Morphology of the Saccular Otoliths of Six Species of Lanternfishes of the Genus Symbolophorus (Pisces: Myctophidae)

BULLETIN OF MARINE SCIENCE, 52(3): 949-960, 1993

MORPHOLOGY OF THE SACCULAR OTOLITHS OF SIX SPECIES OF LANTERNFISHES OF THE GENUS SYMBOLOPHORUS (PISCES: MYCTOPHIDAE)

F. Javier Gaga

ABSTRACT Variation in the morphology of saccular otoliths (sagittae) among six species oflantemfishes of the genus Symbolophorus is analyzed, A cluster analysis based on a dichotomous matrix of 10 morphological characters separates the 6 species into well-defined groups, Whereas the otoliths of S. evermanni and S. rulmus are indistinguishable, those from males of the undescribed "reversus" morphotype are easily discriminated from them. This suggests that "reversus" might be a valid species, The sagittae of S. veranyi and S, boops form a second group separated from S. calzforniensis. This study also points out the problems and possi- bilities of using otolith morphology in helping to resolve the problems within this genus.

The genus Symbolophorus (Bolin and Wisner, in Bolin, 1959) is defined by the following photophore patterns: a markedly angu1ate SAO series, one Pol, two Pre's (the last slightly elevated), five PO's, and four leveled VO's (Wisner, 1976). This genus is composed of about 10 nominal species of worldwide distribution (Becker, 1983; Hulley, 1981). Of these 10 species, 7 to 8 are variably recognized by different authors. The taxonomy of the genus remains unresolved partially because of the overlap in meristic counts and similarities in photophore patterns among species. However, several authors have divided the genus Symbolophorus into several well defined groups. Hulley (1981) divided the genus into two groups according to several morphological characters including the position of the Pol photophore relative to the adipose fin. He included S. californiensis, S. evermanni, and S. rufmus in a group characterized by having a Pol that is situated below the base of the adipose fin and a PLO that is closer to the lateral line than to the base of the pectoral fin. However, S. californiensis should not be included in Hulley's first group since it has a Pol situated in advance of the adipose fin base and a PLO that is equidistant to the lateral line and to the pectoral fin. Both of these two characters may warrant the inclusion of S. californiensis into a separate third group as indicated by Wisner (1976). However, Wisner (1976) partially based his S. californiensis group on the presence of a narrow band of small, villiform, palatine teeth, a character that is also shared by S. evermanni. Wisner (1976) recognized two other groups also present in the eastern Pacific. His S. evermanni group included the valid species S. evermanni plus two undescribed morphotypes. In addition, he characterized his S. boops group as "a group of poorly understood species found only in southern waters." Furthermore, according to their distribution, Becker (1983) divided the genus into three major groups: the tropical species S. evermanni, S. kreffti, and S. rufinus; the moderately cold water northern species S. californiensis and S. veranyi; and those species inhabiting the moderately warm subtropical and temperate southern waters S. barnardi and S. boops. Whereas the species groups mentioned above are easily defined, the problem arises when trying to establish the number of valid species within each group. My original objective was to identify possible diagnostic characters for one of the undescribed species within this group: the "reversus" morpho type of Wisner (1976). The type species S. evermanni is characterized by males with a supracaudal gland made up of chevron-shaped luminous scales that have their concave margins pointing cranially. The "reversus" morphotype in which the concavity faces cau-

