Stegostoma tigrinum, Forster, 1781

Dahl, Rikke Beckmann, Sigsgaard, Eva Egelyng, Mwangi, Gorret, Thomsen, Philip Francis, Jørgensen, René Dalsgaard, Torquato, Felipe de Oliveira, Olsen, Lars & Møller, Peter Rask, 2019, The Sandy Zebra Shark: A New Color Morph of the Zebra Shark Stegostoma tigrinum, with a Redescription of the Species and a Revision of Its Nomenclature, Copeia 107 (3), pp. 524-541 : 3-13

Stegostoma tigrinum

Zebra Shark

Figures 1–9 , Table 1

The following is a revised synonymy based on Kottelat

(2013).

Squalus varius Seba, 1759: p. 105 , pl. 34, fig. 1. Claimed by Compagno (1984, 2001) to be unavailable, as binominal nomenclature was not used consistently. Description was based on two juvenile individuals, one of which measured 406 mm TL. No type locality or specimens, except for an illustration.

Squalus tigrinus Forster, 1781: p. 24 , pl. 13, fig. 2. Description and schematic presentation was based on juvenile individual(s) from Sri Lanka (Indian Ocean), and on Seba’s (1759) specimens.

Squalus fasciatus Hermann, 1783: p. 301 –303. Hermann mentioned the species as Squalo fasciato in a discussion of shark gill slits, referring to Seba (1759). Hermann examined no specimens himself ( Bloch, 1784). No type specimens, no locality.

Squalus fasciatus Bloch, 1784: p. 19 , pl. 113. Bloch (1784) discovered the fifth gill slit and based his thorough description on two specimens (one measures. 300 mm TL) from Tranquebar, India ( Tharangambadi). Kottelat (2013) mentions three specimens: ZMB 4449 (holotype), ZMB 7833, and ZMB 22610. According to Paepke and Schmidt (1988), Bloch (1784) had 15 specimens in his collection. Bloch (1787) published another description of the species with additions to table CXIII presented in Bloch (1784) showing the anterior ventral part of a female next to the original figure.

Squalus tigrinus Broussonet, 1784: 659 . Based on Gronovius, 1754: p. 62, no. 136; Seba (1759) and Forster (1781). Type locality: ‘‘mer des Indes’’ (Indian Ocean), China: ‘‘rivère de Canton’’ (Canton River).

Squalus tygrinus Bonnaterre, 1788: p. 8 pl. 8, fig. 23. Description based on Bloch (1787) p. 17; Forster (1781) and Broussonet (1784). Considered a misspelling from Forster (1781) and Broussonet (1784). Bonnaterre (1788) also referred to the species as S. varius based on Seba (1759). Drawing of specimen is from Bloch (1784): p. 19. No types known.

Squalus fasciatus Bonnaterre, 1788: p. 8 . Description based on Broussonet (1784). Locality: South Africa, Cape of Good Hope: BMNH. Bonnaterre (1788) already described the species under S. tygrinus , but believed that S. fasciatus was a different species. He also used two different common names for the two descriptions; Le requin barbu (the barbed shark) and La galloné (the braided one), the latter name was incorrectly interpreted in Kottelat (2013).

Squalus tigrinus Gmelin, 1789: p. 1493 . Cites Bloch (1784), Gronovius (1754), Gronovius (1763: p. 33, no. 147), Seba (1759), Hermann (1783), and Broussonet (1784).

Squalus longicaudus Gmelin, 1789: p. 1496 . No description other than a few words from Gronovius (1754), Gronovius (1763), and Seba (1759). In the same publication on p. 1493, he described the species as Squalus tigrinus , s uggesting that he wanted to rename it to S. longicaudus .

Squalus tigrinus Pennant, 1791: p. 55 and 92, pl. 16 (XVI). Pennant cited Gmelin (1789), Bloch (1784), Gronovius (1754), Seba (1759), Hermann (1783), and Forster (1781). According to Kottelat (2013), the description was based on syntypes mentioned by Seba (1759). But the schematic drawing of a specimen is from Forster (1781).

