Macroramphosus

Murat Bilecenoglu, 2006, Status of the genus Macroramphosus (Syngnathiformes: Centriscidae) in the eastern Mediterranean Sea., Zootaxa 1273, pp. 55-64 : 60-62

[[ Genus Macroramphosus ]]

Discussion

A total of 16 proportions of morphometric measurements were analyzed in this study. Five parameters (SnL/SL, HL/SL, PBL/SL, SnL/HL, MBD/LBDS) were similar between the two species, while a statistically significant difference was found for the remaining 11 characters (Table 1).

Eye diameter measurements and its proportion in relation to HL, D1-D2 and PHL gave significant results. The most striking ratios were found to be as ED/D1-D2 (eye diameter greater than the length between dorsal fins in M. scolopax versus smaller in M. gracilis ) and ED/PHL (postocular head length smaller than the eye diameter in M. gracilis , larger in M. scolopax ). These ratios were previously used in identification keys to Mediterranean centriscid species by Bauchot & Pras (1980) and Tortonese (1970), respectively. The mean ED/HL indicates a smaller eye diameter for M. gracilis (16.74%), when compared to M. scolopax (19.04%).

In several previous studies, length of dorsal spine (LDS) was considered as a very important character to discriminate Macroramphosus spp. Our findings revealed a slight overlap in the range values of LDS/SL and LDS/PBL; however, their mean values indicate a clear difference between the two species, where M. gracilis has a shorter dorsal spine when compared to M. scolopax . Similar results were obtained from studies conducted in the Atlantic and Pacific Oceans (Brethes, 1979; Assis, 1992; Mathiessen et al., 2003; Miyazaki et al., 2004). The eastern Mediterranean specimens of M. gracilis had conspicuously weaker dorsal spines, which does not reach to caudal base, as also noted by Günther (1889) and Smith (1965).

Maximum body depths of the two species differ significantly. MBD/SL and MBD/ PBL ratios were smaller for M. gracilis , as a result of the slender body shape. Similar results were obtained by Brethes (1979), Assis (1992) and Miyazaki et al. (2004). We determined that body depth is found 3.71-4.60 times in SL of M. scolopax , and 4.53-5.39 times in M. gracilis , in agreement with the findings of Tortonese (1970).

Apart from metric measurements, some other distinctive morphological characters were determined between the two species. The occipital crest located at the anterior part of eye bears prominent spinules in two or three rows in M. scolopax , but no spinule was observed on the occipital crest of M. gracilis (Figure 3). D’Ancona (1933) reported that, both supraorbital and occipital crests are visible in specimens with 3 mm length, becoming conspicuous in fish larger than 9 mm. Thus, a change in occipital crest structure through developmental stages between juveniles and adults is not expected, and this character can be used in distinguishing the two centriscid species.

The relatively straight ventral body profile of M. gracilis is another distinctive feature, which was also mentioned in previous studies (D’Ancona, 1933; Miyazaki et al., 2004). The well-developed elevated scutes between the pelvic and anal fins of M. scolopax are conspicuous, while in M. gracilis scutes are either absent or represented by a single weakly developed scute (Figure 4). This character was stated to be diagnostic for Japanese Macroramphosus spp. (Miyazaki et al., 2004), which seems to be also valid for Mediterranean species.

The number of spinules along the posterior margin of D2 spine of M. gracilis tends to be lower than M. scolopax (0-14 vs 14-23 spinules). The less denticulated dorsal spine structure of M. gracilis was also indicated by Günther (1889) and Tortonese (1970). Miyazaki et al. (2004) examined dorsal spine denticulation based on Japanese coast centriscids, where a degeneration of spinules by 50 mm SL was noted for M. gracilis - which is not the case in Mediterranean specimens. Since both centriscid species are known to be sympatric (i.e. Assis, 1992), the assumption indicated by Matthiessen et al. (2004) that ecological differences in diet composition and habitat use of sympatric species promotes morphological diversification should be concerned.

Macroramphosus scolopax and M. gracilis are two morphologically similar species with some overlapping metric and meristic measurements, and no single character provides precise (Assis, 1992; Mathiessen et al., 2003). Based on the results of our study, the following combination of characters should be used in distinguishing Mediterranean specimens of M. gracilis from M. scolopax : first dorsal fin originating before anus; second spine of first dorsal not extending beyond second dorsal fin base; length between two dorsal fins greater than the eye diameter; ventral scutes not protruding; less developed projecting scute between pelvic and anal fins.