Serranus knysnaensis Gilchrist, 1904

Sithole, Yonela, Heemstra, Elaine & Mwale, Monica, 2021, Revalidation and redescription of Serranus knysnaensis Gilchrist, 1904 (Perciformes: Serranidae) with a new distribution record, Zootaxa 5057 (1), pp. 99-113 : 102-105

Serranus knysnaensis Gilchrist, 1904

English name: African seabass

Figures 1–3 ; Tables 1–5

Serranus knysnaensis Gilchrist 1904: 2 , Pl. 19 (Knysna, Cape Province, South Africa); Smith 1949: 193, Fig. 427 ( South Africa); Heemstra & Heemstra 2004: 174 (unnumb. painting; South Africa); King & Fraser 2014: (unnumb. photograph p. 69; endemic, from False Bay in Western Cape to northern KwaZulu-Natal).

Serranus cabrilla (in part): Smith & Smith 1966: 88 (painting 429; Atlantic species from Mediterranean southwards, colonising South Africa to Natal); Heemstra & Randall 1986: 536 (Fig. 166.76, Pl. 45; Mediterranean, eastern Atlantic, English Channel to South Africa, and round coast to Durban)

Diagnosis. Dorsal-fin X, 14–15; pectoral-fin rays 15–16; gill rakers 6–8+12–15 (18–22 total); circumpeduncular scales 26–34; lateral-line scales 67–76; scales from dorsal fin to lateral line 9–12; scales from the anal-fin origin to lateral line 22–26. Caudal fin emarginate. Scales small, ctenoid and deciduous.

Body elongate, not much compressed; body depth less than head length, about 3.3–4.2 in SL; head length 2.5– 2.9 in SL. Maxilla extending to below middle of the eye; jaws subequal; lower jaw projecting; opercle with three flat spines. Eye diameter greater than interorbital length and less than snout length, eye diameter contained 9.6–13.1 in SL. Snout, interorbital and maxilla naked. Pectoral fin originates before dorsal fin and slightly before the pelvic fin. Distal tip of pectoral fin before anal-fin origin.

Description. Proportional measurements are given in Table 3 . Body elongate; dorsal fin continuous, with spinous membrane moderately incised between spines, becoming less incised posteriorly; dorsal-fin spines graduated, but with first two spines noticeably shorter, and ninth and tenth spines subequal; soft-rayed portion of fin higher than spinous part. First anal-fin spine shortest; second anal-fin spine thicker than others. Anterior lateral-line tubes with ascending branch. Peduncle longer than deep. Patch of small teeth on vomer and palate.

Preopercle margin serrate with more developed spines near the angle. Snout and interorbital naked; preopercle, opercle, operculum and postorbital scaly; chest and belly scaly; snout to eye naked. Lateral line complete, arching slightly from upper end of gill cavity and following body outline to caudal peduncle where it becomes horizontal; lateral line with pores.

Fresh color ( Fig. 1a ). Head and body light (pale) to yellowish brown dorsally, pale pinkish to white ventrally. Head with three yellow to bronze stripes, two below and one behind eye, the lowest stripe from just behind maxilla to preopercle, the second stripe from below anterior of eye to subopercle and occasionally, the pectoral-fin base, the upper stripe from rear ventral part of eye to opercular margin near upper pectoral-fin base; posterior bony ridge of the orbit yellow. Body with two irregular usually darker brown to yellowish-brown longitudinal bands of subequal width (~2/3 eye diameter), or with lower band slightly narrower, the upper band from either side joining on premaxilla and running from snout tip through eye to upper caudal-fin base, with narrow white border (<pupil diameter) above and subequal white band below; lower brown band from pectoral-fin base; around nine faint to dark brown bars may overlay the dark body bands, and which may be reduced to poorly defined dark vertical blotches along the dorsolateral part of the body. Dorsal fin with outer triangular part of interspinous membrane white, remainder of fin hyaline yellowish to pale brownish with midlateral whitish stripe or irregular whitish streaks midlaterally, and pale blue spots on soft dorsal fin. Caudal fin hyaline whitish with pale blue spots and caudal-fin lobe tips deep yellow to orange. Tips of dorsal-fin spines whitish.

Color in preservation ( Fig. 1b ). Specimens often lose color almost completely in preservation. Two irregular dark, longitudinal bands from head to caudal-fin base, and dark stripes below and behind the eye visible in some specimens. Holotype uniformly dark brown, no visible pattern.

Distribution. Serranus knysnaensis occurs in the southwestern Indian Ocean from South Africa (Knysna, Western Cape, rarely southwards) and northwards to Bazaruto, southern Mozambique. New records from north of Durban ( Fig. 2a , Umhlanga; SAIAB 193652, uThukela [formerly Tugela] Banks; Fig. 2b , Sodwana Bay by C. van Jaarsveld sourced from Fishwise Professional:

https://www.fishwisepro.com/pictures/default?Sid=34346&Info=%27 Serranus %20knysnaensis%27

As well as from Mozambique and Madagascar (see Material examined, also Figs. 2c and 2d )), are reported in this study.

Species delimitation with DNA barcoding. The pairwise genetic distances ( Table 4 ) of the COI mtDNA sequences for all Serranus species under the K2P model ranged from 1.60% to 25.30%. The pairwise genetic distance ( Table 4 ) between COI mtDNA sequences for S. knysnaensis from South Africa and S. cabrilla from the Atlantic Ocean and Mediterranean Sea was low at 1.60%. Although, this was among the lowest genetic distances between the analysed Serranus species , it was comparable to that between two distinct species, S. novemcinctus to S. cabrilla (1.61%), and lower than the genetic distance between S. knysnaensis and S. novemcinctus (1.99%). These divergence estimates were also comparable to the COI sequence divergence reported between other serranid species (e.g Victor 2012; Vella & Vella 2016; Iswarya et al. 2018) and other marine fish species (e.g. Uiblein & Gouws 2014). Furthermore, K2P genetic distance more than 1% for the COI gene is considered as a threshold of species delimitation for marine fishes ( Avise 2000; Zhang & Hanner 2011).