949 950 BULLETIN OF MARINE SCIENCE, VOL. 52, NO.3, 1993

DORSAL

POSTERIOR

SULCUS

VENTRAL Figure 1. Generalized diagram of the medial face of a left sagitta showing the major morphological landmarks (modified after Chaine and Duvergier, 1934). dally is very common in the eastern Pacific along the continental margin, but specimens are occasionally found in the central Pacific within the area occupied by S. evermanni. Adult males ofthis species are easily recognized by the "reverse" condition of their supracaudal gland scales, but the females and immature individuals present a problem since their meristic, morphometric, and external morphological characters seem to be indistinguishable from those of the other species of the S. evermanni group. When the saccular otoliths of the "reversus" morphotype are compared with those of other Symbolophorus species it is apparent that not only do the otoliths allow for the further discrimination of the "reversus" males from the S, evermanni group, but that they offer a set of reliable characters for separating the species of Symbolophorus into well-defined groups. This study analyzes the variation in saccular otolith morphology among the following species of the genus Symbolophorus: S. boops (Richardson, 1845), S. californiensis (Ei- genmann and Eigenmann, 1889), S. evermanni (Gilbert, 1905), S. rufinus (Taning, 1928), S. veranyi (Moreau, 1888), and the "reversus" morphotype of Wisner (1976). Further, the importance of using otolith morphology as a possible pool of character features that might help to resolve the taxonomic problems of this genus is presented.

MATERIALS AND METHODS

The series of 236 saccular (sagittae) otoliths used in this study were obtained from the John Fitch otolith collection at the Natural History Museum of Los Angeles County. The terminology for the morphological characters on the medial face of the saccular otoliths (Fig. 1) follows Chaine and Duvergier (1934). The otoliths were lightly rubbed with graphite for enhancement of their morphological details. They were examined under a dissecting microscope, and scanning electron microscopy was performed on representative specimens. The length and depth of 109 right sagittae were measured using an ocular micrometer. The standard lengths of the original specimens ranged from: S. hoops (""45-103 mm), S. californiensis (""35-97 mm), S. evermanni (39-80 mm), S. rufinus (24-89 mm), S. veranyi (35-121 mm), and "reversus" (48-75 mm). The geographic locations of the samples extend from: S. hoops (southern Pacific and Atlantic, 35°-60"S), S. californiensis (eastern north Pacific, CaI- GAGO: SYMBOLOPHORUS OTOLITH MORPHOLOGY 951

ifornia Current), S. evermanni (off Japan, New Caledonia, and equatorial central-north Pacific), S. rufinus (central-north Atlantic), S. veranyi (eastern north Atlantic), and "reversus" (off Hawaii, equatorial north Pacific and tropical western north Pacific). The length of the otolith is here defined as the greatest distance between the posterior (caudal) and anterior (cranial) margins ofthe otolith. The depth is considered as the largest measurement between the dorsal and ventral margins. Model II regression analysis has been suggested in cases where both variables are subject to error (Sokal and Rohlf, 1981). However, Laws and Archie (1981) indicated that when the correlation coefficient between the two variables is high the results obtained from the application of a Model I analysis are little different from those obtained when a Model II regression is used. The otolith length: depth data were analyzed by using simple linear (Model I) regressions since preliminary analyses showed that the regression coefficients (r) of each species ranged from 0.912 in "reversus" to 0.995 in S. rufinus (Table I). Interspecific differences were estimated by comparing the slopes and elevations of each species length: depth regression equations. Tests of homogeneity of slopes and elevations, and Tukey type multiple comparisons' tests were performed according to Zar (1984). All conclusions derived from the statistical analyses were based on a significance level (a) of 0.05. A dendogram of species groups based on 10 otolith characters was developed using a cluster analysis procedure from a microcomputer statistical package (SYST AT 5.0). A correlation matrix based on dichotomous dissimilarity coefficients was developed and a dendogram fitted using hierarchical clustering by averaging all distances between taxa in different groups. The cluster analysis was used to analyze levels of overall similarity of otolith morphology and is not intended to show phylogenetic relationships since the value of auditory structures as non-convergent morphological characters is doubtful (Marshall, 1971; Popper and Coombs, 1982).

