Mylochromis rotundus, Turner, 2025

Turner, George F., 2025, Two new species of Mylochromis (Cichlidae) from Lake Malawi, Africa, Journal of Fish Biology 106 (4), pp. 1124-1133 : 1126-1128

3.1 | Mylochromis rotundus new species urn:lsid:zoobank.org:act:54696C3B-768C-4405-BDA6-9777A3F3A57D .

Holotype: University Museum of Zoology , Cambridge : UMZC 2016.25 .9 apparent male, 99.7 mm SL, collected by scuba at Mphanga Rocks February 23, 2016, by Malawi Cichlid Genomic Diversity Survey ( MCGDS) (Figures 1 – 3).

Paratypes (four): three apparent males: UMZC 2016.5.4, 88 mm SL; 2016.5.5 (lower jaw damaged), UMZC 104.8 mm SL; UMZC 2016.5.11, 93.5 mm SL and one female UMZC 2016.25 .10, 71.5 mm SL collected with holotype .

Etymology: “rotundus ” = round, referring to the rounded body shape and short, rounded snout in particular.

Diagnosis: the lower jaw dentition is “ Placidochromis - type,” with the outer series extending relatively straight toward the posterior end of the jaw. The rounded body, short snout, and large eye enable the species to be readily distinguished from most other known oblique-striped species. Body depth of>38% SL and the lack of enlarged pharyngeal teeth distinguish the species from all other known congeneric species apart from M. semipalatus and M. melanonotus (data from Eccles & Trewavas, 1989). These two are considered conspecific by Konings (1993b). The latter has a distinctively flattened lower jaw at the symphysis. Both of these have relatively much longer snouts than M. rotundus (see Figure 9). In M. melanonotus : snout length 1.5 to almost 2 eye diameter ( Eccles & Trewavas, 1989), M. semipalatus snout 1.5 eye diameter ( Eccles & Trewavas, 1989) versus 0.9 – 1.2 in M. rotundus .

Description: body measurements and counts are presented in Table 1. M. rotundus is a medium sized (<105 mm SL), laterally compressed (maximum body depth 2.5 – 2.6 times maximum width) cichlid fish with a rounded head profile, terminal mouth, and large eyes (31.8% – 38.8% head length [HL]) (Figures 1 and 2). When a melanin pattern is visible, it is dominated by a broad continuous oblique stripe.

All specimens are relatively deep bodied and laterally compressed, with deepest part of body generally around seventh dorsal-fin spine. Anterior upper lateral profile convex, 40 – 45 to horizontal anteriorly, slightly concave above eye, continuing straight to tip of snout, with no obvious bulge made by premaxillary pedicel. Jaws isognathous to slightly retrognathous, jaw teeth prominent even when mouth closed. Tip of snout well above level of upper insertion of pectoral fin and about level with the bottom of eye. Lower profile curves gently from lower jaw tip to insertion of pelvic fins, then almost straight from pelvic to first anal spine. Mouth relatively small, gape angled about 32 – 45 to horizontal and lips relatively thin. Posterior end of maxilla well in front of anterior margin of eye. Eye large, circular, and generally below the head profile in lateral view. Lachrymal wider than deep with five openings.

Flank scales weakly ctenoid, with cteni becoming reduced dorsally, particularly anteriorly above upper lateral line, where they transition into a cycloid state. Scales on chest relatively large (e.g., in type, largest scale 2 mm vs. width across insertion of pelvics, 7.6 mm). Gradual transition in size from larger flank scales to smaller chest scales, typical in non-mbuna Malawian endemic haplochromines ( Eccles & Trewavas, 1989). Caudal fin densely scaled, over at least proximal three-fourths.

Cephalic lateral-line pores fairly inconspicuous, and flank lateral line shows usual cichlid pattern of separate upper and lower portions, with zero to four pored scales after kink in upper lateral line and two to three smaller pored scales after line of flexion of hypurals.

