Mugil cephalus

Pacheco-Almanzar, Eloísa, Simons, James, Espinosa-Pérez, Héctor, Carrara, Xavier Chiappa- & Ibáñez, Ana L., 2016, Can the name Mugil cephalus (Pisces: Mugilidae) be used for the species occurring in the north western Atlantic?, Zootaxa 4109 (3), pp. 381-390 : 383-388

Diagnostic characters of Mugil cephalus

The morphometric data obtained are shown in Table 1 . Adults have a thick layer of adipose tissue around the eye, covering most of pupil; hind edge of preorbital does not extend beyond the corner of the mouth. Preorbital extends almost to the end of the maxilla ( Figure 2 ). Both the second dorsal and anal fins have small scales on the basal parts. Anal fin has 3 spines and 8 soft rays in adults (the first spine is very short, and may be hidden under the overlapping scales). Juvenile fish, especially those ≤ 30 mm standard length, usually have 2 spines and 9 soft rays. Pectoral fin with 1 spine and 15 to 16 (rarely 17) soft rays; its length does not reach the origin of the first dorsal fin. The number of scales of the longitudinal series is generally 38–43 (mean 40) for the samples analyzed (reported range worldwide is 36 to 44), and from 12 to 15 in the transverse series ( Table 1 ). The number of gill rakers on the first gill arch is 53–89, showing a positive relationship with standard length.

Dorsally, specimens are gray-olive or grayish brown colored. Flanks are silver and the abdomen is whitish; 7 to 10 dark longitudinal stripes are visible along the sides. Dorsal and caudal fins are dark; pelvic and anal fins are pale. The pectoral fins have a dark spot at the origin ( Figure 2 ).

Interspecific comparison

The morphometric measures of M. cephalus and M. liza are shown in Table 1 . The mean values of all measurements are similar between the two species except the distance from the snout to the origin of the dorsal fin, which is much larger for M. liza . In this case, the beginning of the dorsal fin is located towards the distal zone while in M. cephalus the beginning of the dorsal fin has an anterior location. Also, the body width is greater in M. liza than in M. cephalus , which appears more robust. This fact has also been recognized by Harrison (2002) who mentions that the ratio of body depth to standard length in M. liza ranges from 17 to 23% while values of 24 to 28% are common in M. cephalus . Even if the majority of the mean values of the morphometric variables are similar, ranges are generally wider in M. cephalus . In M. liza , adipose tissue around eye is almost absent and the hind edge of the preorbital extends beyond the corner of the mouth. The preorbital ends before the end of maxilla ( Figure 3 ). The second dorsal and the anal fin of M. liza have no scales in the basal parts. Anal fin has 3 spines and 8 soft rays in adults; usually 2 spines and 9 soft rays in juveniles about 30 mm or less in standard length in both species.

Although the number of transverse, circumpeduncular, and longitudinal scales overlap between the two species, it is possible to distinguish M. cephalus captured in the Gulf of Mexico from M. liza (according to Menezes et al., 2010) using scale counts. Figures 4 to 6 show that, in general, M. cephalus has a higher number of scales that M. liza ; M. cephalus has between 12 and 15 transverse scales with a mode set at 13, while the number of these scales in M. liza ranges from 11 to 14. The number of scales on the circumpeduncular series of the flathead mullet goes from 18 to 23 with a mode at 20, while in the longitudinal series counts range from 38 to 43, with a mode of 40. In M. liza , scales in the circumpeduncular series range from 16 to 20 and in the longitudinal series from 32 to 39. There is almost no overlap between the number of scales in the longitudinal series of both species; only specimens of M. liza with 39 scales could be confused with M. cephalus .

In the species identification keys of Thomson (1997) and Harrison (2003) the criterion to differentiate between M. cephalus and M. liza is the number of scales in the longitudinal series, being 34 or fewer for M. liza ( Thomson, 1997) and greater than 34 for M. cephalus (Harrison, 2003) . Misidentification of these two species may be due to the fact that among the mullets of the Western Atlantic, these are the largest and attain similar sizes: M. cephalus reaches 120 cm and M. liza reaches 100 cm.