RESULTS Regression analyses indicate a gradual decrease in the slope of the length: depth regression lines, with both S. rufinus and S. evermanni having the largest slopes and "reversus," S. californiensis, S. veranyi, and S. boops the smallest in that order (Fig. 2, Table 1).The slopes of the length: depth regression lines are arranged in decreasing order and a continuous line drawn under those slopes that are not statistically different at the 0.05 significance level (Table 1). The multiple comparison Tukey test of the slopes among the different lines shows three groups of species where there is no statistically significant difference of the slopes within each group (Table I). The first group includes S. evermanni, S. rufinus, and the "reversus" morphotype. An intermediate group composed of S. evermanni, "reversus", and'S. californiensis overlaps with both the first group and a group composed of "reversus," S. californiensis, S. veranyi, and S. boops. Whereas the multiple comparison analysis of the slopes shows no disjunction between the groups, the same Tukey type analysis among the elevations separates the group formed by S. evermanni and S. rufinus from the rest of the species studied (Table 1, P < 0.001). In addition to the analysis of the otolith length: depth relationship, a study of the medial face morphology (Fig. 1) provided data for the construction of a table of variation in 10 characters among the species of Symbolophorus studied (Table 2). The shape of the otoliths varies from circular to elongated on an anterior- posterior axis (the two groups separated by the elevations of their regression lines; Figs. 2, 3, Table 1). Otoliths vary along their medial-lateral axis from very thin and delicate, such as those of S. californiensis, to very thick and robust like those of S. evermanni, S. rufinus, and "reversus." The latter three characterized by the presence ofa well-developed lateral dome (Fig. 4). This character is also illustrated in the otolith ventral view of S. reversus by Schwarzhans (1980). All otoliths have well-developed rostra and antirostra except for those of S. evermanni and S. rufinus, which have no antirostrum or a poorly delineated excissura (Fig. 3A, B). The illustrations of Schwarzhans (1980) show the presence of better developed rostra and antirostra in the otoliths of S. evermanni and S. rufinus. In this study, only 3 out of the 14 otoliths of S. evermanni and 2 out of the 19 otoliths of S. 952 BULLETIN OF MARINE SCIENCE, VOL. 52, NO.3, 1993

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Figure 2. Regression analysis of otolith length and depth of the different species of Symbolophorus used: S. boops (black diamonds), S. californiensis (black triangles), S. evermanni (clear squares), S. rufinus (black squares), S. veranyi (clear diamonds), "reversus" (clear triangles). rufinus showed such condition. However, in these few cases the antirostra were never as prominent as in the other species studied. A postcaudal trough is absent in all species studied, except in S. californiensis, which are usually characterized by the presence of a small groove that connects the cauda with the posterior margin (Fig. 3F). The crista superior (dorsal crista) varies from being very prominent, particularly at the caudal end (Fig. 3A-C), to underdeveloped (Fig. 3D-F). The posterior collicula of S. californiensis, S. evermanni, "reversus," and S. rufinus are more or less square in shape (Fig. 3A-C, F) when compared to the rectangular shapes in the species S. boops, and S. veranyi (Fig. 3D, E). The sulcus can be 954 BULLETIN OF MARINE SCIENCE. VOL. 52. NO.3, 1993

Figure 3. Scanning electron micrographs of the medial faces of the left sagittae of Symbolophorus spp.; black scale bar = 2 mm; (A) S. evermanni (67 mm SL), (B) S. rufinus (56 mm SL), (C) "reversus" morphotype (75 mm SL), (D) S. veranyi (102 mm SL), (E) S. boops (103 mm SL), (F) S. californiensis (97 mm SL). either relatively straight (Fig. 3D-F), or slightly curved with the cauda bent dorsally (Fig. 3A, B). Most ofthe otoliths of the Hreversus" morphotype show curved sulci such as those of S. evermanni and S. rufinus. However, a few (3 out of 38 in this study) tend to have straight sulci (Fig. 3C). All of the otoliths studied were ornamented both ventrally and dorsally, but only those of S. veranyi, S. boops and S. californiensis had ornamented posterior margins. In addition, the posterior margin of the otoliths in all the species studied is rounded or slightly truncated, except in that of S. boops which is pointed (Fig. 3E). A cluster analysis based on a matrix of dichotomous dissimilarity coefficients derived from the above otolith characters resulted in the dendogram of Figure 5. Two groups can be separated at the -0.27 dissimilarity level; as. evermanni-S. GAGO: SYMBOLOPHORUS OTOLITH MORPHOLOGY 955

