Cytheridella Daday, 1905; Gomphocythere Sars, 1924. Note: the genus Gomphodella De Deckker, 1981 is now lodged in the tribe Gomphodellini Danielopol et al. 2018.

Cytheridella ilosvayi Daday, 1905.

Syn.: Metacypris ometepensis Swain & Gilby, 1964 (fide Purper 1974; Martens and Behen 1994).

Syn.: Onychocythere alosa Tressler, 1939 (fide Purper 1974; Cohuo et al. 2017).

Syn.: Gomphocythere argentinensis Ferguson, 1967 (fide Karanovic 2009).

Syn.: Cytheridella boldii Purper, 1974 (fide Danielopol et al. 2018).

(partly derived from the extensive analysis of Danielopol et al. 2023). Cp largely sexually dimorphic. Males: CpV and CpD laterally rather flattened, with greatest width slightly behind the middle, both anterior and posterior sides pointed. Females: CpD and CpV with highly developed brood chamber, occupying 2/3 of the posterior part of the Cp, posterior margin almost straight, anterior margin pointed. In both sexes with well-developed lateral sulci and external valve surfaces heavily ornamented, with pits, rimmed pores with setae, and, especially anteriorly and posteriorly, with long and stiff setae and pores on conical elevations with setae (Porenwarzen). In inner views, both valves with well-developed anterior and posterior selvages, largely inwardly displaced; anterior calcified inner lamella of both valves set with two connected rows of long and fine cuticular filaments (setulae). Hinge adont. Central Muscle Scars consisting of a vertical row of four scars.

A1 with second segment bearing a long seta on the proximo-ventral side; penultimate segment fully or partly fused (segments 4+5); one of dorso-apical setae on this segment shaped as a trident. A2 with three distal claws. T1 and T2 with segment En4 fused with end claw. T3 a reflexed “cleaning limb”, with segment 4 not fused with end claw, seta h3 a spine. In females, with elongated CR (“organ fourchu”), with bifurcated tip. Hp with DL hinging on basal part, copulatory process coiled, short or (very) long. In males, CR simple but robust setae.

C. damasi Klie, 1944 (Congo, syn.: C. chariessa Rome, 1977 (Congo, in Rome and De Deckker 1977, fide Karanovic 2009)); Cytheridella monodi Klie, 1936 (Cameroon) Cytheridella tepida Victor, 1987 (Nigeria).

Cytheridella americana (Furtos, 1936) Danielopol (1981 in Colin and Danielopol 1980) from Yucatan (Mexico) is here considered an uncertain species (see below).

Holotype • 1 ♂ (adult); dissected and stored on a permanent microscopic slide and valves stored dry in a micropalaeontological slide (nr INV323000). Allotype • 1 ♀ (adult); dissected and stored as the holotype (nr INV323001). Paratypes • 3 ♂♂ adult Cp used for SEM (nrs INV323002-323004). 1 ♂ dissected and stored as the holotype (nr INV323005). 3 ♀♀ adult Cp used for SEM (nrs INV323006, INV323008, INV323010,). 1 ♀ dissected and stored as the holotype (nr INV3230014). Thirty ♀♀ and ♂♂ in EtOH (INV3230021).

USA • Massachusetts, Cape Cod, Grews Pond, Goodwill Park, Falmouth. Coordinates: N: 41.5696816, W: 70.6146054. Altitude: 5 m a.s.l. Collected on 27 July 2023. Leg.: Koen Martens and Isa Schön. Measurements at the time of collecting: Electrical Conductivity: 49 µS/cm, pH: 7.4, Water Temperature: 28 °C (holotype, allotype, and paratypes are all from the type locality).

(details on ecology will be provided elsewhere). Woods Hole: Miles Pond. Falmouth: Mares Pond, Deep Pond, Coonamessett Pond. Mashpee: Wakeby Pond, Peters Pond, Pemlico Pond. Barnstable: Lorells Pond, Snake Pond, Mystic Pond, Middle Pond, Hamblin Pond, Shubael Pond, Wequaquet Pond, Dennis Pond. Bourne: Flax Pond. Sandwich: Laurence Pond, Spectacle Pond. Yarmouth: Long Pond.