The number of transverse, circumpeduncular, and longitudinal scales of M. cephalus from the Mediterranean coincide with specimens from the Gulf of Mexico ( Menezes et al., 2010). The exception is the upper limit of the range of the number of longitudinal scales, which is greater in individuals caught in the Mediterranean, reaching 38 to 46 ( Figures 4 to 6).

The number of gill rakers is similar between these two mullets. Within the same length range, the number of gill rakers of M. cephalus varies from 50 to 90 while for M. liza counts fall between 53 and 81. The relationship between the number of gill rakers and standard length was linear, which differs from Menezes et al. (2010) because in this study small specimens of sizes between 10 and 100 mm, which have fewer gill rakers ( Figure 7 ), were analyzed.

Harrison (2003) illustrates the presence of M. cephalus in the Gulf of Mexico, which is supported by data presented here; Álvarez-Lajonchere (personal communication) does not accept that its distribution range reaches the Caribbean Sea. The presence of M. liza in the Caribbean has been shown by Álvarez-Lajonchere (1978a; 1978b), and its presence is confirmed in the Gulf of Mexico with specimens catalogued as CNPE-IBUNAM 757 captured in the coastal waters of Veracruz, Mexico, as well as other evidence documented by Castro-Aguirre et al. (1999).

The meristic and morphometric measures of M. liza from the Gulf of Mexico are contained within the ranges showed by Menezes et al. (2010). No comparison of Mexican samples was included in figures and tables here since only seven specimens were reviewed in the Gulf of Mexico and this could bias the information.

Variation in the number of scales of Mugil cephalus in the Gulf of Mexico

Morphometric measurements were not significantly in different individuals from different sites along the coast of the Gulf of Mexico. However, the number of scales of the transverse and longitudinal series (p <0.05) was significantly different among sites. Although the median number of scales in the longitudinal series is identical in six of the seven sites, there is a pattern that emerges considering specimens obtained from sites in Florida (FL), Alabama (AL), and Mississippi (MS) and those caught in Louisiana (LA), Texas (TX), and Veracruz (VE). Differences seem to be determined geographically by the coastal areas east and west of the Mississippi River. The number of scales in the longitudinal series of specimens caught in Yucatan (YU) is different from the others ( Figure 8 ).

The number of scales in the transverse series shows differences in individuals obtained east and west of the Mississippi River. The most frequent number of scales in the transverse series for individuals from locations in Louisiana, Mississippi, Alabama, and Florida was 13 scales, found in 62.8 to 79.2% of the specimens analyzed. In the southern areas, such as Texas, Tamaulipas, and Veracruz, 14 transverse scales were counted in 77.2%, 91.1%, and 69.5% of analyzed specimens. In individuals from Yucatán, 13 transverse scales were counted more frequently ( Table 2 ).

According to our data, a pattern of variation exists in individuals obtained east and west of the mouth of the Mississippi. Those from Yucatan have unique characteristics relative from specimens obtained in other Gulf sites particularly when considering the number of scales in the longitudinal series. A microsatellite study conducted by Colin (2014) suggests that there is a trend in the genetic differentiation that supports the hypothesis that, within the Gulf of Mexico, this species is structured with at least three subgroups: the Mississippi, the Caribbean, and the Gulf, possibly separated by the prevailing circulation pattern. Zavala-Hidalgo et al. (2003) noted that there is a seasonal current from Louisiana and Texas towards Tamaulipas and Veracruz, which is well established from September to March; a reverse direction is observed from May to August. Different counts in the scales both of the longitudinal and transverse series were found in individuals from these two areas.

In conclusion, morphometric and meristic results are evidence of the presence of M. cephalus in the Gulf of Mexico and clarify doubts on its distribution in the northwest Atlantic. The sympatric presence of M. liza is confirmed although its abundance seems to be low.

