Siamcyclops cavernicolus

Boonyanusith, Chaichat, Sanoamuang, La-orsri & Brancelj, Anton, 2018, A new genus and two new species of cave-dwelling cyclopoids (Crustacea, Copepoda) from the epikarst zone of Thailand and up-to-date keys to genera and subgenera of the Bryocyclops and Microcyc, European Journal of Taxonomy 431, pp. 1-30: 4-14

publication ID

publication LSID

persistent identifier

taxon LSID

treatment provided by


scientific name

Siamcyclops cavernicolus

gen. et sp. nov.

Siamcyclops cavernicolus  gen. et sp. nov.

Figs 1–5View Fig. 1View Fig. 2View Fig. 3View Fig. 4View Fig. 5


The specific epithet cavernicolus  meaning ‘organism living in a cave’, indicates its habitat. The epithet is a noun in the nominative singular masculine.

Material examined


THAILAND: ♀ (adult), 414 µm long, Ratchaburi Province, Chom Phon Cave , 13°37′32.1″ N, 99°35′14.2″ E, 100 m a.s.l., filtering bottle, 21 Nov. 2009, C. Boonyanusith leg., completely dissected then mounted on a slide in glycerol and sealed with nail polish ( NHM 2011.2080View Materials). View MaterialsGoogleMaps 


THAILAND: ♂ (adult), 394 µm long, collected with the holotype, completely dissected, mounted on a slide in glycerol and sealed with nail polish ( NHM 2011.2081View Materials).GoogleMaps 


THAILAND: 2 ♀♀ without egg sacs, 1 ♀ with spermatophore, sampled with the holotype, whole specimens stored in 70% alcohol ( NHM 2011.2082‒2084) View Materials  ; 1 ♀ with spermatophore and egg sac, 1 ♀ with spermatophore, 2 ♀♀ without egg sacs, sampled with the holotype, whole specimens stored in 70% alcohol ( KKU-COP-2011-002);GoogleMaps  2 ♀♀, 2 ♀♀ with egg sac, sampled with the holotype, whole specimens stored in 70% alcohol (NIB).GoogleMaps 

Type locality

The Chom Phon Cave is located in Chom Bung district, about 30 km west of the town of Ratchaburi. The cave is located in an isolated, small, limestone hill. It is a fossil cave, about 290 m long, with horizontal galleries only and the origin of the sampled water is exclusively drips of percolating water from the epikarst. The cave has two major openings. The first is an entrance located about 10 m above the valley floor at an elevation of 100 m a.s.l. The coordinates of the entrance are 13º37′32.1″ N, 99º35′14.2″ E. Beyond the entrance is a long horizontal gallery at the same level as the valley floor and accessible without any special equipment except a lamp. The gallery is about 10 m wide and 8–10 m high and was formed by a subterranean river. The second opening, about 25 m in diameter, is located above the end-hall of the cave. There is a reclining Buddha statue and in a semi-illuminated place there are several plastic and aluminium buckets (= Buddha pots according to Brancelj et al. 2010) collecting dripping water from the cave walls and roots of trees for the whole year around. This is the type locality of Siamcyclops cavernicolus  gen. et sp. nov. On the sampling date (21 Nov. 2009), no water was dripping from the cave walls or tree roots. The volume of water in the plastic buckets was about 5 l and had a brown colour. The water temperature was 23.8°C, pH 8.74, and conductivity 435 µS cm-1. There was some guano and other organic debris in the buckets.