Squalus zebra Shaw, 1804: p. 352 , pl. 148. First entry of the common name ‘‘Zebra Shark’’ in the literature. Figure is from Bloch (1784). Shaw cites Seba (1759), Gmelin (1789), and Bloch (1784).

Scyllia quinquecornautum van Hasselt, 1823: p. 315 (see Alfred, 1961: p. 81 for English translation). Based on specimens found by Kuhl and van Hasselt in Java, Indonesia, measuring. 8 feet (2400 mm) with reference to Seba (1759). Van Hasselt was aware that S. tigrinum , which had previously only been portrayed in the juvenile stage, was in fact the same species as the. 2400 mm specimens he had described. Van Hasselt placed this species in the order Chondropterygiens in the family Cyclostomes and the genus Scyllia (based on Cuvier’s Scyllium , 1816 and today accepted as Scyliorhinus Blainville, 1816 ).

Scyllium heptagonum Rüppel, 1837: p. 61 , pl. 17 fig. 1. Description was based on a single specimen from the Red Sea, Didda (Jeddah), Saudi Arabia; Holotype: SMF 3152. Kottelat (2013) mentioned that Klausewitz (1960: p. 290) designated lectotypes from Rüppel’s types, but this information is inaccurate as Rüppel explicitly based his description on a single specimen. The specimen is 1060 mm TL, and the figure reveals a mix of transitional and adult patterning.

Stegostoma carinatum Blyth, 1847: p. 725 , pl. 25, fig. 1. Based on an adult specimen caught in India (holotype possibly preserved at the Zoological Survey of India). Blyth (1847) described the shark as a new, second species of the genus Stegostoma . He portrayed the specimen in pl. 25, fig. 1 as the adult stage with details of the enlarged dermal denticles on the dorsal ridge.

Stegostoma fasciatum Bleeker, 1852: p. 23 . Based on a single female of 425’’ caught off Saramang in 1846. Bleeker was aware that Blyth (1847) already described this species, as he included Blyth’s name S. carinatum in a synonymy. He noted the developmental stage of each specimen (juvenile/ adolescence), which indicates that Bleeker was aware of the ontogenetic pattern difference.

Squalus pantherinus Kuhl and van Hasselt , mentioned by Bleeker (1852) in synonymy.

Squalus cirrosus Gronow , in Gray, 1854: p. 6. Described the species with references to Gronovius (1763), which is based on Gronovius (1754) and Seba (1759). Description is brief and Gray (1854) suggested S. cirrosus as a new name for the species.

Stegostoma tigrinum Günther, 1870: p. 409 . Provided a synonymy briefly describing the skin colors observed in the species and mentioning specimens preserved in collections at the time.

Stegostoma varium Garman, 1913: p. 59 . Based on two specimens of 300 mm TL (juvenile) and 1500 mm TL (adult), with details on the juvenile and adult patterns and their individual variation (based on other literature). Includes a synonymy. Location: East Indies to Africa. Syntypes are MCZ 55-S (1 specimen, Philippines: Manila), 33437 (1 specimen, Mauritius), and 1 uncat. specimen ( Eschmeyer et al., 2019).

Stegostoma tigrinum naucum Whitley, 1939: p. 229 , fig. 2. Mentioned that type specimens from the Indian Ocean are banded, while other types are spotted. Never suggested that the two morphs belong to a single species. Suggested that a spotted example from Hawkesbury River, New South Wales, Australia, was a new subspecies naucum, based on morphometric differences, such as a longer distance between the anal fin and subcaudal lobes, fin proportions, larger spots and thicker nasal cirri. Holotype: AMS I.4174 ( Paxton et al., 1989).