Pectoral fins relatively short, extending beyond vent but not to first anal spine, whereas pelvics occasionally just reach first anal spine base. Filaments of dorsal and anal fins reaching just past base of caudal fin. Caudal emarginate.

Lower jaw relatively sturdy and broad, with marked mental process. Outer series of teeth in lower jaw stout, erect, prominent, subequally bicuspid with two large rounded cusps, generally deeply implanted in fleshy gums (Figure 3). Upper jaw outer series similar, but in some specimens relatively long shafts visible. In both upper and lower jaws, three to five irregular inner series of relatively large, erect, stout tricuspid teeth with blunt tips.

Lower pharyngeal bone small, lightly built, Y-shaped, carrying small, short, slender blunt teeth (Figure 3). None notably enlarged, F I G U R E 3 Mylochromis rotundus , holotype, lower pharyngeal (left) and oral dentition (lower jaw, right) .

meristics of Mylochromis rotundus sp.

nov., holotype and four paratypes. Standard length (SL, mm)

As % SL

Body depth

Head length

Dorsal-fin base length Anal-fin base length

Predorsal length

Pre-anal length

Prepelvic length

Preventral length

Caudal peduncle length

Caudal peduncle depth

As % head length

Head width

Interorbital width

Snout length

Lower jaw length

Premaxillary pedicel length Cheek depth

Eye diameter

Lachrymal depth

Ratios

Holotype Paratypes: mean Minimum Maximum

99.7 89.5 71.5 104.8

40.7 39.8 38.6 40.4

32.2 32.9 31.9 35.0

56.5 55.9 53.4 57.6

19.1 18.7 18.3 19.1

37.2 38.1 36.8 40.4

71.2 69.9 68.7 71.8

34.8 34.2 32.1 36.5

42.5 40.6 38.8 42.0

14.1 14.8 13.6 15.9

13.1 12.4 11.5 13.4

49.2 48.6 44.4 51.0

29.6 26.9 22.8 28.8

37.7 36.0 34.8 37.4

35.2 33.8 30.6 38.0

28.0 26.6 25.2 27.8

23.1 17.0 16.3 17.6

31.8 34.8 32.0 38.8

26.2 23.9 21.2 25.3 Maximum depth/head width 2.57 2.49 2.46 2.54 Caudal peduncle length/depth 1.08 1.20 1.11 1.31

Meristics

Upper gill rakers

Lower gill rakers Dorsal-fin spines Dorsal-fin rays

Anal-fin rays

Longitudinal-line scales Cheek scales

3 4 5

13 10 11

16 16 17

12 10 11

9 9 9

36 32 33

4 3 3 although central teeth tend to be larger than lateral ones. Gill rakers widely spaced, pointed, finger-like with thick bases.

Preserved and fresh colouration of female beige, paler on chest and belly, flank scales with a brownish spot anteriorly (Figure 2). Strong dark oblique stripe from nape to the caudal-fin base, extending into fin rays. Stripe is continuous but has a “stepped” form of thinner lines joining elongated midlateral and suprapectoral blotches. A large dark blotch on upper half of operculum. Dorsal fin with dull orange spots throughout, and caudal, anal, and pelvic fins dusky. Preserved and freshly collected mature males darker with golden spots on majority of flank scales (Figure 1). Dorsal, caudal, anal, and pelvic fins dark, dorsal and caudal with numerous orange spots. Dorsal-fin lappets white with orange tips. It is possible that this is not the full breeding dress and that fully courting males are bright blue (Figure 4).

Distribution and ecology: known only from a collection from shallow water at Mphanga Rocks near Chilumba in the northwestern part of Lake Malawi. Photographs under the name Mylochromis sp. “mollis north,” which may or may not be of this species, are shown from the coast between Ruarwe and Chilumba ( Konings, 2016; Figure 4). It appears to be a species of rocky habitats.