Body length, measured from tip of rostrum to posterior margin of caudal rami, 391‒414 µm (mean: 404 µm; n = 10); prosome/urosome ratio about 2.0 ( Fig. 1AView Fig. 1). Body dorsoventrally compressed. Preserved specimens transparent; naupliar eye not discernible; rostrum small, triangular. Cephalothorax anteriorly oval, as long as wide, with greatest width at posterior end of cephalosome; representing 41% of body length. Posterior margins of Pd1–Pd4 smooth. Integument smooth, not strongly chitinized, with no visible cuticular windows. Body width/length ratio 2.4. Genital double-somite well developed, about 1.4 times as wide as long, with pair of refractile, sclerotized, rounded, dorsolateral lobes ( Fig. 1CView Fig. 1); as long as following urosomites, including caudal rami; with well-discernible incision between anterior and posterior half of segment and tapering posteriorly ( Fig. 1A–CView Fig. 1). Copulatory pore small, oval, situated near middle of somite; copulatory duct short, narrow and well sclerotized. Seminal receptacle small, representing about ⅓ of double-somite length, with anterior and posterior expansions; both expansions short but anterior one slightly longer, sclerotized and much wider compared to posterior one ( Fig. 1BView Fig. 1). Ovipores situated lateroposteriorly at about ½ length of somite, covered with reduced P6 ( Fig. 1DView Fig. 1). Posterior margin of genital double-somite and two subsequent somites with hyaline fringes with irregular serration both ventrally and dorsally. Anal somite with well developed operculum, reaching ½ length of caudal rami; distal half of free margin serrated; two large sensilla at base of operculum ( Fig. 1CView Fig. 1).

CAUDAL RAMI ( Fig. 1B–DView Fig. 1). Slightly divergent; each about 1.5 times as long as wide. Anterolateral accessory seta (I) reduced. Anterolateral seta (II) bare, implanted at ⅔ length of ramus; slightly shorter than ramus. Posterolateral seta (III) slim, about 1.2 times as long as ramus, implanted at ¼ of ramus length; insertion of seta ornamented with few spinules. Outer apical seta (IV) plumose; inner apical seta (V) longest; both setae bipinnatae, without fracture planes; inner one about twice as long as outer one and about 0.5 times as long as body. Apical accessory seta (VI) bare, spiniform, curved outward; about 0.5 times as long as ramus. Dorsal seta (VII) bipinnate, inserted at distal inner corner of ramus, about twice as long as ramus.

ANTENNULE ( Fig. 1EView Fig. 1). 11-segmented, not reaching posterior margin of cephalothorax. Armature formula: Fifth segment with short spine ventrally. Penultimate segment with aesthetasc near insertion of outer seta; aesthetasc as long as outer seta. Terminal segment with acrotheck subapically.

ANTENNA ( Fig. 2AView Fig. 2). 4-segmented; with coxobasis and 3-segmented Endp. Coxobasis with one smooth seta on distal inner corner; seta representing Exp absent. Endp-1 with longitudinal row of spinules along distal half of inner margin; with one smooth seta at ½ length of margin. Endp-2 about 1.5 times as long as wide, with longitudinal row of minute spinules; along inner margin five smooth setae increasing in length (three laterally, one subapically, one apically). Endp-3 twice as long as wide, with longitudinal row of minute spinules; seven smooth setae apically (two of them robust and curved).

MANDIBLE ( Fig. 2B–CView Fig. 2). With coxa and short basis partly fused with coxa. Gnathobase with strong chitinized teeth; ventralmost teeth very robust and slightly obtuse, with pinnate seta dorsally. Basis with one short and two long setae representing Exp and Endp, respectively; long setae about seven times as long as short seta.

MAXILLULE ( Fig. 2DView Fig. 2). With robust praecoxa and 2-segmented palp; proximal segment of palp coxobasis; distal one Endp. Arthrite of praecoxa with six strong spines laterally, five of them smooth; apically three claw-like spines decreasing in length and one weak seta. Coxobasis with three elements; one robust bipinnate seta accompanied by two weak smooth setae apically. Endp with two setae apically and one seta subapically. Exp represented by one seta.

MAXILLA ( Fig. 2EView Fig. 2). 5-segmented. Endite of praecoxa prominent, with two plumose setae. Proximal endite of coxa with one plumose seta; distal endite elongate, with two bipinnate setae apically. Basis with clawlike basal endite, with row of spinules along concave margin; two setae at base of claw-like expansion; longest one as long as claw; other one shorter, 0.5 times as long as longer one. Endp 2-segmented; proximal segment with two robust setae; distal segment with one robust seta apically, as long as clawlike expansion on basis, accompanied by two slender, shorter setae.

