Nothobranchius sylvaticus, Nagy, Bellstedt & Luke, 2025

Nothobranchius sylvaticus, Nagy, Bellstedt & Luke , new species

( Figs 5A–D View FIGURE 5 )

Nothobranchius elongatus View in CoL : de Rham 1991: 43.

Nothobranchius sp. Southern Kenya: van der Merwe et al. 2021: 10.

Holotype. NMK, ICH FW/6014/1 , male, 33.6 mm SL; Kenya: ephemeral swamp in the Mkurumudzi system, about 4 km west southwest of Gazi village, 04°25.860' S, 39°28.138' E; 26 m a.s.l. D.U. Bellstedt, 4 July 2018, preserved in the field. GoogleMaps

Paratypes. NMK, ICH FW/6013/1-2 8, 4 males, 29.2–34.3 mm SL & 4 females, 25.2–26.5 mm SL; same as for the holotype, preserved in the field GoogleMaps .

Diagnosis. Nothobranchius sylvaticus is distinguished from all other members of the genus by the following combination of characters in males: scales on trunk iridescent light blue covering the entire caudal peduncle; dorsal portion of head grey with distinct red lobes along supraorbital canal; posterior scale margins on postorbital portion of opercle creating two pronounced brown-red oblique bars; dorsal and anal fins golden, marked with dark grey dots and stripes proximally and medially, merging into uniform dark grey zone distally; caudal fin dark red to red proximally, plain red medially, and with slender black distal band.

Furthermore, N. sylvaticus differs from its geographically closest congeners, N. elongatus and N. interruptus , by colouration of the brown-red scale margins, a series of morphometric and meristic differences, and in the structure of the cephalic sensory system as detailed below.

Description. General body features illustrated by Figures 5A–D View FIGURE 5 . Morphometric and meristic characters of holotype and paratypes summarized in Table 1 View TABLE 1 .

Male: Medium-sized Nothobranchius species, maximum observed length of specimens examined 34.3 mm SL. General body shape robust and laterally compressed. Greatest body depth at vertical passing through pelvicfin origin: 26.4–27.9 % SL. Greatest body width at pectoral-fin base with body progressively narrowing towards caudal-fin base. Dorsal profile concave from tip of snout to nape and convex from nape to base of last dorsal-fin ray; postdorsal profile straight to slightly concave on caudal peduncle. Ventral profile convex from lower jaw to base of last anal-fin ray; postanal profile straight to slightly concave on caudal peduncle from insertion of posteriormost anal-fin ray to caudal-fin base. Caudal peduncle shallow, length 155–160 % of its depth. Anus just anterior to anal-fin origin.

Head moderately long (27.0–29.6 % SL), laterally compressed, deeper than wide (head width 74–81 % of its depth). Snout slightly pointed, smaller than eye diameter. Mouth supraterminal, slightly oblique in profile. Jaws subequal, lower jaw longer than upper, posterior end of rictus at same level or slightly ventral to centre of eye. Premaxilla and dentary with many irregularly distributed conical, slightly curved teeth. Orbit large (28–32 % HL), entirely in anterior half of head (snout to eye end length 39–45 % HL), at median portion of head side. Branchiostegal membrane projecting posteriorly from opercle.

Dorsal-fin origin anterior to anal-fin origin, both fins originating posterior to mid-length of body. Distal border of dorsal and anal fins rounded, with small contact organs in form of papillae on fin rays and distal margin with short filamentous rays. Posterior extremity of dorsal fin reaching caudal-fin base. Dorsal fin, 14–15; anal fin, 13–15. Pectoral fin subtriangular; its insertion slightly posterior to margin of opercular opening; base slightly oblique, upper fin rays placed slightly anteriorly to lower fin rays, tip reaching or slightly surpassing base of pelvic fin. Pelvic fin subabdominal, its origin at about mid-length of body, short, bases medially separated, tip reaching urogenital papilla. Caudal fin subtruncate, with 15–18 branched rays, plus three to five procurrent rays at dorsal and ventral origins.

Scales cycloid, body and head entirely scaled, except for ventral surface of head. Scales on head, trunk and caudal peduncle iridescent light blue with brown-red scale margins creating irregular vertical chevron-shaped stripes on trunk ( Fig. 6 B and C View FIGURE 6 ). No scales on dorsal and anal fins. Scales in mid-longitudinal series, 27–29 plus two or three small scales on caudal-fin base. Transverse rows of scales in front of dorsal-fin origin, 11; scale rows around caudal peduncle 12–14.

