Rivulus mahdiaensis, W. H. Suijker & G. E. Collier, 2006

W. H. Suijker & G. E. Collier, 2006, Rivulus mahdiaensis, a new killifish from central Guyana (Cyprinodontiformes: Rivulidae)., Zootaxa 1246, pp. 1-13 : 3-12

Rivulus mahdiaensis , new species

(Figure 1)

Holotype. ZMA 123.720 (Allotype, ZMA 123.721; Paratype, ZMA 123.722) Guyana, Mazaruni-Potaro District, Blackwater Creek, tributary of the Potaro River, on 5 miles from Mahdia crossing the road from Mahdia to Garroway Stream, 5° 21’ 11” N and 59 ° 08’48” W. Field code Guy 95/11. Coll W. Suijker & Y. Suijker-Jansen, 1995 April 12 at 14.00 hours.

3 Paratypes. FMNH 113582, FMNH 113583 and FMNH 113584. Guyana, Mazaruni-Potaro District, Blackwater Creek, tributary of the Potaro River, on 5 miles from Mahdia crossing the road from Mahdia to Garroway Stream, 5° 21’ 11” N and 59 ° 08’48” W. Field code Guy 95/11. Coll W. Suijker & Y. Suijker-Jansen, 1995 April 12 at 14.00 hours.

3 Paratypes. ZMA 123.853, ZMA 123.854 and ZMA 123.855 Guyana, Mazaruni-Potaro District, Blackwater Creek, tributary of the Potaro River, on 5 miles from Mahdia crossing the road from Mahdia to Garroway Stream, 5° 21’ 11” N and 59 ° 08’48” W. Field code Guy 95/11. Coll W. Suijker & Y. Suijker-Jansen, 1995 April 12 at 14.00 hours.

Diagnosis

Males of Rivulus mahdiaensis differ from all other species of the genus Rivulus by their unique long and lyre-shaped caudal fin with filaments and by the elongated unpaired fins.

Females show a narrow dark lateral band and share this distinctive characteristic only with Rivulus frenatus , R. xiphidius and perhaps R. campelloi (Huber 1992). Females of Rivulus mahdiaensis have an ocellus on the dorsal part of the caudal peduncle. This is unusual because the small species of the Guyana plateau have no ocellus on the caudal peduncle. This ocellus is often present in females of many species of Rivulus and hence is often called a “ Rivulus spot”. The meristics (D 6-7, A 8-9 and D/A 4-5) (Table 1) are lower than those of the other small species on the Guyana plateau (Table 2).

Comparison of mitochondrial DNA sequences place this species as a member of a monophyletic clade which includes R. agilae , geayi Vaillant , frenatus , xiphidius Huber and lyricauda Thomerson, Berkenkamp & Taphorn . R. mahdiaensis possesses a unique karyotype (2n=38) among members of this group.

Description

This new species is small and slender and has a uniquely shaped caudal fin. All rays of the caudal are proportionately longer than for most Rivulus while the dorsal and ventral rays are extended to an even greater degree in mature males. Thus, it is the first record of a Rivulus species with an exaggerated lyre tail. The dorsal and anal fin rays are also uniquely elongated. The longest anal fin rays of mature males are at least 150% of the body depth at anal fin origin, whereas in all other known species of the genus they are less than 100% of the body depth.

Coloration

Live colors. Male: The sides are blue reflective and show five or six continuous lines. The three upper lines are red. The lower lip is black and a wide stripe runs via the eyes to the gill covers where it splits into two lines and ends at anal fin height. These two lower lines are more dark red to black on the sides before they merge to one red band posterior to the anal fin and continue through the mid-lower portion of the caudal fin. The lyre shaped caudal fin has the following color pattern. There is a yellow stripe along the dorsal margin which continues to the end of the dorsal extension, followed interiorly by a blue stripe and then a red one. The ventral margin of the caudal is also yellow which continues to the end of the ventral extension, followed interiorly by a blue stripe and then a red one. The middle part of the caudal is greenish. The elongated dorsal fin is reflective blue with a many red spots. The elongated anal fin is yellow distally and blue basally with a few red spots. The ventral fins are yellow with some red spots. The pectoral fins are clear. The dorsum is brown and the belly is dirty white.

Female: dorsum and sides are light brown with irregular dark spots that form a pattern of diagonal bands. There is a black band on the lower lip, which runs, as in the males, via the eye to the lower part of the base of the caudal fin. Above this band there are two lines of indistinct red spots. The caudal fin is clear and shows irregular black markings that form three vertical-lines. On each fin ray between the vertical-lines is a light yellow spot. Females also have an ocellus on the dorsal part of the caudal peduncle. This ocellus is often present in females of many species of Rivulus and hence is often called a “ Rivulus spot”. The proximal 1/3 of the anal fin is light blue with two rows of dark spots. The distal 2/3 is dark red. The dorsal fin is clear with irregular red spots. The ventral and pectoral fins are clear.

Color in alcohol. Males: Sides are light brown, on the lower part there are two or three rows of dark red spots, no longitudinal band is visible. All fins are clear without spots. Females are light brown with darker brown spots with no visible longitudinal stripe. All fins are clear, except the caudal fin, which has the same pattern as in life.

The holotype and allotype have a D type pattern of frontal scalation (sensu Hoedeman 1958). The other specimens were not examined for this character as it is no longer used in diagnostic keys.

Etymology

Rivulus mahdiaensis is named after the village Mahdia near where it was discovered.

Distribution and habitat

The type locality of Rivulus mahdiaensis is a small black water creek, called Blackwater Creek, near the village of Mahdia. This creek is a tributary of the Potaro River, 4 miles above Kangaruma landing, on the right bank of the Potaro River, Mazaruni-Potaro District, Guyana. The location is about 250 meters above sea level.