MAXILLIPED ( Fig. 2FView Fig. 2). 4-segmented; syncoxa and basis with two pinnate setae each; basis with one row of spinules on distal outer margin; additional row of spinules laterally. Proximal segment of End with one strong seta. Distal segment armed with three setae, shortest one smooth.

P1–P4. With un-ornamented intercoxal sclerite, with deeply concave posterior margins; coxa rectangular; basis relatively triangular with slender outer seta; 2-segmented Exp and Endp ( Fig. 3A–DView Fig. 3). Coxa of P1– P3 each armed with one plumose seta on inner distal corner. Exp-2 spine/seta formula: Endp-2 spine/seta formula: Complete armature of P1–P4 as follows (Roman numbers = spines; Arabic numbers = setae):

P1 ( Fig. 3AView Fig. 3). Exp-1 with one smooth, blunt, curved spine on outer corner. Exp-2 about twice as long as wide, apical setae as long as Exp-1 and -2 combined. Endp-1 shorter than wide, with sharply pointed extension on distal outer corner. Endp-2 1.3 times as long as wide, with one seta between 2 claw-like extensions on outer margin; very strong, blunt, hook-shaped spine apically.

P2 ( Fig. 3BView Fig. 3). Similar to P1, slightly less robust; apical spine on Endp-2 only slightly curved, as long as segment bearing it. Exp-2 1.5 times as long as wide.

P3 ( Fig. 3CView Fig. 3). Coxa, basis and Exp similar to those of P2. Endp-2 1.5 times as long as wide; terminal spine on Endp-2 straight, as long as segment bearing it.

P4 ( Fig. 3DView Fig. 3). Exp-2 twice as long as wide, with relatively weak pinnate spines on outer margin; apical setae longer than Exp-1 and 2 combined. Endp-1 large, 1.5 times as wide as long. Endp-2 1.3 times as long as wide, with short apical setae, only slightly longer than segment bearing them; apical spine short, about 0.5 times as long as segment bearing it.

P5 ( Figs 1B–D, F–GView Fig. 1, 2G–HView Fig. 2). Reduced to 2 cuticular lobes, completely fused to Pd5, with three slender setae. Dorsal lobe broad, with one seta; ventral lobe small, with two unequal setae apically; longer seta about twice as long as shorter one.

P6 ( Fig. 1DView Fig. 1). Small, forming simple cuticular plate, inserted laterodorsally on genital double-somite, with two minute spines ventrally and one short seta dorsally.

SPERMATOPHORE ( Fig. 2G–HView Fig. 2). Paired; each forming 3-dimensional structure running in anterior-posterior direction ventrally, bending laterally at level of seminal receptacle and bending dorsally (as double flipped L); walls very thick.

EGG SAC ( Fig. 1AView Fig. 1). Extruded from gonopores laterodorsally, with two large eggs.


Body length, measured from tip of rostrum to posterior margin of caudal rami, 381‒402 µm (mean: 391 µm; n = 10); prosome/urosome ratio about 2.0. Habitus ( Fig. 4AView Fig. 4) slightly smaller and more slender than in female. Naupliar eye not discernible; rostrum as in female. Cephalothorax and Pd 2–4 similar to those of female. Cephalothorax anteriorly oval, 1.1 times as long as wide, with greatest width at posterior end, representing 43% of body length. Posterior margins of Pd1‒Pd4 smooth. Body length/width ratio about 2.3. Genital somite large, globular, about 0.9 times as long as rest of urosome, including caudal rami ( Fig. 4A–DView Fig. 4). Hyaline structures on dorsolateral part of genital somite well developed ( Fig. 4A–DView Fig. 4). Posterior border of genital somite with broad hyaline fringe with irregular serration dorsally. Subsequent three urosomites narrower than genital somite, with irregular serrated free hyaline fringes posteriorly. Anal somite and operculum as in female ( Fig. 4A–DView Fig. 4).