Diagnosis

Stegostoma tigrinum is characterized by a long caudal fin (49.9–54.2% TL) and five dorsolateral ridges along the body, visible even in hatchlings. Spiracles bean shaped, large (length 0.4–1.7% TL); eyes small (length 0.9–2.1% TL); barbels two, short (0.6–2.8% TL); gill slits five, but fourth and fifth partly fused so only four noticeable from a distance; pectoral fins large (anterior margin length 10.4–19.1% TL), broad and rounded; first dorsal fin originates far posteriorly above pelvic fins. Two color morphs, with a three-stage ontogenetic color and pattern change. Zebra morph: juveniles with dark brown background and cream colored bands (zebra-like); transitionals light brown with dark bands, broken up by dots; adults beige to yellow with spotted pattern (can be leopard-like). Sandy color morph: transitionals light beige background with swirly pattern of narrow, darker brown bands with tiny spots breaking up the pattern; adults uniformly sandy beige with tiny dark brown freckles. Maximum length 2050 mm TL, hatchlings approx. 250 mm TL; pectoral-fin rays of semi-plesodic structure, reaching approx. 66–88% of pectoral fin. Total vertebrae 207–262, monospondylous precaudal vertebrae 43–49, diplospondylous precaudal vertebrae 38–50, diplospondylous caudal vertebrae 120–175, precaudal vertebrae 81–101. Tooth rows upper jaw: 13–30, lower jaw: 22–30, and series count, upper jaw: 7–27, lower jaw: 8–16. Ring-type intestine with 18–20 valvular turns.

Description

Body long and slender. Caudal fin approximately half TL.

Head truncate and rounded with an almost straight, subterminal mouth; mouth corners framed with an upper and a lower labial furrow, lower furrows intervened by chin (a trilobate lower lip); nasal flap covers teeth and originates from two large nostrils, each with a small barbel; long and strongly developed nasoral grooves below nasal flap; teeth tricuspid and homodont; eyes small and oval, positioned laterally on head prior to a large bean-shaped spiracle; no nictitating eyelid and no subocular pockets, but eye can be almost closed by muscle contraction if disturbed. Four gill slits visible externally, as the fourth and fifth slit share the same opening; no filter screens internally. Pectoral fins originate below second gill slit and are large and rounded; pectoral-fin web semi-plesodic, so that fin radials extend further distally in the fin web than in the aplesodic type; mesopterygium and metapterygium are separated, and according to Compagno (2001), the propterygium is small and separated from the latter ones; first dorsal-fin origin at level of post spiracle, but a strong dorsal ridge obscures exact origin; ridges are made of enlarged and strengthened dermal denticles; two additional ridges on each lateral flank, all ridges clearly visible from neonate stage, upper lateral ridge originates above mid pectoral fin and ends below second dorsal fin, where the lower lateral ridge originates after pectoral fin and ends at caudal-fin origin, first dorsal fin triangular in shape with rounded edges, immediately followed by an elevation of the dorsal skin which turns into the second dorsal fin, the latter half the size of the first but proportions are similar; pelvic fins positioned below first dorsal fin but end further caudally; pelvic fins are smaller than first dorsal fin but larger than second dorsal fin; anal-fin origin below mid second dorsal fin and ends at caudal-fin origin; anal fin larger than second dorsal fin; caudal peduncle smooth with no keel or precaudal pits; caudal fin long, broad and distally narrowed in a strong, single terminal lobe with a subterminal notch; consists of a dorsal lobe only. Mature males express an outer clasper size of minimum 6.9% of TL and an inner size of 12.5% of TL. Vertebral count highly variable but no indication of differences between the two morphs ( Table 1); total vertebrae 207–262, monospondylous precaudal count 43–49, diplospondylous precaudal count 38–50, diplospondylous caudal count 120–175, precaudal count 81–101. Valvular intestine is of ring type and turns range from 18–20. For more details on head cartilage (cranium, jaws) and jaw muscles, see Compagno (2001) and Goto (2001).

Genetics

A comparison of the ND4 and COI sequences between the sandy morph (own sequences) and the zebra morph (own and GenBank sequences) revealed a 99.49–100% and 99.69–100% identity, respectively. Thus, the genetic results indicate that the two color morphs represent the same species. Nucleotide sequences for the COI and ND4 genes can be found in GenBank under accession numbers MK495838–MK495846.