The species is named after Dr Charles Otis Whitman (1842–1910), professor at the University of Chicago, and the first director of the Marine Biological Laboratory (MBL) at Woods Hole (Ma, USA), after whom one of the present Research Centres at MBL and a series of fellowships are named (https://en.wikipedia.org/wiki/Charles_Otis_Whitman). The name is a noun in the genitive singular.

Cp as typical for the genus and in dorsal view most similar to the type species, but significantly smaller. Valves in inner view both with largely inwardly displaced selvage, especially in the poster-ventral corner of the RV. Posterior flanges of both valves on the inner side set with a series of rimmed pores, each bearing a simple seta. A1 with ventro-apical seta strong and claw-like. Mx1 palp apically with four claws and one seta. T3 a cleaning limb, with endopodal segment 4 fused with terminal claw, seta h3 a spine. Hp with DL elongated, sub-rectangular with bluntly pointed ventro-distal edge, and a long narrow, coiled copulatory process, distally pointed.

Male. CpRL (Fig. 2A, B) view rectangular, with widely rounded posterior and anterior margins, the latter slightly ventrally produced; dorsal margin straight for more than half of its length, ventral margin sinuous slightly anteriorly to the middle; with in both valves a clear lateral dorso-medial sulcus (reaching from dorsal side to more than half the height of the valves) and an antero-ventral sulcus (reaching from ventral side to more than half the height of the valves). CpD (Fig. 2C) and CpV (Fig. 2D–F) with pointed anterior and posterior margins and unevenly rounded lateral sides, the latter interrupted by the lateral sulci, greatest width situated slightly behind the middle. External valve surface heavily ornamented, set with circular and longitudinal pits and rimed pores, the latter especially anteriorly and posteriorly with long and stiff setae (Porenwarzen).

RVi (Fig. 3B, E, F) with shape as for the CpRL, with straight dorsal and slightly sinuous ventral sides, both anterior and posterior margins widely rounded; with well-developed selvage widely inwardly displaced along anterior and posterior margins, especially in the postero-ventral part; this part of the flange with a circular line of rimmed pores with setae.

LVi (Fig. 3A, C, D) almost symmetrically to RV, but with selvage less inwardly displaced, especially in the postero-ventral part.

A1 (Fig. 4A). Five-segmented. First segment slightly longer than wide. Second segment slightly shorter than the first one, with one long ventral and distally plumose seta, sub-basically inserted and reaching tip of penultimate segment. Third segment sub-quadrate, with a single dorso-apical seta reaching mid-length of fourth segment. Fourth segment (fusion of ancestral 4^th and 5^th segments) approximately twice as long as basal width. Setation of ancestral fourth segment: two unequal dorso-medial setae, one ventro-medial seta approximately as long as the shortest dorso-medial seta. Setation of ancestral fifth segment: three dorso-apical setae: one long, one short and one approximately one third the length of the long one; this latter seta broad and distally with two spines, almost looking like a trident, consisting of apical point and two subapical spines) and one seta of intermediate length, slightly longer than half the length of the long seta; further with one long ventro-apical claw-like seta. Fifth (terminal) segment approximately 1.5 times as long as the basal width, apically with aesthetasc Ya and its longer accompanying seta, fused at the base with that of the Ya, one long seta, almost as long as the accompanying seta of Ya and a shorter, but stout claw.

A2 (Fig. 4B). Protopodite two-segmented. First segment short. Second segment approximately twice as long as basal width. Exopodite a long, one-segmented spinneret seta, reaching beyond tips of end claws. Endopodite three-segmented. En1 skewed rectangular, approximately as long as basal width, with a short ventro-apical seta, reaching halfway along the second segment; dorso-apically with some pseudochaeta. En2 approximately five times as long as basal width; mid-ventrally with a short aesthetasc Y, flanked on each side by a subequal seta; dorsally with two sub-apical setae, one approximately half the length of the other and ventro-apically with a large claw, more than two times the length of the third segment. En3 (terminal segment) small, skewed sub-quadrate, apically with three large and subequal pectinate claws.