CAUDAL RAMI ( Fig. 4B–DView Fig. 4). More slender than in female, about 1.6 times as long as wide. Anterolateral accessory seta (I) reduced.Anterolateral seta (II) bare, implanted at about ½ length of ramus, slightly shorter than ramus. Posterolateral seta (III) bipinnate, about 1.2 times as long as ramus; insertion ornamented with few spinules. Outer apical seta (IV) plumose; without fracture plane. Inner apical seta (V) longest, plumose, about twice as long as seta IV and about 0.5 times as long as body length, without fracture plane. Apical accessory seta (VI) bare, spiniform, curved outward; about 0.5 times as long as ramus. Dorsal seta (VII) bipinnate, inserted at distal inner corner of ramus, about twice as long as ramus.

ANTENNULE ( Fig. 4EView Fig. 4). 15-segmented, geniculate. Armature formula: 7+3A.4.2.2+A.1.2+A.3.1+A+I. Terminal segment with acrotheck; short spine on eighth segment. Seta on ninth and eleventh segments robust, spiniform; seta on tenth, eleventh and twelfth segments very short, bipinnate.

ANTENNA, MOUTHPARTS, P1, P2 AND P5. As in female.

P3 ( Fig. 5AView Fig. 5). Exp as in female. Endp-2 with apical spine modified as spoon-like element; bent inward and ornamented with minute transverse denticles along distal ⅓ of margin; tip of spine bent. Inner subterminal seta modified as claw-like spine, bare and bent toward apical spoon-like spine. Sub-terminal seta on outer margin very short.

P4 ( Fig. 5BView Fig. 5). Exp and Endp similar to those of female; Endp with relatively longer setae and spines compared to female.

P6 ( Fig. 4B‒CView Fig. 4). Positioned ventrally; modified to large cuticular plate with three setae; middle one shortest; ventral one longest.


No significant variability was observed in females except minor variation in size, shape and number of spinules on some segments or somites. Variability not observed in males.

Differential diagnosis and remarks

Based on the number of segments of the antennules, the progressive oligomerization of P1–P4, and the shape and armature of P5, Pesce (1996) classified 33 genera in the subfamily Cyclopinae  into 6 groups, including the Bryocyclops  and Microcyclops  groups, both presented in this paper. Most of the 33 classified genera also include stygobiotic members. Their morphological adaptations for life in subterranean habitats are related to specific environmental characteristics (i.e., lack of light and food scarcity) and include a reduction of body size, 10- or 11-segmented antennules, P1–P4 with three or two segmented Exp, two or one segmented Endp rami, and partly or completely fused P5 to Pd5. The level of reductions express a grade of a transformation of appendages from a plesiomorphic state (P1–P4 with three segmented Exp/Endp and P5 clearly separated from Pd5) to an apomorphic one (P1–P4 with reduced number of segments in Exp/Endp and P5 fused with Pd5) ( Pesce 1996).

In stygobiotic Cyclopinae  , intensive reductions of P5 have a strong effect on establishing a genus level ( Reid & Ishida 2000). For that reason, some other morphological characters should also be included to support the erection of a new genus as well as its position within six groups in the subfamily Cyclopinae  . Examples of the effect of inclusion of other morphological characters in erecting a new genus and its positioning within six gropus of Cyclopinae  include the morphologicaly similar genera Speocyclops Kiefer, 1937  (a member of Microcyclops  group) and Bryocyclop s Kiefer, 1927 (a member of Bryocyclops  group). They are similar in body shape and both have very reduced P5 or even have it fused to Pd5 ( Pandourski 1992; Dussart & Defaye 2001; Galassi & De Laurentiis 2004). The main discrimination character between both genera/groups is the presence/absence of a seta on the inner corner of the coxa in P4. In the genus Bryocyclops  it is absent, while in the genus Speocyclops  it is present.