Morphometrics

Ontogenetic changes within both morphs.— Barbel length of the zebra color morph is significantly different among all three stages. There is a negative allometric change in barbel size, meaning that the barbels become proportionally shorter with increased body size (P ¼ 0.0002, Fig. 2A ). Juveniles of the zebra color morph have the longest preorbital length (P ¼ 0.0001, Fig. 2B ), the longest prepectoral-fin length (P ¼ 0.0002, Fig. 2C ), and the longest pre-ridge length (P ¼ 0.0001, Fig. 2D ), compared with transitionals and adults. This implies that the heads of juveniles have longer dimensions and become shorter and more compact with age. For the zebra color morph, the head width (P ¼ 0.001, Fig. 2E ) and the prefirst dorsal-fin length (P ¼ 0.001, Fig. 2F ) are significantly shorter in the transitional stage. Adults of both morphs have significantly smaller eye length than transitionals and juveniles (P ¼ 0.002 zebra morph and P ¼ 0.005 sandy morph, Fig. 3A ). Adults of both morphs have a significantly shorter caudal-fin length than both transitionals and juveniles (P ¼ 0.0001 zebra morph and P ¼ 0.027 sandy morph, Fig. 3B ). There is a tendency that adults of both morphs have a longer prepelvic-fin length (P ¼ 0.20 zebra morph and P ¼ 0.092 sandy morph, Fig. 3C ) and a longer snout to vent length (P ¼ 0.0003 zebra morph and P ¼ 0.049 sandy morph, Fig. 3D ) than transitionals and juveniles. Adults of the zebra morph have a longer distance from vent to caudal-fin tip than transitionals and juveniles (P ¼ 0.024, Fig. 3E ).

Differences between the two morphs.— No diagnostic differences between the two morphs were found, though the sample size of the sandy morph was below the ideal size and the results should be interpreted as tendencies. A few characters appear to be different when comparing average values; for instance, the sandy morph displayed a longer distance from vent to caudal-fin tip than the zebra morph (P ¼ 0.055, Fig. 3E ), while the zebra morph displayed a larger snout to vent (P ¼ 0.055, Fig. 3D ) and prespiracular length (P ¼ 0.017, Fig. 3F ). These differences, however, are in all likelihood a result of the ontogenetic variation in those characters and the different average TL in the two groups.

Oral teeth

No differences in the oral teeth were found between the two color morphs. Stegostoma tigrinum has homodont teeth with no apparent sexual dimorphism. Each tooth has a strong, elongated medial cusp and two smaller lateral cusplets ( Fig. 4A, B, E–L ). The cusplets are rounded, whereas the cusp has blade-like edges. The tooth sheets reveal that the teeth are positioned closer to each other in the series than in the rows ( Fig. 4E–H ). The oral dentition expresses individual variation, where the curvature of the cusps can be either concave or convex ( Fig. 4J, L ). The tooth root is mildly bilobed with small lateral and a large labial/centered nutrient groove.

Dermal denticles

Elevated lateral ridges along the flanks of S. tigrinum consist of denticles that appear thicker, broader, and more densely positioned in ridge areas compared to non-ridge areas. There appear to be no differences between the zebra and the sandy color morph in the dermal denticles ( Fig. 4 ).

Three types of dermal denticles are present, identified by the number of scutes and the width of the dorsal denticle cusp ( Fig. 4C ). The first type, a, is a simple droplet-shaped denticle, with a single medial scute on the dorsal cusp. The second type, b, has a broader cusp with one medial and two lateral scutes protruding from the surface. The medial scute can vary in thickness by being either broad and flat or high and narrow. The third type, c, has an even broader dorsal cusp than type b, with one medial scute and two lateral scutes on each side. Type c denticles appear to be thicker, and thus stronger, than type a and type b (see Fig. 4C for a schematic overview of the denticle types), and the width of the root increases with the width of the crown.

The ridges consist of more type b and c denticles compared with the non-ridge areas. Non-ridge areas consist mainly of type a and b denticles, but all examined specimens also expressed intermittent pairs of enlarged denticles in the non-ridge area of type b and c ( Fig. 4M ). These were found in a straight line between the two lateral ridges always with one denticle overlapping the other.

