A new species and depth record of bopyrid (Crustacea, Isopoda) from a squat lobster in the Kuril-Kamchatka Trench

The branchial parasitic isopod Pleurocryptella altalis sp. nov. (Bopyridae: Pseudioninae) is described from the squat lobster host Munidopsis petalorhyncha Baba, 2005. The new species is morphologically similar to Pleurocryptella formosa Bonnier, 1900 and P. wolffi Bourdon, 1972b but can be distinguished based on male characters (differences in head, pleon and uropods) and female characters (differences in barbula, pleopods and pleotelson). The parasite specimens (a female and male pair) were collected with the squat lobster host at a depth of 5060–5130 m from the Kuril-Kamchatka Trench, representing the deepest record for any of the 850+ described bopyrid isopod species and for any record of an infested host. Dichotomous identifi cation keys to females and males of Pleurocryptella species and subspecies are provided.


Introduction
The genus Pleurocryptella Bonnier, 1900 is generally considered to be the most primitive genus in Bopyridae Rafinesque, 1815(Shiino 1965Bourdon 1979;Markham 1986;Boyko & Williams 2009, 2010. Females of species in this genus all possess fully developed oostegites on pereomeres 1-5 and a rudimentary oostegite on each of pereomeres 6 and 7, whereas all other bopyrids lack oostegites on the posterior two pereomeres. Some families of isopods (e.g., Aegidae White, 1850 and Cymothoidae Leach, 1818) that may be basal to bopyrids (Boyko et al. 2013) contain species that possess seven oostegites.
However, the oostegite number and morphology is not thought to be useful in systematic analyses at higher phylogenetic levels (Brusca & Wilson 1991) and the presence of oostegites on pereomeres 6 and 7 could actually be a derived feature in Pleurocryptella, but further analysis, ideally using molecular data, is needed.
Although in many genera of bopyrids female characters are of primary taxonomic importance, males of Pleurocryptella provide taxonomically important characters at the genus and species levels. Males of all species of Pleurocryptella have well-developed pleopods, either small and rounded or broad and subquadrate or conical, and articulated uropods; uropods occur on males in only a few other bopyrid genera (e.g., Gigantione Kossmann, 1881;Parapleurocryptella Bourdon, 1972a). Both males and females of species in Pleurocryptella have biarticulated maxilliped palps. While unusual, this condition also occurs in a few other genera (e.g., males and females in species of Parapleurocryptella; males in the species of Goleathopseudione Román-Contreras, 2008 and Pagurocryptella Boyko & Williams, 2010). To date, ten species and subspecies have been described in Pleurocryptella from galatheid and paguroid hosts (Boyko et al. 2008 onwards), but P. paguri Bourdon, 1979 Shiino 1937). Examination of the morphology of all nine described species and subspecies of Pleurocryptella, based on published and unpublished descriptions and illustrations, particularly of males, suggests that more than one phylogenetic lineage is present in the genus. The type species, P. formosa Bonnier, 1900, andP. wolffi Bourdon, 1972b appear to be more closely related to species of Paragigantione Barnard, 1920 than to the other taxa currently placed in Pleurocryptella. In the present paper, we describe a new species of Pleurocryptella that is closely related to P. formosa and P. wolffi. It is not only the deepest identified bopyrid ever reported, but it is from deeper water than all reported bopyridiform swellings on any decapod hosts.

Material and methods
The host specimen of Munidopsis petalorhyncha Baba, 2005 and the parasitic isopods were collected in the Kuril-Kamchatka Trench (45°18′ N, 156°00′ E) during the 39 th cruise of the R/V 'Vityaz' by Sigsbee trawl at abyssal depths of 5060-5130 m in August 1966 (Birstein & Zarenkov 1970) and deposited in the collection of the Zoological Museum of Moscow State University (ZMMU), Moscow, Russia. The original fixative used was not recorded; however, according to the museum staff, primary fixation was made with formalin followed by storage in 70% ethanol that allowed for preservation of the samples in very good condition for a long period of time. For the present study, the parasitic isopods (female and attached male) were carefully extracted from the host's branchial cavity and their morphology was drawn using a camera lucida attached to an Olympus SZX10 light microscope. Measurements were taken to 0.1 mm with a caliper: the size of the isopods is given as total length (TL) from the front edge of the head to the posterior edge of the pleotelson (exclusive of uropods); carapace length (CL) (with rostrum) and carapace width (CW) are provided for the host specimen (see Komai et al. 2017). The isopod type material and their host are deposited in ZMMU. For those taxonomic authorities not specifically cited in the text, we herein refer the reader to them for ease in locating original descriptions of the taxa (Rafinesque 1815;Latreille 1817;Kossmann 1881;Bonnier 1900;Barnard 1920;Nierstrasz & Brender à Brandis 1923;Codreanu 1967;Bourdon 1972aBourdon , 1976Bourdon , 1981Román-Contreras 2008).

