Kentrophoros magnus sp. nov. (Ciliophora, Karyorelictea), a new flagship species of marine interstitial ciliates

The karyorelictean ciliate Kentrophoros lacks a defined oral apparatus but has a dense coat of symbiotic bacteria that it consumes by phagocytosis. Body size, shape, and nuclear characters are variable in this genus. We formally describe a new species, K. magnus from Elba (Italy), which has unusual folding of its symbiont-bearing surface into pouch-like compartments, a body form that we term "pseudotrophosomal". K. magnus cells are large (2100 ± 700 × 170 ± 23 µm in vivo), but contain only one micronucleus and two macronuclei, although these are much bigger than other Kentrophoros (widths 20 ± 2.5 and 31 ± 4.0 µm respectively in K. magnus). We also present morphological observations on a close relative from Twin Cayes (Belize), which also has relatively large nuclei (micronuclei 13 ± 1.5 µm, mature macronuclei 20 ± 2.8 µm), but unlike K. magnus, it has on average 22 nuclei per cell, with different developmental stages of the macronuclei present simultaneously, and lacks pouch-like folding. Nuclear number and arrangement are important characters for karyorelicts. We suggest the use of a "nuclear formula" to simplify descriptions. Our discovery of large and morphologically distinctive new species underlines the incompleteness of our knowledge about meiofaunal ciliates.