The denticles change with the growth of the individual shark, ranging from type a to type b in a non-ridge area, and from type b to type c in a ridge area. In juveniles, the non-ridge area is mainly made up of type a denticles ( Fig. 4M ), while the ridge area is of type b ( Fig. 4N ). As the transitional stage is long, and the animal grows extensively in this period, the denticle composition was different between a small (630 mm TL, ZMUC P2395470 ) and a large transitional specimen (885 mm TL, ZMB 5258 ), the latter being more reminiscent of an adult in the denticle composition. The smallest specimen exhibited type a and type b denticles in the non-ridge area ( Fig. 4O ) and type b denticles in the ridge area ( Fig. 4P, Q ) .

For the adults, the non-ridge denticles were of type b and c ( Fig. 4R ), and the ridge area was mainly of type c with only sporadic occurrences of type b ( Fig. 4S ).

Coloration

Zebra morph of Stegostoma tigrinum . — The juvenile coloration consists of a dark brown background on the dorsal side and lateral flanks, with 18–25 cream white to yellowish bands stretching from flank to flank, only interrupted by large round or elongated dots of the same color ( Fig. 5A ). The bands show clear individual variation, but there are still some repeated features seen on most of the specimens: the third anterior-most band branches distally into two branches on each flank; the first or the second anterior-most band continues onto the pectoral fin; one or two large round dots or short bands and two elongated dots or short bands are seen on each pectoral fin; the upper dorsal lobe has short- to dot-like bands repeated to the tip of the fin. Correspondingly, the lower dorsal lobe displays round or elongated dots in either a horizontal or vertical direction; there are one or two round dots on the first dorsal fin and a single dot on the second dorsal fin. The ventral side is cream colored from snout to lower caudal-fin origin.

The breaking up of the juvenile pattern marks the transitional stage ( Figs. 5B–F , 7 ). Only some of the Kenyan specimens have the third anterior-most band distally split in two ( Fig. 7 : no. 3, 4, 5, 6, 13, 15, 16, 19, 21). The most noticeable repeated features are those of the pectoral fins: two elongated, slightly curved dots/short bands are positioned anteriorly, stretching from the lateral flank toward the anterior margin of the pectoral fin; and a single dot in the middle of the distal-most half of the fin. In the transitional stage, small dots appear around the edges of the cream colored bands on the whole body of the shark, along with dots on the dark background of the pectoral fins, the lower lateral flanks, and the dorsal lobe of the caudal fin. The dark brown color becomes lighter and is much more variable between transitionals than between juveniles. In some individuals, the small dots also cover the dark areas between the bands prior to the first dorsal fin. The snout is always pale without dots. Over time, the bands disappear and merge to a uniform background color and the dots spread out evenly over the body ( Fig. 5F ). Toward the end of the transitional stage, the largest specimen in this study (975 mm TL) had pectoral fins completely covered by dots ( Fig. 7 : no. 24), and the remnants of the two short bands and the single dot were very vague.

The adult coloration is a uniform sandy beige to yellowish background, with a spotted pattern spreading out from above the eyes to the caudal-fin tip ( Fig. 5G ). The spots can vary greatly in shape, size, and frequency, from small dots to larger leopard-like shapes. The two dried adults examined in this study had visible but faint coloration due to the state of preservation. Both these adults had medium sized dots covering the body in a spacious pattern. The remaining four adult specimens ( DBP1–4 ) displayed dark brown, uniformly shaped dots, though one of the females ( DBP2 ) expressed a cream colored ring around each dot .

Sandy morph of Stegostoma tigrinum . — The juvenile coloration of the sandy color morph is currently not described. The long-term observations of ZMUC P2395374 (transitional) and ZMUC P2395374 (adult) in this study reveal the pattern ontogeny from transitional to adult ( Fig. 6A–H ). The transitional coloration is a light brown/beige background with a darker brown pattern. The pattern is comprised of many narrow, twisted bands in swirly formations ( Fig. 6A–D ). The pattern begins dorsally above the eyes and covers the shark dorsolaterally, leaving the lower lateral flanks and the ventral side cream white and covered with tiny dots. Despite pattern differences compared to the zebra color morph, there are similarities in how the pattern transforms to the next stage. In the sandy color morph, the pattern also breaks up into dots, though the dots are tiny and freckle-like. These freckles are found on the edge of every curved and twisted band along the body, on pectoral fins spreading inward from all margins, along the lower lateral flanks from below the spiracle to lower caudal-fin origin, including the pelvic fins. When the freckles break the bands, a thin line of cream color appears within the bands. Along the lower dorsal lobe on the caudal fin, the freckles have also broken the pattern and been replaced by the same cream color. Over time, as seen in the long-term observations, the cream/light beige color replaces the pattern from the ventral to the dorsal side, and the bands expand in width with the overall growth of the animal. Observations of the specimen ZMUC P2395374 (1070 mm TL) revealed almost fully dotted pectoral fins with scarcely any pattern left. The anal fin was never seen with dots.