Note on host identification
The host was originally identified as the holotype of Munidopsis subsquamosa latimana Birstein & Zarenkov, 1970, but, because that name is a junior homonym of M. latimana Miyake & Baba, 1966, Baba (2005 proposed the replacement name M. petalorhyncha Baba, 2005.

Remarks
The new species appears most closely related to Pleurocryptella formosa and P. wolffi. Males of these three species all have low, broad midventral tubercles on pleomeres 1-4 that cover nearly all of the space between the pleopods, whereas males of all other species in the genus have small, semispherical tubercles on the pleomeres that do not cover the space between the pleopods. Both sexes of Pleurocryptella altalis sp. nov. are distinguishable from those of P. formosa and P. wolffi. Males of P. altalis sp. nov. have the lateral margins of the pleomeres curled laterally and covering the pleopods in ventral view (also curled in P. wolffi, but splayed laterally and pleopods visible in ventral view in P. formosa), the head nearly as broad as pleomere 1 (also as broad in P. wolffi but much narrower than pleomere 1 in P. formosa), and the uropods larger than pleopod 5 (uropods smaller than pleopod 5 in P. wolffi and the same size as pleopod 5 in P. formosa). Females of P. altalis sp. nov. have the barbula with two acute, slender lobes of approximately the same length (barbula with outer lobe tapered but much broader, inner lobe much shorter than outer and rounded in P. wolffi and P. formosa), pleopod 5 less than half as large as pleopod 4 and uniramous (pleopod 5 more than half as large as pleopod 4 and biramous in P. wolffi and P. formosa), and pleomeres smoothly curved with pleomere 5 not surrounding pleotelson (pleomeres smoothly curved, but surrounding pleotelson in P. wolffi and not smoothly curved (angular middle portion) and not surrounding pleotelson in P. formosa).  (Richardson, 1910) Bourdon, 1981 * Pleurocryptella latimellaris (Nierstrasz & Brender à Brandis, 1931) not included in the key. ** Data for P. laevis based on Bourdon (unpublished MS). *** Based on published and unpublished data, the males of P. laevis and P. infecta tuberculata are indistinguishable.

Discussion
In light of the marked differences between the males of the various species of Pleurocryptella, a thorough revision of the genus is warranted. Richardson (1910) incompletely described and figured P. laevis, but Bourdon (unpublished) later redescribed it on the basis of the types and other material. Fortunately, one of us (CBB) was given a copy of Bourdon's MS during a visit to the Muséum national d'histoire naturelle (MNHN) in 2002, which has aided in the production of our key to species (see above). Likewise, Pseudione latilamellaris was incompletely described and illustrated by Nierstrasz & Brender à Brandis (1931) and was placed in Pleurocryptella by Bourdon (1979) without comment as to the reason for this placement. Several characters of the male and female of P. latilamellaris suggest that this species is not congeneric with the other species of Pleurocryptella, even if the females possess the rudimentary oostegites on pleomeres 6 and 7.
Pleurocryptella altalis sp. nov. represents the deepest record for any bopyrid parasite, including all described species and unidentified bopyridiform swellings on hosts. Although the depth of the host's collection has previously been given as 5035-5210 m (Birstein & Zarenkov 1970, 1972Baba 2005) or 5025-5210 m (Boyko et al. 2012), the actual depth is 5060-5130 m (Komai et al. 2017; Marin 2020). The host species, Munidopsis petalorhyncha, is also recorded from the east slope of the Iwaki Seamount off the coast of Japan at greater depths (5353-5380 m; see Komai et al. 2017), and it is possible that the bathymetric and geographic ranges of the parasite are similar to those of its host, although parasites often occur only in a portion of the range of a host species (Pielou 1974