Rivulus mahdiaensis was collected in the shallow parts of this black water forest-creek between the roots of plants growing at the bank of the creek (Fig. 2). We also caught them in little pools along the banks of the creek. They prefer the dark parts of the biotope. Rivulus waimacui , Apistogramma ortmanni , Nannacara bimaculata , Poecilocharax bovalli were caught in the same creek. The temperature of the water was 25° C. at 14.00 hours. The pH was 4.5 and the Total Dissolved Solids was <1DH.

A second location of this new species was found in 1997 along the road between Bartica and Garraway Stream by the author and his wife Y. Suijker-Jansen, F. Vermeulen and M. Vermeulen. This location is another blackwater creek, north of the Potaro River, which crosses the road from Bartica to Mahdia at 77 miles from Bartica (Fig. 3). No specimens were preserved from this second location. This species may have a very limited distribution or be uncommon where it is found as no other collections of R. mahdiaensis were made at the eight different sites around Mahdia and along the road from Bartica to Mahdia visited during collecting trips of 1991, 1995, and 1997.

Aquarium behavior

Rivulus mahdiaensis is a peaceful species, which can be kept in a group of several pairs. Sometimes there is some aggression or competition between the males but they do not appear to damage each other. They do not like bright light. The pH of the water must be around 6 and temperature around 23° C. Under aquarium conditions they lay their eggs (~2 mm in diameter) in plants with fine leafs, like Vesicularia dubyana (Java moss). They will also spawn in artificial spawning mops made of yarn depositing eggs in the top and at the bottom. The eggs develop in 18 to 21 days. The newly hatched fry are immediately able to take Artemia nauplii. Young fish have the plainer pattern of females until ten to twelve weeks of age when the color pattern of males begins to appear.

Discussion

One of the hallmarks of the genus Rivulus is morphometric homogeneity (Huber, 1992). However, R. mahdiaensis is easily distinguished from related small species of Rivulus by the following criteria. It differs from Rivulus breviceps , Rivulus lyricauda , Rivulus agilae , Rivulus torenticola Vermeulen & Isbruecker , Rivulus gransabanae Lasso, Taphorn & Thomerson , Rivulus frenatus and Rivulus xiphidius by its lower number of dorsal and anal fin rays as well as a more anterior insertion of the dorsal fin relative to the anal fin (D/A metric of Huber, 1992). The low number of lateral line scales (30) in R. mahdiaensis is similar to that of R. lyricauda (30.6) and R. xiphidius while this number averages greater than 31 for the other species listed above (Huber 1992) (Table 2). Females of R. mahdiaensis have a caudal ocellus as do R. agilae and R. geayi . However, it is lacking in females of R. lyricauda , R. frenatus , R. xiphidius , R. gransabanae , and R. torenticola . R. mahdiaensis also differs from all of these species by body shape, shape of anal and dorsal fin and shape and color pattern of the caudal fin.

R. mahdiaensis represents a unique and distinct phenotype within the genus Rivulus . The color pattern, especially that of the female, suggested a relationship with R. frenatus and R. xiphidius , as females of these species also are characterized by a broad, dark longitudinal stripe on the sides. The broad dark longitudinal stripe also appears in R. lyricauda when it is stressed (Thomerson et al. 1991). Although R. lyricauda has modest extensions to the dorsal and ventral rays of the caudal, these extensions are not as long as those of R. mahdiaensis nor are the rays of all unpaired fins as elongated. All four of these species are also characterized by having very dark to black lips.

The molecular phylogeny of R. mahdiaensis and some of its related species reveals considerable congruence with the morphological features discussed above (Fig. 4). The closest relative among the species sampled is R. lyricauda , the only other species of Rivulus with a clear tendency to form a lyre shaped caudal fin. These two sister taxa are members of a monophyletic group that also includes R. frenatus , R xiphidius , R. agilae , R. geayi , and R. strigatus . [We are aware that R. strigatus Regan has been synonymized with R. geayi (Huber 1992), however the two geographically distinct populations used for the DNA sequencing (Guyana and Belem, Brazil) displayed significant sequence differences and we retained the name strigatus in Fig. 4]

The ten taxa represented in Figure 4 and rooted in that figure with R. cylindraceus and R. roloffi when placed in the larger context of the family Rivulidae, represents a monophyletic clade that diverged from other members of the family early in its history (Murphy et al., 1999: Hrbek and Larsen, 1999). Rivulus breviceps , and R. gransabanae are also members of this group based upon independent sequence data (Hrbek et al., 2004), but were not available for inclusion in this analysis. Rivulus torenticola may also be a member of this group.

The reduced chromosome number seen for R. mahdiaensis (Fig. 5a) is an unusual karyotype in that Rivulus karyotypes rarely show reduction in chromosome number below 2n=44 (Scheel 1972, Collier, unpublished data). However, within the group of ten taxa shown in Figure 4, most members have reduced chromosome numbers. This reduction is minimal for three species in this clade which have most recently diverged from each other [ R. agilae is 2n=44 (Zoch et al. 1989), R. geayi is 2n=42 (Fig. 5f), R. strigatus from Belem is 2n=42 (Fig. 5e)], whereas it is more pronounced for R. mahdiaensis and the three other related species [ R. mahdiaensis is 2n=38 (Fig. 5a), R. lyricauda is 2n=40 (Fig. 5b), R. frenatus and R. xiphidius are both 2n=30 (Fig. 5c & d),]. Within this latter group R. mahdiaensis is also distinctive in possessing a pair of an exceptionally large acrocentric chromosome not evident in the other karyotypes examined. One of the remaining species, R. rectocaudatus (Fels & de Rham, 1982) , is reported to also be 2n=38.