Of the two adult specimens, one was a dark brown female ( ZMUC P6268 , Fig. 6E, F ) and the other a light brown male ( ZMUC P2395375 , Fig. 6G, H ), both with dark brown freckles covering the dorsolateral body. On both adults, it was possible to see remnants of the transitional pattern above the pectoral fins ( Fig. 6F ) and on the tip of the caudal fin ( Fig. 6G ). Over time, the adults express fewer freckles, though the freckles still cover most of the body except the ventral side, the anal fin, and the head close to the snout .

Size and sexual maturity

The specimens examined in the present study measured between 185–2000 mm TL. Embryos were identified by the presence of a yolk sac and included two specimens of 185– 195 mm TL. Juveniles were identified by the lack of a yolk sac and by the black and white striped pattern, and measured 255–562 mm TL. The transitional specimens were identified by the appearance of dots in the coloration pattern and measured between 600–1535 mm TL. Mature male individuals were recognized by the clasper length exceeding the length of the pelvic fins and by the state of calcification of the claspers, and measured 1935–2000 mm TL. The long-term observations of the larger male sandy color morph (ZMUC P2395375) showed considerable growth of the claspers from first to last observation, though the animal was still not fully mature at the time of the last observation. A live, mature male of the zebra color morph (DBP4) of 1935 mm TL had claspers well exceeding the pelvic fins, almost reaching the anal-fin origin, with internal calcification ( Fig. 8A ). Outer clasper length measured 135 mm and inner length 243 mm, which corresponds to 6.9% and 12.5% of TL, respectively. Each clasper of this male also carried a short, triangular spike extruding from the dorsal terminal of the clasper gland ( Fig. 8B ). The live, immature male of the sandy color morph (ZMUC P2395374) had a TL of 1270 mm. Outer clasper length measured 21 mm and inner clasper length was 69 mm, which corresponds to 1.7% and 5.4% of TL. An immature specimen (ZMB 5258) of 885 mm TL had an outer clasper length of 107 mm and an inner length of 532 mm, corresponding to 1.2% and 3.9% of TL, respectively. These examples show an incremental increase in clasper length as a percentage of TL with animal growth. The female sandy color morph of 1395 mm TL (ZMUC P6268) was dissected to establish the state of maturity, which showed that the ovarian follicles were small and equal in size, and the uterus width measured 25 mm, indicating an immature female.

Eggs

Over the last 3.5 years, approximately 10– 20 eggs per year have been found in a tank with mature female S. tigrinum of the zebra morph at The National Aquarium of Denmark. It has not been established whether one or both of the females have laid eggs, as egg-laying has not been observed directly. In this period, the contents of all the eggs were emptied by other species within the tank; however, two eggs have yielded two juveniles in April and May of 2019. The eggs are large, dark green, and taper off bluntly at each end. No hair-like structures extrude from the corners. The eggs measure 175 mm in length and 93 mm in width at the broadest point ( Fig. 8C ). The lack of hair-like structures is inconsistent with previous descriptions of Zebra Shark eggs and could be a result of unfavorable conditions for the ovulating females .

Distribution

The sandy color morph of S. tigrinum has in the present study been documented from a single location, off the coast of Kenya, ca. 10 km north of Malindi ( Fig. 9 ). Six specimens of this morph were caught in June 2016 (rainy season) at a depth of 12 m , on a sandy substrate. A seventh specimen was caught in May 2017, 15 km north of Malindi on sandy bottom, also at 12 m depth .

Etymology

The name Stegostoma tigrinum is made up of the Greek stego meaning cover, and stoma meaning mouth, and can thus be translated to ‘covered mouth’ ( Froese and Pauly, 2018). The epithet tigrinum refers to the juveniles’ banded pattern.

