The Cyclopinae are among the most common and numerous freshwater cyclopid copepods. They are found in various aquatic biotopes, most of them are planktonic, typical of pelagic lakes, other ones are bottom-dwelling inhabitants of different freshwater bodies and moist semiterrestrial habitats; numerous species and some genera have been reported specifically from subterranean aquatic habitats, where they usually live within small spaces of coarse substrates rather than in pools.

Some large, epigean species, such as Acanthocyclops robustus (G. O. Sars, 1863), A. vernalis (Fischer A., 1853), Diacyclops bicuspidatus odessanus (Schmankevitch, 1875), Eucyclops serrulatus (Fischer, 1851) and Macrocyclops albidus (Jurine, 1820) can mantain permanent stygophilic populations in interstitial biotopes, as well (Pesce 1985; Pesce & Maggi 1983).

Although the Cyclopinae phenotype tends to be quite conservative (Chappuis 1920; Reid 1994), several morphological modifications involve the stygobitic members of this subfamily. Primary changes are related to reduction of the body size and somites. Particularly, most stygobitic species bear a large genital double-somite, generally much wider than long, short caudal rami, reduced antennula and antenna, as well as they undergo attenuation and shortening of the swimming legs, accomplished by fusion or loss of primitive segments.

In some cases reduction in number and length of setae/spines may also occur following oligomerization of the swimming legs (Reid 1994). Recently, Boxshall et al. (1993) described a secondary pseudosomite anterior to the genital double-somite in females of Diacyclops biceri Boxshall et al. 1994, interpreting this feature as an adaptation allowing a greater range of urosomal movements in the intertitial habitat.

At present the subfamily comprises 42 (+1; Fiers, in press) genera and some subgenera which, according to the the number of articles of the antennula, the increasing oligomerization of swimming legs 1-4 and the construction and armature of leg 5, could be classified into six distinct groups, including phenetically allied species viz. Orthocyclops Forbes, 1897, Cyclops (Kiefer, 1939), Austriocyclops Kiefer, 1964, Mixocy-' clops Kiefer, 1944, Microcyclops Claus, 1893 and Bryocyclops Kiefer, 1927.

The genera Diacyclops and Acanthocylops (Diacyclops-Acanthocyclops complex), owing to the still preoccupied and debated taxonomy, are provisionally placed partly in the Cyclops and Mixocyclops groups. Kiefer (1927) firstly subdivided the above complex into three distinct genera, viz. Acanthocyclops, Megacyclops and Diacyclops; subsequently, the separation of these genera has been questioned by Reid 1993); other authors (Monchenko 1974; Dussart & Defaye 1985; Einsle 1993) still accept both as valid genera. Anyway, due to the noteworthy variation among species and the clear overlap between the genera, the complex Diacyclops-Acanthocyclops is in urgent need of exhaustive revision.

Pesce (1996) reviewed the subfamily Cyclopinae, suggesting six morphological groups. The first group (Orthocyclops-group) is monogeneric, containing only the genus Orthocyclops. Members, which are inhabitants of epigean water bodies, are characterized by the most plesiomorphic condition for all the nominate characters, viz. they show a 16-segmented antennule, swimming legs 1-4 with 3-segmented rami and a 3-segmented leg 5.

Likewise, swimming legs with both rami 3-segmented, are shared by the genera Austriocyclops, Ponticyclops Reid, 1986 and Australocyclops Morton, 1985 , whose member species have a variously reduced segmentation of the antennule and leg 5 1-segmented or completely fused to somite (Austriocyclops-group).

Other primitive genera [Cyclops, Megacyclops, Mesocyclops, Thermocyclops, Kieferiella Lescher-Moutoué, 1976, Diacyclops (partim), Acanthocyclops (partim)] share a plesiomorphic state of most appendages, such as the 12-17 (rarely 11) segmented antennula, the swimming legs with 3-segmented rami, but the leg 5 consisting of two distinct segments. The members of these genera are generally large species, mostly distributed in surface fresh waters, only a few stygobitic or stigophilic species occurring in groundwater biotopes (Cyclops-group).