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' .. " .... " ~ ~~ Figure 4. Ventral view drawings of the left sagittae of Symbolophorus spp. (lateral face towards the bottom of the page); scale bar = 2 mm; (A) S. evermanni (67 mm SL), (B) S. rufinus (76 mm SL), (C) "reversus" morphotype (66 mm SL), (D) S. veranyi (94 mm SL), (E) S. boops (91 mm SL), (F) S. californiensis (95 mm SL). rufinus-"reversus" group, and a S boops-S. veranyi-S californiensis group. S evermanni and S. rufinus are extremely similar and cannot be discriminated based on otolith morphology. However, the "reversus" morphotype is discriminated from S. evermanni and S. rufinus at a dissimilarity coefficient of -0.70. Within

Dissimilarity Coefficient

-1.0 -0.9 -0.8 .fJ.7 ·0.6 ·0.5 -0.4 .fJ.3 .fJ.2 ·0.1 0.0

S. veranyi

s. boops

S. californiensis

"reversus"

S. evermanni

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Figure 5. Cluster analysis dendogram based on a matrix of dichotomous dissimilarity coefficients of 10 otolith characters using the average linkage method. GAGO: SYMBOLOPHORUS OTOLITH MORPHOLOGY 957 the second group S. boops and S. veranyi separate from S. californiensis with a -0.75 coefficient of dissimilarity.