There are some similar morphological details between the Microcyclops  and Bryocyclops  groups, also. An example is the modification of the P3 Endp-2 spine in the males observed in representatives of Bryocyclops  and the Alaskan population of Itocyclops yezoensis (Itô, 1953)  (member of the Microcyclops  group). However, according to Reid & Ishida (2000), there are no phylogenetic relationships between the genera Bryocyclops  and Itocyclops Reid & Ishida, 2000  , and the modification of P3 might be a result of random mutation within the Alaskan population.

Absence of a seta on the inner corner of the coxa in P 4 in the male and female as well as the modification of the P3 Endp-2 spine in the male place Siamcyclops  gen. nov. in the Bryocyclops  group. So far, representatives of genera Bryocyclops  and Siamcyclops  gen. nov. only differ from other genera within the Bryocyclops  group by morphological modifications in armature elements on the male P3 Endp-2. The lack of a seta on the inner corner of the coxa of P 4 in the male and female, combined with modifications of elements of the P3 Endp- 2 in the male, indicate a close relationship between S. cavernicolus  gen. et sp. nov. and the representatives of Bryocyclops  groups I, II, V and VII (in group VII it is not modification but a difference in the size of the spine between the male and female) ( Lindberg 1956; Reid 1999; Fiers 2002; Watiroyram et al. 2015).

A similarity in the shape of the P4 Endp-2 and the presence of six armature elements there (five in other Bryocyclops  species) indicate that the new taxon most resembles B. jankowskajae Monchenko, 1972  , known from the Kyzylkum Desert in Uzbekistan (formerly part of the USSR). The species was recently moved from the subgenus Palaeocyclops Monchenko, 1972  into Bryocyclops  s. str. ( Walter 2015). For that reason, we suggest the establishment of a new group, i.e., group VIII, to accommodate it within the genus Bryocyclops  .

All representatives of the genus Bryocyclops  (except B. bogoriensis (Menzel, 1926))  and the former member of the subgenus Palaeocyclops  have a pointed prominence on the intercoxal sclerite of P4, while in S. cavernicolus  gen. et sp. nov. it is obtuse. However, the armament on the mandibular palp and distal segment of P4 Exp found in representatives of the genus Bryocyclops  , including a former member of the subgenus Palaeocylops, show a pattern of reduction of armature elements. The presence of only one or no seta can be observed in several species within the genus Bryocyclops  where the mandibular palp was described or illustrated, as in B. saqotraensis Mirabdullayev, Van Damme & Dumont, 2002  , B. muscicola (Menzel, 1926)  , B. maewaensis Watiroyram, Brancelj & Sanoamuang, 2012  and B. jankowskajae  . The presence of 3 setae on the mandibular palp in S.cavernicoulus  gen. et sp. nov. shows a plesiomorphic state of the character, which clearly differentiates it from members of the genus Bryocyclops  .

Furthermore, the spine/seta formula found in the new taxon differs clearly from the formulae of the species within the genus Bryocyclops  , where the spine/seta formula varied from (group V) to 5(4).4.4.3 (group VI), (group III), (groups I and VII), /5(4).5.5.4 (group II), and / (group IV) ( Lindberg 1947; Watiroyram et al. 2015). It also differs considerably from B. jankowskajae  whose spine/setae formula is

Normally, the spermatophores within the subfamily Cyclopinae  are bean- or kidney-shaped. They have also been illustrated in some species of Bryocyclops  such as B. caroli Bjornberg, 1985  and B. absalomi Por, 1981  . The unique, three dimensional L-flip feature of the spermatophore observed in S.cavernicolus  gen. et sp. nov. differs from those of other members of the genus Bryocyclops  as well as from other members of the subfamily Cyclopinae  .

For the reasons listed above, we propose the establishment of the new genus Siamcyclops  gen. nov. to accommodate the new stygobiotic species from Thailand. The main differences between the three related taxa, Bryocyclops  spp., Bryocyclops jankowskajae  and Siamcylops  gen. nov., are listed in Table 1.