Md coxa (Fig. 5A) long and curved, apically with eight strong teeth, some doubled, interspaced with thin setae, ventro-apically with a short, reflexed plumose seta; sub-apically with a long and stout seta, not reaching the tips of the claws.

MdPalp (Fig. 5B) three-segmented. First segment ventrally set with two large, plumose sub-apical setae, one approximately 3/4 the length of the other, and a respiratory plate (not illustrated). Second segment (fusion of two segments) mid-ventrally with two large and stout setae, almost equally long and plumose in the distal half; mid-dorsally with one long and smooth seta, reaching beyond all other setae, ventro-apically with one long and stout seta, plumose in the distal half and one short, thin and largely smooth seta; dorso-apically with a bunch of three long, sub-equal setae, mostly smooth. Third (terminal) segment very small, approximately twice as long as the basal width; with three apical setae, one long, one of intermediate length and one shorter than the other two.

Mx1 (Fig. 5C) consisting of a basis, a large respiratory plate (not illustrated), three endites and a one-segmented palp. First endite with three subequal, slender setae. Second endite with five subequal, claw-like setae. Third endite with five claw-like setae, four large and one half the size of the others. Palp with four long claws, distally plumose and one small smooth seta, approximately half the length of the claws. Respiratory plate (exopodite – not illustrated) with approximately 16 plumose rays.

T1 (Fig. 6A) a four-segmented walking leg. Basal segment (Basis) long and broad, with one long ventral seta dp, almost reaching distal tip of segment, two short, subequal dorso-apical setae and a two mid-dorsal seta, the most distally inserted one approximately twice as long as the proximal one and reaching distal tip of segment. Segment En1 with one stout ventro-apical seta (e seta) reaching tip of En2. Segment En2 with length approximately 1.5 times basal width and without setae. En3 with length similar to that of second endopodal segment and also without setae; apically with one long and curved distal claw (h2), distally pectinate and basally incorporating segment En4.

T2 (Fig. 6B) also a four-segmented walking leg, slightly larger than the first thoracic limb. Basal segment (Basis) with long and thin mid-ventral seta dp, plumose in the distal 2/3 of its length; one short dorso-apical seta and two mid-dorsal setae, the most distally inserted one approximately three times as long as the proximal one and reaching beyond the distal tip of segment. Segment En1 with ventro-apical seta (e seta) approximately as long as the segment itself. En2 and En3 subequal and without setae, distal claw (h2), basally incorporating segment En4, longer and slightly more arched than equivalent claw on T1.

T3 (Fig. 6C) a cleaning leg. Basal segment (basis) elongated, ventrally with a long basal seta dp; apically with a single, long (as long as the segment itself) and smooth seta, mid-dorsally with two setae, the most distally inserted one approximately three times as long as the proximal one and reaching beyond the distal tip of segment with half of its length. En1 the longest endopodal segment, with subapically a long e seta, plumose in its distal third. En2 shorter than En1 by approximately one third, devoid of setae. En3 short, approximately half the length En2 and devoid of setae. En4 even smaller than En3, slightly obliquely inserted on the latter, carrying a long and curved claw h2 (but not fused with it) and a spine-like h3, fitting in a ventro-apical space of En2, thus forming a cleaning pincer.

Hp (Figs 6D, 7) with broad, elongated and sclerotised muscular body, comprising three or four main bundles of muscles (see Polscope illustration, Fig. 7), an elongated and sub-rectangular distal lobe (DL), with bluntly pointed ventro-distal edge, with a short seta inserted in the middle of the basal part of the lobe DL, and a long narrow, coiled copulatory process, distally pointed. CR (Fig. 6E) consisting of two stout setae at base of each Hp, but not fused with them.

Female (only sexually dimorphic features mentioned).