Figures

FIG. 1. Schematic presentation of the morphology of S. tigrinum, including 52 morphometric measurements performed in the present study. Each number refers to a specific measurement further elaborated in Table 1.

FIG. 2. Six plots of morphometric characters of the zebra morph of S. trigrinum measured in % of TL. (A) Barbel length: decreases with growth of individual shark (P ¼ 0.0002). (B) Preorbital length: longer in zebra juveniles (P ¼ 0.0001). (C) Prepectoral-fin length: longer in zebra juveniles (P ¼ 0.0002). (D) Pre-ridge length: longer in zebra juveniles (P ¼ 0.0001). (E) Head width: shorter in zebra transitionals (P ¼ 0.001). (F) Pre-first dorsal-fin length: shorter in zebra transitionals (P ¼ 0.001). ‘–’ Juveniles, ‘þ’ transitionals, ‘3’ adults.

FIG. 3. Six plots of morphometric characters of the zebra and the sandy morphs of S. tigrinum measured in % of TL. (A) Eye length: smaller in adults of both morphs (zebra P ¼ 0.002, sandy P ¼ 0.005). (B) Caudal-fin length: shorter in adults of both morphs (zebra P ¼ 0.0001, sandy P ¼ 0.027). (C) Prepelvic-fin length: longer in adults of both morphs (zebra P ¼ 0.02, sandy P ¼ 0.092). (D) Snout to vent length: longer in adults of both morphs (zebra P ¼ 0.0003, sandy P ¼ 0.049), and longer in zebra morph as a group (P ¼ 0.055). (E) Vent to caudal-fin tip: longer in zebra morph adults (P ¼ 0.024), and for the sandy morph as a group (P ¼ 0.055). (F) Prespiracular length: larger in zebra morph as a group (P ¼ 0.017). ‘–’ Zebra juveniles, ‘þ’ zebra transitionals, ‘3’ zebra adults, ‘*’ sandy transitionals, ‘D’ sandy adults.

FIG. 4. Oral teeth and dermal denticles of S. tigrinum: schematic presentation and nomenclature of oral teeth (A–B) and the three types of dermal denticles, type a, b, and c (C–D). Oral teeth from a juvenile specimen (ZMUC P2394733, 325 mm TL) from upper (E–F) and lower jaw (G–H); oral teeth from an adult (ZMUC P6268, 1395 mm TL) upper (I–J) and lower (K–L) jaw; dermal denticles from a juvenile specimen (ZMUC P2394733, 325 mm TL) from a non-ridge area (M) and a ridge area (N); a transitional specimen (ZMUC P2395470, 630 mm TL) non-ridge (O–P) and ridge (Q); and of an adult specimen (ZMUC P6268, 1395 mm TL) non-ridge (R) and ridge (S).

FIG. 5. The three ontogenetic stages of the external morphology of the zebra morph of S. tigrinum: juvenile (A), transitional (B–F), and adult (G).

FIG. 6. The sandy morph of S. tigrinum: transitional (A–D) and adult (E–H). Close up of the skin pattern on the right lateral flank of a transitional specimen (B–C) and of remnants of the transitional pattern on flank and caudal-fin tip of adults (F–G).

FIG. 7. Specimens of S. tigrinum from Kenya in the transitional stage, shown from smallest to largest body size (no. 1–24) in dorsal and lateral view. No. 1–6: 565–670 mm TL, no. 7–12: 675–710 mm TL, no. 13–18: 740–800 mm TL, and no. 19–24: 802–975 mm TL.

FIG. 8. Claspers of a mature, male S. tigrinum, DBP4, 1935 mm TL (A) with close up of the short, triangular spike extruding from the dorsal terminal of the clasper gland (B) and an unfertilized egg from a female, DBP2/3 (C).

FIG. 9. Point map of catch location for the sandy morph of S. tigrinum (shark icon), marking 10 km north of Malindi, off the coast of Kipini, Kenya, Africa. Map is modified and based on UN Office for the Coordination of Humanitarian Affairs (OCHA) map.