A leg 5 composed of two distinct segments distinguishes also the genera Caspicyclops Monchenko, 1986, Mixocyclops, Diacyclops (partim) and Acanthocyclops (partim), but their member species are characterized by 10-11 (rarely 16) segmented antennula and swimming legs with variously reduced number of segments of both exopodite and endopodite. The reduction occurs at first in the anterior swimming legs and endopodites, then in the posterior legs and exopodites. In some apomorphic Diacyclops species (virginianus-group) the reduction regards anterior and posterior legs, all with 2-segmented rami, or the exopodite of legs 3-4, 3-segmented in some species. The greatest reduction state in the above genus is shown by D. trajani (Petkovski, 1954), which has all the swimming legs completely 2-segmented (Reid 1994). The members of the above genera can be found both in surface freshwater bodies and in ground waters, with stygophilic or stigobitic species (Mixocyclops-group).

The fifth group (Microcyclops-group) comprises genera with leg 5 consisting of a single free segment due to the fusion of the proximal segment to the somite. The representatives of this group show a reduced antennula (10-11 segments) and swimming legs 1-4 with both endopodite and exopodite 2-segmented. In the genus Hesperocyclops Herbst, 1984 the female leg 4 endopodite is 1-segmented. In the same group the free segment of leg 5 can bear one (Cryptocyclops G. O. Sars, 1927, Idiocyclops Herbst, 1987, Microcyclops Claus, 1893), two [Hesperocyclops, Graeteriella Brehm, 1926, with subgenus Paragraeteriella Rylov, 1948/63, Fimbricyclops, Reid 1993, Menzeliella Lindberg, 1954, Metacyclops Kiefer, 1927, Apocyclops Lindberg, 1942, Speocyclops, Kiefer, 1937, Muscocyclops Kiefer, 1937, Goniocyclops Kiefer, 1955 (= Psammophilocyclops Fryer, 1956), and probably Cochlacocyclops Kiefer, 1955] or three (Psammocyclops Kiefer, 1955) spines/setae.

In some genera the free segment of leg 5 may be also ornamented with an unsocketed spinula, along the middle margin; in the genus Psammocyclops, the same segment is composed of fusion of the original segments, therefore it bears 3 setae. Other genera lack the seta on the fifth thoracic somite. In some species of the genus Speocyclops the free segment of leg 5 may be partially fused with the somite. In other genera of this group, including Goniocyclops, Muscocyclops and Speocyclops, the anal operculum is considerably produced and, sometimes, toothed or serrate.

As regard the inadequately characterized, monospecific genus Teratocyclops Plesa, 1981, long since Dussart & Defaye (1985) noticed its incomplete description and illustrations; later on Reid (1993) too, remarked the absence of adequate figures in Plesa's description, hypothesizing as well that both segments of leg 5 could be distinct in this genus. Nevertheless, recent re-examination of the type material (Plesa in litt.) revealed that the leg 5 is composed of a single free segment, therefore the nominate genus fits well into the above group, being very close to the genus Metacyclops. All the nominate genera are for the most part stygobitic inhabitants of different groundwater habitats.

The last group (Bryocyclops-group) includes more derived genera whose representatives can be found mostly in ground waters. The members of this group (Yansacyclops Reid, 1988, Allocyclops Kiefer, 1932, Bacillocyclops Lindberg, 1956, Bryocyclops Kiefer, 1927 s. 1.) show both segments of leg 5 fused to the-somite, 1-2 setae, sometimes a little knob, representing the distal segment remaining. In the same genera the antennula and the swimming legs are strongly reduced, viz. the antennula is 10-11-segmented and the swimming legs with both endopodite and exopodite 2-segmented or the endopodite and exopodite of leg 4, 1-segmented.

The genus Bryocyclops was firstly divided by Kiefer (1927, 1952) into the subgenera Bryocyclops s. str. and Haplocyclops Kiefer, 1952. Subsequently, Lindberg (1956) splitted the same genus into six groups (subgenera) according to such characters as the sexual dimorphism, the armature of the leg 1 basipodite, the morphology of the couplers of leg 4, the spines/setae formula of legs 1-4 and the segmentation of the leg 4 exopodite. At present, according to Dussart (1982), the genus should be divided at least into three morphological subgroups, viz. Bryocyclops s. str.-subgroup, whose members are characterized by coxopodite and basipodite of leg 1 with inner spine, and couplers of leg 4 with pointed protuberances; Rybocyclops-subgroup, without spine on both coxopodite and basipodite of leg 1, and couplers of leg 4 with little developed, rounded protuberances; Haplocyclops-subgroup, with coxopodite and basipodite of leg 1 without or with inner spine, respectively, and couplers of leg 4 little produced, rounded.