CpRL (Fig. 8A, B) sub-rectangular, with widely rounded posterior and anterior margins, the latter slightly ventrally produced; dorsal margin not straight, but strongly indented behind the middle at the start of the lateral sulcus, ventral margin slightly sinuous; with a clear dorso-medial sulcus and an anterior ventro-medial sulcus in both valves, as in the male. CpD (Fig. 8C) and CpV (Fig. 8D–F) with pointed anterior margin and posteriorly with highly developed brood chamber, occupying two-thirds of the posterior part of the Cp, posterior margin almost straight. External valve surface heavily ornamented, set with circular and longitudinal pits, rimed pores, especially anteriorly and posteriorly with long and stiff setae in Porenwarzen.

RVi (Fig. 9B, E, F) with shape as for the CpRL, but with straight dorsal and slightly sinuous ventral sides, both anterior and posterior margins widely rounded; well-developed selvage widely inwardly displaced along anterior and posterior margins, especially in the postero-ventral part, this part of the flange with a series of rimmed pores with single setae. Posterior brood pouch most prominent.

LVi (Fig. 9A, C, D) almost symmetrically as the RV, but with selvage slightly less inwardly displaced, especially in the postero-ventral part.

A1 (Fig. 10A) with “trident” aspect of short dorso-apical seta on fourth segment pronounced (consisting of apical point and two subapical spines); ventro-apical seta in this segment a long seta, not a claw; accompanying seta to aesthetasc Ya on terminal seta twice as long as aesthetasc itself.

A2 (Fig. 10B) with exopodal seta shorter than in the male, not reaching tips of end claws; these three claws more (sub-) equal than in the male.

Md coxa (Md) (Fig. 11A) less sinuous than in the male. Palp (Fig. 11B) with setae in first segment more subequal in length.

Chaetotaxy of endites and palp of Mx1 (Fig. 11C) highly similar to that in the male, but one distal claw on the palp significantly shorter than the three others.

T1 (Fig. 12A), T2 (Fig. 12B) and T3 (Fig. 12C) largely as in the male, but with En4 in T3 even more obliquely inserted on the tip of En3.

Posterior part of body (Fig. 12D) with CR (“organ fourchu” in Rome and De Deckker 1977) composed of an elongated ramus, ending in bifurcation, with one short, bluntly pointed branch and one longer, hook-like branch; one additional caudal lobe set with pseudochaeta.

See Table 2.

The species is abundant in the permanent lakes on Cape Cod. It occurs on different types of sediments with detritus and was mostly found at ca 0.5–1 m depth.

This species is especially characterised by the shape of the Cp and of the DL and the cop of the Hp, by which it can be distinguished from all living Cytheridella species. The selvage is more inwardly displaced than in other species, especially so in the postero-ventral corner of the RV of both genders, which also allows distinction from fossil species. Cytheridella whitmani can be further distinguished from C. ilosvayi by the fact that it is significantly smaller (female length approximately 800 µm against 1000 µm or more in C. ilosvayi), by the less widely developed posterior brood pouch in the female, and by the fact that the setae on the rimmed pores on the posterior inner flanges (named peripheral marginal infold (pmi) by Danielopol et al. 2023) are simple, whereas these are bi- or multifurcated in C. ilosvayi. In addition, the valves and CpRL in males and females of C. whitmani have a straight dorsal margin over more than half the length, unlike in C. ilosvayi where this margin is curved. The fossil species, C. martingrossi Danielopol & Piller, 2023 (in Danielopol et al. 2023), is significantly larger than the new species (female length approximately 1100 µm), and the shape of the CpRL is different in that it is posteriorly upturned. Females of C. tepida are between 1100 and 1640 µm long, and as such are the largest species in the genus, much larger than C. whitmani. Cytheridella damasi (syn. C. chariessa), C. monodi, and the fossil C. danielopoli Purper, 1979 are of similar sizes as C. whitmani, but have different valve shapes. Whereas the dorsal margin in males and females in C. whitmani is straight and running parallel with the ventral margin, the dorsal margin in C. monodi is curved, while in C. damasi it is sloping towards the posterior side. In C. danielopoli, the shape of the male valves in inner view resembles that of C. whitmani, but the female brood chamber in the latter species is much more developed than in C. danielopoli in both lateral and dorsal view (see Purper 1974: pl. 7, figs 23, 24).