Chaiyaphum Province sits in the westernmost edge of this plateau, mostly covered by Triassic-Tertiary sedimentary rocks and Permian limestone (Singtuen and Won-In 2018). The Prakai Phet cave of Chaiyaphum Province, located in the Phetchabun Mountain Range, is made of dolomitic limestone karst formed in the Permian Period and it served as a refuge for several Pleistocene mammals (Filoux et al. 2014). This cave is about 1.5 km long, ending in a 45 m-deep abyss and only 15 m of the cave’s anterior is open to tourists. The present author visited the cave in the rainy season once a year – September and October 2017–2019 – and a single population of the new copepod on the floor and stalagmites, which are formed by dripping water, was collected at about 4–5 m from the cave’s entrance (Fig. 1).

Samples were collected using a hand net (60 µm) from drip pools on the stalagmites and the cave floor and then fixed immediately in ca. 70% ethanol. Adult animals were picked out and preserved with 70% ethanol in 1.5 ml microtube. Adult specimens were dissected under an Olympus SZ51 stereomicroscope in a mixture of glycerol and 70% ethanol (ratio ~ 1:10 v/v). Dissected specimens were mounted in pure glycerol and sealed with transparent nail polish. Permanent slides with dissected animals were examined with an Olympus compound microscope (CX31) at 1000× magnification. Pencil drawings were made with a drawing tube (an Olympus U-Da) mounted on a compound microscope, then the final drawings were scanned for correction in the CORELDRAW 12.0 graphic programme. Specimens for scanning electron microscopy (SEM) were dehydrated in progressive ethanol concentrations (70%, 80%, 90%, 95%, 100% and 100% absolute ethanol) for 15 min each concentration. Specimens were dried in a critical point dryer using liquid carbon dioxide as the exchange medium. Dried specimens were mounted on stubs using adhesive tape under the stereomicroscope. Specimens were coated with gold in a sputter-coater. The SEM photographs were made using a scanning electron microscope (LEO 1450 VP).

The morphological terminology follows Huys and Boxshall (1991). Specimens were deposited at the Natural History Museum, London, United Kingdom (NHMUK) and the Nakhon Phanom University, Faculty of Science, Thailand (NPU).

The following abbreviations are used throughout the text and figures:

A aesthetasc;

Enp endopod;

Exp exopod;

Exp/Enp-n exopodal segment n/endopodal segment n;

P1–P6 swimming legs 1–6;

Sp spine/spines.

Kiefer (1927) separated three Java species from the genus Cyclops Müller O.F., 1785: C. anninae Menzel, 1926; C. bogoriensis Menzel, 1926; and C. muscicola Menzel, 1926. From these, he erected the genus Bryocyclops Kiefer, 1927, the species of which were characterised by P5 being completely fused to the thoracic somite, having three setae. Earlier species, described before 1937 and having this reduced P5, were considered members of Bryocyclops.

In 1937, Kiefer provided the additional four characteristics of the genus: 1) a genital segment that is wider than it is long, 2) a P4 coxa without inner seta, 3) a male P3 with a transformed spine and 4) an acute distal margin of the intercoxal sclerite of the legs (unspecified legs, but probably P1–P4).

Later, Lindberg (1953–1954) revised the genus and lumped all species into Bryocyclops s. lato by using the following characteristics: 1) reduced P5, 2) sexual dimorphism of P3–P4 (male P3 with/without a transformed spine), 3) P4 coxa without inner seta, 4) P1 basis with/without inner spine, 5) round or acute distal margin of the intercoxal sclerite of P4, 6) variable spine and setae formulae for P1–P4 Exp-2, 7) variable armature on P4 Enp, 8) male P6 with 2–3 elements and 9) bulging anterior receptaculum seminis, which is more developed than the posterior receptaculum seminis. At the same time, Lindberg also divided Bryocyclops s. lato. into six groups (Groups I–VI).

The diagnosis of the genus Bryocyclops s. lato, as well as the status of the groups comprised in it, has been much debated recently and its members have been separated into five genera: Bryocyclops s. str.; Haplocyclops Kiefer, 1952; Palaeocyclops Monchenko, 1972; Rybocyclops Dussart, 1982; and Thalamocyclops Fiers & Van Damme, 2017. Although some authors believe that the Palaeocyclops genus should have remained the subgenus Bryocyclops (Palaeocyclops) Monchenko, 1972, the status of the genus Palaeocyclops is indubitable when considering the possibility of convergences and the morphological differences (Fiers 2002; Dussart and Defaye 2006; Ranga Reddy and Defaye 2008; Fiers and Van Damme 2017).