Figures

FIGURE 2. Mugil cephalus, Madre Lagoon, Tamaulipas, Mexico 376 mm total length.

FIGURE 4. Comparative plots of number of transverse scale rows among Mugil cephalus from the Gulf of Mexico, Mugil cephalus and Mugil liza according to Menezes et al. (2010).

FIGURE 7. Regression of number of gill rakers vs. standard length for Mugil cephalus specimens from the Gulf of Mexico.

FIGURE 8. Box plot of the number of longitudinal scale rows of Mugil cephalus of the Gulf of Mexico. Code: Florida (FL); Alabama (AL); Mississippi (MS); Louisiana (LA); Texas (TX), Veracruz (VE); Yucatán (YU).

FIGURE 3. a) Mugil cephalus showing preorbital extending beyond corner of mouth and size of the scales. b and c) M. liza preorbital position and bigger size of the scales.

Tables

TABLE 1. Morphometrics of Mugil liza and Mugil cephalus. Length expressed in mm; measurements through head length are percentage of standard length; last seven entries are percentages of head length. Data for M. liza were taken from Menezes et al. (2010).

		Mugil liza		Mugil cephalus
Characters	n	Range Mean	SD	n Range Mean	SD
Standard length	155	24.0–412.0 179.3		341 24.0–412.0 138.1	101.0
Body depth	132	22.0–32.5 26.0	2.8	341 18.8–27.2 25.8	3.0
Snout to dorsal-fin origin	156	70.0–79.0 74.7	1.6	341 45.8–52.2 50.9	2.0
Snout to pectoral-fin origin	156	25.3–32.7 28.2	1.9	341 20.8–44.8 29.4	2.7
Snout to pelvic-fin origin	156	35.2–45.3 40.6	2.1	341 33.3–40.0 41.7	3.0
Snout to anal-fin origin	156	69.0–74.7 72.5	1.4	341 56.1–83.3 71.2	2.8
Caudal peduncle depth	156	9.0–11.8 10.6	0.5	341 6.0–17.9 10.3	1.1
Pectoral-fin length	155	16.4–21.7 18.5	1.1	341 8.3–20.7 17.4	2.1
Pelvic-fin length	155	13.6–19.1 16.4	1.0	341 8.0–20.0 14.5	1.6
Head length	155	24.0–30.8 27.3	1.9	341 20.8–35.8 27.7	2.4
Head width	155	60.0–71.7 66.6	2.5	341 28.6–81.3 62.2	7.4
Head depth	155	60.0-70.0 64.8	2.8	Data not available
Horizontal orbital diameter	153	16.5–26.6 22.0	1.9	341 10.3–50.0 24.7	4.1
Least interorbital width	155	20.0–25.5 22.3	1.3	341 9.2–40.0 24.7	3.7
Snout length	155	32.5–38.8 35.7	1.6	341 14.3–52.2 36.9	5.2
Upper jaw width	155	27.0–32.0 29.6	1.1	341 10.0–60.0 30.6	4.8
Mouth width	155	20.0–25.0 23.4	1.0	292 16.7–33.0 26.4	3.9

TABLE 2. Percentage frequency of transverse scale number for Mugil cephalus from the Gulf of Mexico. Higher values in bold.

			Transverse scale rows
Sample zone	N	12	13 14	15
Florida, USA	164	9.1	62.9 25.0	3.0
Alabama, USA	24	20.8	79.2 0.0	0.0
Mississippi, USA	27	37.0	63.0 0.0	0.0
Louisiana, USA	21	19.0	76.2 4.8	0.0
Texas, USA	114	3.5	16.7 77.2	2.6
Tamaulipas, MEX	56	0.0	0.0 91.1	8.9
Veracruz, MEX	82	2.4	24.4 69.5	3.7
Yucatán, MEX	15	0.0	80.0 20.0	0.0