The maximum likelihood tree ( Fig. 3 ) based on 92 COI mtDNA sequences of 18 species including the outgroup ( Cephalopholis nigripinnis ) recovered monophyletic clusters corresponding to all species including the WIO species ( S. knysnaensis and S. novemcinctus ). All these three clades were strongly supported as monophyletic groups with bootstrap>75%.

The S. knysnaensis and S. cabrilla clusters ( Table 5 ) had the highest Intra/Inter ratios (0.28 and 0.38 respectively), suggesting that the divergence within these lineages was high relative to the divergence with their closest species. The small values (<0.03) observed among the other 15 species comparisons indicate that intraspecific differences are small relative to interspecific and the nearest species, although it is important to note the low sample sizes (See Table 1 ). The three species form monophyletic clusters although the phylogenetic relationships among them are unresolved using this gene dataset. There was also evidence of possible genetic substructuring in S. knysnaensis .

Remarks: Serranus knysnaensis can be distinguished from the other Serranus species in the SWIO, S. novemcinctus , by having 12–15 lower limb gill rakers (versus 21–23) and 26–34 circumpeduncular scales (versus 34–36) ( Table 2 ). Serranus knysnaensis also differs from S. novemcinctus by two dark prominent longitudinal bands and is endemic to southern Africa while S. novemcinctus has nine dark vertical bars dorsally on the body and it is distributed in Amsterdam and St. Paul Islands, and Madagascar. Serranus knysnaensis differs from S. cabrilla by having 26–34 circumpeduncular scales (versus 34–38) and 18–22 total gill rakers (versus 22–24) ( Table 2 ). The color pattern of S. knysnaensis is very similar to that of S. cabrilla , however, S. knysnaensis can be distinguished by two dark prominent longitudinal bands, whereas S. cabrilla has two or three longitudinal bands often partially broken into dark blotches. The two species also have different distribution range; S. knysnaensis is a southern African endemic from Knysna to Bazaruto, southern Mozambique and Madagascar, with rare waifs reported south of Knysna to False Bay. Serranus cabrilla is known from the British Isles to Angola, including Azores, Madeira, Canary Islands, São Tomé and Príncipe, and Cape Verde Islands, also through Mediterranean and Black Sea, and in the Red Sea ( Heemstra & Heemstra 2004; Heemstra & Anderson 2016; Iwamoto & Wirtz 2018). Heemstra & Heemstra (2004) described S. knysnaensis as a mainly rocky bottom species occurring in depths of 1–200 m, it is also often found at the mouth of estuaries ( Smith & Smith 1966). Whereas S. cabrilla is reported over rocky as well as soft bottom, from shore to 450 m ( Heemstra & Anderson 2016; Iwamoto & Wirtz 2018). Furthermore, the S. knysnaensis clade differs from S. novemcinctus and S. cabrilla by 1.99 to 1.60% genetic distance, respectively.

Based on differences in the morphological characters, distribution and genetic data, S. knysnaensis is a valid species distinct from S. cabrilla and South African records of S. cabrilla are misidentifications of S. knysnaensis . Available genetic data, from the two WIO species ( S. knysnaensis and S. novemcinctus ) shows that they are closely related to each other, as well as S. cabrilla . The low genetic diversity (approx. 1.60%) was significant, suggesting that these lineages could represent ecomorphs or lineages that have only recently diverged (speciated) in this genus.

Figures

FIGURE 1. Serranus knysnaensis (a) Underwater picture from Umhlanga Ruby Snow (29°44’ S, 31°15’ E), Photograph by Ryan Palmer (DST/NRF/ACEP Spatial Solutions Project), and (b) in preservation (SAIAB 17159, 64.9 mm SL, Tsitsikamma, South Africa).

FIGURE 2. New distribution records of S. knysnaensis (a) north of Durban, uThukela Banks, KwaZulu-Natal, photograph by Allan D. Connell (NRF-SAIAB). (b) Sodwana Bay, KwaZulu-Natal, photograph by Christo van Jaarsveld. (c) SAIAB 207782, 105.1 mm SL, off Lagoa Polela, Inhambane Province, Mozambique, Photograph by Oddgeir Alvheim (IMR). (d) SAIAB 189784, 82.0−92.0 mm SL, Sud-Ouest Cap Andavaka, southwestern Madagascar, photograph by Martyn Lee (AV2010).

FIGURE 3. A maximum likelihood tree based on 92 COI mtDNA sequences (597 bp) of 17 Serranus species based on the General Time Reversible model. All sequences were downloaded from BOLD and GenBank (Table 1). The bottom scale bar represents branch lengths measured in the number of substitutions per site. Study species clusters are also highlighted: Serranus cabrilla (SC), S. knysnaensis (SK) and S. novemcinctus (SN).

Tables

TABLE 2. Frequency distribution of the number of circumpeduncular scales, the number of lower limb and total gill rakers (both including rudiments) for specimens of Serranus cabrilla (SC), S. knysnaensis (SK), and S. novemcinctus (SN).

Meristic trait	Species	12	13	14	15	16	18	19	20	21	22	23	24
Circumpeduncular scales	SC (n=18)
SK (n=38)
SN (n=16)
Lower limb gill rakers	SC (n=18)			1	8	9
	SK (n=42)	2	18	17	5
	SN (n=16)									4	8	3	1
Total gill rakers	SC (n=18)										5	7	6
	SK (n=42)						2	5	12	14	9
SN (n=16)