Members of Bryocyclops s. str. have acute intercoxal sclerites on their free distal margins, at least on P4. The swimming legs of many early known species (before 1972) are incompletely illustrated and described, except for P4. Regarding the available information, all species of Groups III–VI are characterised by round distal margins in all legs (P1–P4) versus the acute forms on P4 in Groups I, II and VII. In addition, amongst Groups I, II and VII, the anterior legs tend to have rounder distal margins compared to the posterior legs. For example, P1 is round in the females of B. anninae (Menzel, 1926) and B. asetus Watiroyram, 2018 (Watiroyram 2018a; Sanoamuang et al. 2019).

The presence of an inner coxal seta is also, so far, unified for the system of cyclopine genera (Reid and Spooner 1998). This seta and a medial spine at the basis are present on P1, but lacking on other legs in all known Bryocyclops s. str. species and both of these characteristics are identical in the Bryocyclops s. str., Palaeocyclops and Thalamocyclops genera. However, the inner coxal seta is absent on the P1 of some Bryocyclops species, such as B. absalomi Por, 1981, B. asetus, B. maholarnensis and B. jayabhumi sp. nov. Thus, the presence of the inner coxal seta on P1 is a non-significant characteristic for generic diagnosis of Bryocyclops s. str.

All Bryocyclops s. str. species exhibit specialised sexual dimorphism on P3 and P4, which other cyclopids usually express only on the urosomal segmentation, antennules, P5 and P6 (Fiers 2002). Rocha and Bjornberg (1987) note that the transformed spine of the male P3 Enp-2 is traditionally used to determine genus because it is present amongst all Bryocyclops s. str. Additionally, the discovery of new species has confirmed that the apical spine of the male P3 Enp-2 varies in the degree of transformation (its tip, position and ornamentation of medial swelling). For example, B. maewaensis has the greatest medial swelling at the distal half of the spine, produced in the outer margin, similar to most of its congeners; B. jayabhumi sp. nov. has less medial swelling at about one-half of the spine length and is enlarged in both margins; and B. maholarnensis presents the lowest level of transformation. These three species have no ornamentation (spinules) on the surface of their expended parts, but these spinules are present in B. asetus, B. muscicola, B. muscicoloides and B. trangensis. This transformed spine on the male P3 Enp-2 has also been reported in the Thalamocyclops, Palaeocyclops and Siamcyclops genera (Monchenko 1972; Fiers and Van Damme 2017; Boonyanusith et al. 2018). This characteristic has traditionally been used to recognise Bryocyclops and to reasonably conclude that the genera Thalamocyclops, Palaeocyclops and Siamcyclops are more closely related to Bryocyclops s. str. than are Haplocyclops and Rybocyclops. Reid and Spooner (1998) propose that the female P4 Enp of Bryocyclops, which has five elements in most previously-described species (e.g. B. bogoriensis, B. campaneri, B. caroli, B. muscicola, B. muscicoloides, B. trangensis etc.) is considered plesiomorphic. Thus, the presence of two instead of five elements on the P4 Enp in B. maholarnensis and B. jayabhumi sp. nov. (Group VII) is probably apomorphic and new to the genus.

The male P6 of the new species and of all Bryocyclops s. str. (except B. africanus, Group III) and Thalamocyclops species, has three setae. These are considered to be plesiomorphic formations in cyclopines, which have been reduced to two setae in closely-related genera, including Palaeocyclops, Rybocyclops and Haplocyclops. The female P6 of Bryocyclops s. str. and Thalamocyclops are also similar, but they show more reduction than the male forms; the P6 vestiges have three elements in spiniform and setiform, but they always appear in Haplocyclops and Rybocyclops with setae (unknown amongst Palaeocyclops). Amongst seven species collected in Thailand, the P6 of B. trangensis is most similar to Thalamocyclops (but B. trangensis and other Bryocyclops are easily distinguished from Thalamocyclops by the structure of P5), having two setae and one spinule, versus six other species armed with one seta and two spinules (B. jayabhumi sp. nov. with shorter and stronger setae than B. asetus, B. maewaensis, B. maholarnensis, B. muscicola and B. muscicoloides).