Independently of the above morphological groups, further reductions may occurr in some species or genera; the most important of them are the mandibular palp reduced to 1-2 setae (Allocyclops kieferi Petkovski, 1971, Diacyclops imparilis Monchenko, 1985, Fimbricyclops), or completely absent [Speocyclops demetiensis (Scourfield, 1932), Caspicyclops mirabilis Monchenko, 1986, Muscocyclops], the reduced number of setae on the basipodite and second endopodal segment of the antenna, and the reduced or lacking armature of the basis, coxa and couplers of the swimming legs.

The setation of the antenna was firstly recognized as fundamental character by Fiers & Van de Velde (1984), who pointed out distinct patterns of armature (spines/teeth) on the basipodite of numerous species of cyclopinae, as well as they stated that the primitive condition for this appendage is "probably when the whole surface is covered with teeth and /or spines". Successively, Reid (1991) found losses of one or more setae from the basipodite of the same appendage, including the exopodal vestigial seta, pointing out the great importance of these reductions in the specific diagnosis of the cyclopinae as well as of cyclopoid copepods.

The second endopodal segment of the antenna retains 9 setae (likely plesiomorphic condition) only in few primitive genera, in others the same segment has undergone repeated reduction: eight setae can be found in some species of the genus Acanthocyclops and Megacyclops, 6-7 setae in species of the genera Microcyclops, Megacyclops, Cryptocyclops and Diacyclops, five in Graeteriella unisetigera (Graeter, 1908) and Hesperocyclops venezuelanus Pesce & Galassi, 1992. The most apomorphic states can be found in Diacyclops dimorphus Reid, 1994 and Diacyclops paolae Pesce & Galassi, 1987, with four and three setae, respectively.

Unfortunately, in modern taxonomic accounts of cyclopinae there is almost no description or illustrations of the antenna, as well as of other mouthparts, so that these important features are generally overlooked by the authors. Einsle (1985) pointed out a further criterion for identification of species in the complicated genus Cyclops, considering the patterns of spines on the coxa of leg 4, and caming to the conclusion that the patterns are relatively uniform within the species, but noteworthy variation and reductions exist among species.

Recently, it has been shown that some species of the genera Bryocyclops, Haplocyclops, Graeteriella, Hesperocyclops, Metacyclops and Diacyclops show a sexual dimorphism in the swimming legs consisting in a reduction occurring in the female and primarily in the endopodites of the posterior legs (Reid 1994). Reid (1991) discovered also elaborate ornamentation of swimming legs couplers and increasing thickness of the same legs in benthic cyclopids.

Some species of the genera Bryocyclops and Muscocyclops and the species Fimbricyclops jimhensoni Reid, 1993 show elaborate arrays of spines and hairs on the swimming legs, anal somite and caudal rami, which Reid (1993) interpreted as a parallel adaptation for life in semiterrestrial habitats. After all, many species of copepods, including the cyclopinae, are known to possess integumental perforations on the antennule and other appendages (Bresciani 1986), but their function is for the better part unknown.

As regard the cyclopinae, circular pits on the articles of the antennule of some Mesocyclops species were firstly pointed out by Von Daday (1906), who interpreted them as integumental tubercules. Reid & Saunders (1986) recognized similar structures in M. aspericornis (Daday, 1906) and in some species of the genus Thermocyclops, and considered them as sensory structures, advantageous in spatially restricted habitats. Reid et al. (1989) reported integumental pits also in Diacyclops navus Herrik, 1882, confirming as well that such structures can be commonly found in members of the genus Diacyclops. Successively, Pesce et al. (in press) described circular integumental pits both in the antennule and antenna of Mesocyclops species from ground waters of Australia.

Archaeodiacyclops okhensis Chaban & Alekseev 2024 [Russia]
Identification key for subgenera and species of Archaeodiacyclops genus

Zealandcyclops Karanovic 2005

Walter, T.C.; Boxshall, G. (2021). World of Copepods database. Cyclopidae

AUSTRIOCYCLOPS Kiefer, 1964

Austriocyclops vindobonae Kiefer 1964 [Austria; ground waters]

FAUREA Labbé, 1927

Faurea princeps Labbé, 1927

LIST OF CYCLOPIDAE GENERA FROM: invertebrates.si.edu/copepod

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NAUPLIAR DEVELOPMENTG IN CYCLOPINAE COPEPODS (by Chullason et al., 2009)

LINK TO THE COPEPOD WORLD AT SMITHSONIAN INSTITUTION

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