwestern Pacific for the first time, are presented. The diagnostic significance of the setation of the maxillules, maxillae and maxillipeds in Scolecitrichidae, Tharybidae and Diaixidae is analyzed. The proportions of maxillule expressed as indices showing length or width of its different parts in relation to the width of the first inner lobe are proposed here as the main distinguishing characters separating Scolecitrichidae and Tharybidae, more informative than some diagnostic features of its setation. The relationships between the Bradfordian families and between sections of the family Scolecitrichidae are discussed, mainly on the basis of setation of the maxilla and maxilliped. The systematic positions of A. aspinosa and the genera Heteramalla, Puchinia, Cenognatha, Plesioscolecithrix, Rythabis and Parkius are discussed. A detailed comparative analysis of characters of the genus Heteramalla is provided. Heteramalla and Puchinia belong to the group of early offshoots of the family Scolecitrichidae, not to the central core, where they were placed by Ohtsuka with coauthors (2003) and by Boxshall & Hulsey (2004). Rythabis has the setation of maxillule closer to the ancestral type common to all Bradfordian families, but the proportions of maxillule typical to Tharybidae. Since the indices of maxillule do not overlap in Scolecitrichidae and Tharybidae, and the maxillule and other mouthparts show different evolutionary trends of specialization, Rythabis is considered as a member of the Tharybidae, where it was placed by Schulz (in Schulz & Beckmann, 1995). Parkius has the proportions of the maxillule intermediate between those in Scolecitrichidae and Tharybidae and unique apomorphic features recorded earlier by Ferrari & Markhaseva (1996). These unique progressive features of Parkius can be a result of important biological transformations, compared with the other Bradfordian families; therefore the status of the separate family Parkiidae, fam. dist., is restored. The male considered as Racovitzanus sp. (Grice & Hulsemann, 1967) actually belongs to Parkius. Swimming legs 5 of this male are peculiar, fitting those neither of Scolecitrichidae nor of Tharybidae. Their structure shows a separate evolutionary trend and also justifies the family status of Parkiidae.