The new species, depicted in this study, is ornamented with refractile points or pits on the body surface, which is another characteristic presented mostly in Bryocyclops s. str., except for B. asetus (Watiroyram 2018a). Although the possession of refractile points is not confined only to Bryocyclops, it is useful for separating Bryocyclops species from those in closely-related genera, which lack this ornamentation (J. Reid, personal communication), including Rybocyclops (= B. pauliani), Haplocyclops (= B. correctus, B. neuter) and Thalamocyclops (= B. soqotraensis) (Reid 1999; Fiers 2002; Fiers and Van Damme 2017; Watiroyram 2018b). Actually, this characteristic is either present or absent amongst copepod species of the same genus in both harpacticoid and cyclopoid copepods, especially those living in groundwater, such as Halicyclops Norman, 1903 (H. maculatus Rocha & Hakenkamp, 1993, from wells); Speocyclops Kiefer, 1937 (S. orcinus Kiefer, 1937, from caves); Elaphoidella Chappuis, 1929 (E. bromeliaecola (Chappuis, 1928), from caves and phytotelmata); Moraria T. & A. Scott 1893 (M. affinis Chappuis, 1927, from moss; M. cristata Chappuis, 1929, from caves; and M. laurentica Willey, 1927, from litter) (Rocha and Hakenkamp 1993; Fiers and Moldovan 2012; Watiroyram 2018a). The possession of refraction points is also probably associated with weak swimmers in groundwater, scleractinian and parasitic, rather than true planktons; it may also depend on the species lineage, rather than on convergent evolution forced from dependency in the same habitats. This idea is supported by the presence of two cyclopines in the same cave from Vietnam: Pseudograeteriella longiaesthetascus Sanoamuang, Boonyanusith & Brancelj, 2019, which does not have this characteristic and Bryocyclops anninae, which possesses the characteristic and has been found outside the cave in phytothelmata and wet moss. This characteristic is also lacking in the other stygobitic copepods recently reported in various Thailand caves: Boholina laorsriae Boonyanusith, Wongkamhaeng & Athibai, 2020; Fierscyclops tanaosriensis Boonyanusith, Brancelj & Sanoamuang, 2013; F. solaris Boonyanusith, Brancelj & Sanoamuang, 2013; Metacyclops thailandicus Boonyanusith, Sanoamuang & Brancelj, 2018; and Siamcyclops cavernicolus Boonyanusith, Sanoamuang & Brancelj, 2018. Likewise, the blunt or obtuse setae on P2–P3 Exp-2 (and, rarely, on P1) were always present in the early observations of Bryocyclops s. str. compared to other cyclopines (Reid 1999; Sanoamuang et al. 2019). However, the Bryocyclops species collected recently in Thailand differ from previous reports in that B. asetus, B. muscicoloides and B. trangensis lack blunt setae (Watiroyram 2018a, b).

The receptaculum seminis is also useful for determining species and genera in the Bryocyclops s. lato. Although this characteristic is still unknown for many precise species, it seems that Bryocyclops species have a developed anterior part of the receptaculum seminis, as do Haplocyclops, Thalamocyclops species: B. anninae; B. caroli Bjornberg, 1985; B. campaneri Rocha & Bjornberg, 1987; and B. jayabhumi sp. nov. (Fiers and Van Damme 2017; Sanoamuang et al. 2019; the present study). On the contrary, both the anterior and posterior portions of this structure are developed in Palaeocyclops and Rybocyclops species (Dussart and Defaye 2001; Ranga Reddy and Defaye 2008). The shape of the spermatophore has, therefore, been used as a generic characteristic to differentiate Siamcyclops from Bryocyclops s. str. Siamcyclops have L-shaped spermatophore structures versus the kidney or bean-shaped spermatophore structures found in most Bryocyclops species (Boonyanusith et al. 2018). However, this characteristic does not fit B. trangensis and B. asetus, which present two other shapes (see Watiroyram 2018a, fig. 3C; Watiroyram 2018b, figs 8C, 9A). It seems, therefore, that the spermatophore structure takes a unique form in Bryocyclops s. str. and, thus, it must be re-observed amongst the earlier discovered species, whose descriptions are lacking this information. The shapes of spermatophore structures vary more amongst cyclopoids than do those in harpacticoids and calanoids, which are generally cylindrical (Williamson and Reid 2009); in this way, spermatophore shapes may be used to determine what species belong to the Bryocyclops s. str. genus.