Frontal scalation pattern variable. Anterior nostril at anterior tip of snout, tubular opening lateral to upper lip. Posterior nostril in front of orbit, with oval aperture. Cephalic sensory system represented in Figure 6 View FIGURE 6 . Cephalic sensory system at frontal level in shallow groove, with one exposed neuromast. Sensory system at supraorbital level fragmented into two discontinuous portions, with one exposed neuromasts in shallow groove at anterior level, continued at posterior supraorbital level over orbit with two exposed neuromasts, and with pronounced lobes along neuromasts. Supratemporal canal in slightly curved shallow groove, with four exposed neuromasts. Preorbital canal in shallow groove with two exposed neuromasts; postorbital canal in deep groove with one long exposed neuromast; infraorbital level with about 15 small buttons at ventral and posterior margin of orbit. Preopercular system with six exposed neuromasts in deep groove on preopercle portion, whereas four neuromasts in shallow groove on ventral portion. Mandibular canal in shallow groove with four neuromasts along lateral margin running parallel to outer margin of lower jaw and four to five small buttons anteriorly positioned in lower jaw ventral portion. One neuromast on each scale along trunk mid-longitudinal series.

Female: Smaller than male, maximum observed size 26.5 mm SL. Branchiostegal membrane not projecting posteriorly from opercle. Dorsal and caudal fins rounded. Anal fin subtriangular, tip rounded, central rays longer and more rigid. Anal fin positioned more posteriorly than in male (preanal length 62.5–64.2 % SL vs. 61.0–61.8 %, predorsal length 61.1–62.6 % SL vs. 57.7–60.3 %, respectively). Base length anal fin smaller than in male (dorsal-fin base length 20.0–23.0 % SL vs. 23.4–26.5 %, anal-fin base length 14.8–16.0 % SL vs. 16.5–18.9 %, respectively). No papillae or epidermal tissue present on dorsal and anal fins. Pelvic fin short, tip reaching anus. Caudal fin length greater than in male (29.8–31.3 % SL vs. 23.1–27.1 %). Caudal peduncle narrower than in male (caudal peduncle length 167–171 % of its depth vs. 155–160, respectively).

Colouration. Live male ( Figures 5B and C View FIGURE 5 ): Scales on head, trunk and caudal peduncle iridescent light blue with brown-red scale margins creating irregular vertical chevron-shaped stripes on the trunk. Scale margins wider on dorsum, resulting in darker general appearance, whereas scale margins at posteroventral portion of trunk creating chevron shaped reticulation. Snout and dorsal portion of head grey, throat light blue. Lobes along supraorbital canal red ( Fig. 6d View FIGURE 6 ). Posterior scale margins on post-orbital portion of opercle creating two pronounced brown-red oblique bars. Exposed branchiostegal membrane white to light blue. Iris light blue, with dark grey spots on upper and lower parts in some specimens, creating dark vertical bar through centre of eye. Dorsal fin golden, densely marked with dark grey dots and irregular stripes proximally and medially, merging into uniform dark grey zone distally, and with narrow light blue distal band at fin tips. Anal fin golden with dark grey stripes parallel to fin rays and dark grey dots proximally and medially, merging into uniform dark grey zone distally. Caudal fin dark red to red proximally, plain red medially, and with slender black distal band. Pelvic fin golden with dark grey dots with blue distal margin. Pectoral fin hyaline with light blue posterior distal margin.

Live female ( Figure 5D View FIGURE 5 ): Scales on trunk and head pale grey-brown, slightly darker on dorsum and lighter to silver on venter. Scales with dark grey margin, creating reticulation on trunk and forming vertical chevron stripes on posteroventral portions of flank. Snout, throat and dorsal portion of head pale grey-brown, with lobes along supraorbital canal dark red ( Fig. 6e View FIGURE 6 ). Light blue iridescence on opercular region and some scales on trunk. Iris golden. All fins hyaline.

Distribution. Nothobranchius sylvaticus is endemic to south-eastern Kenya. It is currently only known from ephemeral swamps in open areas of the Gongoni Forest in the drainage of the seasonal Mkurumudzi River system ( Fig. 1 View FIGURE 1 ). The Mkurumudzi drains the south-eastern slopes of the Shimba Hills and flows into the Indian Ocean at Gazi Bay.

Ecology. The narrow coastal plain in south-eastern Kenya is situated at an elevation below 200 m a.s.l. Rainfall increases from north to south along the coast ( Hughes & Hughes 1992). Annual rainfall in the Gongoni area ranges from 900 to 1300 mm per year. Rainfall occurs during two distinct periods, the long rains between April and June, and the short rains usually between October and December ( Nicholson 2017). At the type locality, Nothobranchius sylvaticus was the only fish species found.

Two sites were investigated during the course of this study: the type locality during 2018 ( Fig. 7A View FIGURE 7 ) and another seasonal pool situated about 1 km west-northwest of that, first discovered during 2017 ( Fig. 7B and C View FIGURE 7 ) and having similar ecological characteristics. The type locality is an ephemeral swamp formed in open areas of the Gongoni Forest in the drainage of the seasonal Mkurumudzi River ( Fig. 7A View FIGURE 7 ). The habitat belongs to category 1.3, as defined by Watters (2014), representing a marsh associated with a seasonal stream in an area of low gradient. At the end of the first peak of the rainy season, the habitat was a shallow marsh of about 50 m long and 25 m wide, and about 30 cm at the deepest point. The marsh was situated in an open grassy area, completely surrounded by forest. At the type locality, all monitoring took place in the same general habitat. Aquatic vegetation comprised Nymphaea nouchali , and the habitat was densely vegetated/overgrown with grass. The water was turbid, the bottom of the seepage area was not visible. Figure 7B View FIGURE 7 shows the second locality on 5 May 2017, shortly after it had been filled by the seasonal rains. This picture shows the eastern side of the pond, the western side is obscured by trees to the right in the picture. Figure 7C View FIGURE 7 shows the same pond on 3 July 2018 with typical Nymphaea nouchali waterlilies in the section of the pond shown in Figure 7B View FIGURE 7 . Surficial ground cover in the general area of both localities largely comprises decomposed calcareous rock derived from ancient coral reefs. Much of the substrate within the confines of the habitats, overlying the calcareous surficial material, is composed of a surface layer rich in dark grey to black or brown swelling clays, providing a suitable substrate for Nothobranchius populations to persist.

A water sample, taken by de Rham at the locality where he originally found this species, in December 1982 ( Figure 7D View FIGURE 7 ), was analysed six weeks later, in Switzerland, with the following results: pH = 8.1; conductivity = 465 µS; total hardness = 11.2 °F; carbonate hardness = 7.7 °F; Ca/l = 25.6 mg; Mg/l = 11.7 mg; Cl/l = 98.0 mg; Na/l = 57.0 mg ( de Rham 1991). The water was, therefore, alkaline and relatively well mineralized. The abundance of chlorine and sodium points to a slight saline infiltration of marine water, which presumably percolates through the relatively coarse surficial calcareous material. The precise location of de Rham’s 1982 site is not known; however, the nature of the habitat and its setting in association with the Gongoni Forest suggests that very similar water quality characteristics would prevail at the two sites investigated in this study.

Conservation status. We recommend that Nothobranchius sylvaticus be listed as Critically Endangered. The species is currently known only from the type locality and one additional nearby site (possibly two additional sites if de Rham’s site represents a distinct location), both belonging to one threat-based location. The range of the species is currently known to be restricted to the Gongoni Forest Reserve. The forest is characterized by the presence of small seasonal forest pools and wetlands within the drainage of the small seasonal Mkurumudzi River system. The species might exist at a few more sites within the drainages, but its distribution will remain strongly restricted as the area surrounding the forest reserve is an agricultural zone, with livestock rearing and smallholder crop farming. The maximum estimate of extent of occurrence (EOO) is around 20 km ², whereas maximum estimate for the area of occupancy (AOO) is at 10 km ², belonging to a single threat-based location. Using IUCN version 3.1 (2012), the species meets the criteria B1ab(iii)+2ab(iii) for Critically Endangered.

This species inhabits small seasonal wetland habitats situated in a forest ecosystem. This makes it highly vulnerable due to: the surrounding area being an agricultural zone for small-holder farming including rice farming ( Figure 7B View FIGURE 7 ), for which furrows had been dug from the pond in which the species was first found, and livestock grazing; the planting of Eucalyptus sp. (which produce toxic humic acids) and mango trees adjacent to the above-mentioned pond ( Figures 7B and C View FIGURE 7 ); burning of the vegetation in the forest; firewood extraction and building material removal from the forest resources carried out by adjacent communities; damming of the Mkurumudzi River, leading to drainage modification and excessive sedimentation; nearby mining activity involving mineral sands and for which considerable volumes of groundwater are abstracted from various boreholes in this forest; the slimes dam constructed for the mining activity, from which dust and sand can blow into the forest and lead to associated pollution; population increase in the villages surrounding the forest leading to an increasing demand on the land resources. Consequently, habitats in the currently known area of distribution may be modified in ways that render them unsuitable to support the seasonal life cycle of the species. The increasing pressure on land and water resources will continue to be a major factor in habitat degradation and represent a significant threat to the species, raising vulnerability in terms of conservation status. In combination with its seasonal life cycle in ephemeral habitats, its dependence on the clay-rich vertisol-type substratum for embryonic development makes the species very exposed to threats ( Nagy & Watters 2020, 2021).

No conservation measures are in place for this species. There is a need for improved habitat protection at the locations where the species is known to occur. Additional research targeting suitable habitats within the drainage of the Mkurumudzi River system and within the Gongoni Forest Reserve should be conducted in order to better understand the geographic distribution of this species and to elaborate appropriate measures of conservation.

Biology. Aquarium maintenance of selected specimens was undertaken for observation of breeding behaviour and biology. Nothobranchius sylvaticus has an annual mode of reproduction, typical of the genus. Eggs are deposited in the habitat substrate, where they survive the dry season and hatch with the onset of the wet season. Under captive conditions, peat moss was used successfully as an artificial spawning substrate. An embryonic development period of three to four months was observed at a temperature of about 22–24 °C.

Etymology. The specific name sylvaticus is a Latin participial adjective formed from ‘sylva’ (forest), and ‘-aticus’ (pertaining to) and is given in reference to the distinctive habitat preference of the species, restricted to ephemeral pools and seeps associated with seasonal forest-streams.