DISCUSSION It has been demonstrated that interspecific variation in otolith morphology is substantial (Harkonen, 1986). Intraspecific variation in otolith morphology is dependent on the ontogenetic stage of the fish, among other factors. However, certain morphological features of the otoliths are laid down early in the ontogeny of the fish (Brothers, 1984), and are very conservative within species. Works such as those of Chaine and Duvergier (1934), Frizzell and Dante (1965), Harkonen (1986), and Nolf (1985, 1988), describe some of the landmark morphological features of the saccular otoliths that are helpful in taxonomic studies (Fig. 1). Among teleosts the saccular otoliths (sagittae) are the most widely used in comparative taxonomic studies because of their large size and degree of interspecific variation. In this study the cluster analysis based on the morphological characters of the sagittae separates two groups at the -0.27 level of dissimilarity. The Symbolo- phorus evermanni-S. rufinus-"reversus" group is characterized by having otoliths with a lateral dome, a prominent crista superior, a curved sulcus, and a posterior margin with no ornamentation (slight in some of the "reversus" specimens). The S. boops-S. veranyi-S. californiensis group is defined by sagittae with a flat lateral face, an underdeveloped crista superior, a straight sulcus, and an ornamented posterior margin. The S. evermanni-S. rufinus-"reversus" cluster is further divided into two groups at the -0.70 level of dissimilarity. The multiple comparison tests of both the slopes and the elevations of the length: depth regression lines failed to discriminate between the otoliths of S. rufinus and S. evermanni. In addition the morphology of the medial face of the otoliths of these two species seems to be identical. The otoliths of these two species are indistinguishable from each other and are characterized by having poorly defined rostra and antirostra, and a more rounded overall shape. Symbolophorus rufinus is found in the Atlantic Ocean between the 30° parallels and in the equatorial waters of the Indian and Pacific oceans where its distribution overlaps with that of S. evermanni (Becker, 1983). Nafpaktitis and Nafpaktitis (1969) enumerated three differences in photophore patterns and in the length of the pectoral fin that seemed reliable to discriminate between specimens of S. evermanni and S. rufinus from the Indian Ocean. However, they mentioned that "morphological differences between S. evermanni and S. rufinus are very subtle, and peripheral, and immature individuals of the two species may be easily con- fused." Furthermore, they mentioned the extensive overlap of their meristic and morphometric characters. The otolith analysis in this study also indicates the strong similarity between these two species. Whereas the morphology of the saccular otoliths of Symbolophorus evermanni and S. rufinus is very similar, the cluster analysis separates "reversus" from these two species (Fig. 5). The sagittae of "reversus" are different from those of S. evermanni and S. rufinus in having well-defined rostra and antirostra (reduced antirostra only in a few small specimens), and a more elongated overall shape. Although, the present study does not allow for the complete discrimination of "reversus" from S. evermanni, the otoliths do indicate that "reversus" might indeed be a valid species. The otolith collection data did not provide the sex of the specimens from which the otoliths were obtained. It can only be assumed in 958 BULLETIN OF MARINE SCIENCE, VOL. 52, NO.3, 1993 the case of the S. evermanni-"reversus" group specimens that the otoliths be- longed to males since the discrimination of the females is impossible at this time. A preliminary study comparing morphometrics and photophore patterns between specimens of S. evermanni and "reversus" was unsuccessful and more work needs to be done to identify diagnostic characters between the females of the two morphotypes. The sagittae of Symbolophorus boops and S. veranyi are grouped together at a dissimilarity coefficient of -0.90 (Fig. 5). While their otoliths are very similar, those of S. boops have a more pointed posterior (caudal) margin. S. californiensis is separated from these two species in the dendogram at a level of -0.75 (Fig. 5). The otoliths of S. californiensis are characterized by the presence of a postcaudal trough. The Symbolophorus boops-S. veranyi group, as defined by the cluster analysis in this study, agrees with that previously described by Hulley (1981), who used different morphological characters. However, the clustering of these two species must await further analysis including the species S. barnardi (Hming, 1932), S. hookeri (Whitley, 1953), and S. kreffii Hulley, 1981, which were not examined. The tight clustering of S. evermanni and S. rufinus supports previous obser- vations of the subtle morphological variation between these species (Nafpaktitis and Nafpaktitis, 1969). Furthermore, the cluster of S. evermanni-S. rufinus- "reversus" supports the groups of both Hulley (1981) and Wisner (1976). The exclusion of S. californiensis from the group composed of S. evermanni and S. rufinus in the work ofHulley (1981) is supported not only by photophore position data but also by the otolith data presented in this study. Otolith morphology suggests a closer relationship between S. californiensis and the S. veranyi-S. boops group than to the S. evermanni-S. rufinus-"reversus" group. However this is in disagreement with Wisner's (1976) study, which placed S. californiensis in a separate group. The cluster analysis in this study is not intended to show phylogenetic relationships among the taxa studied, but simply to diagram the level of overall similarity in otolith morphology among these species. The use of otolith morphological features as characters for the analysis of phylogenetic relationships is debatable. Some authors believe that the similarity in many features of the auditory system among different species of fishes might be due to convergent evolution (Marshall, 1971; Popper and Coombs, 1982). However, no conclusive evidence on the convergent nature of otolith morphology has been proposed. Following Hennig's (1966) Auxiliary Principle, in a cladistic analysis the deter- mination of character convergence is based on the a posteriori optimization of characters on a phylogenetic tree. Exclusion of otolith morphology from phylogenetic analysis is thus unwarranted. Combination of otolith morphology with other types of data to construct a large matrix of characters for cladistic analysis may give us stronger evidence about the convergent regularity of otolith morphological characters. The value of otoliths in ichthyological research, and spe- cially as a tool for the taxonomic diagnosis of taxa is well established (Brothers, 1984). The qualitative characters identified in this study for the cluster analysis are mostly based on sagittae from adult specimens. The presence or absence of some of these characters depends on the ontogenetic stage of the fish. Features such as the rostrum and antirostrum might not be yet developed in otoliths from small individuals. Furthermore, otoliths can be subject to erosion and deterioration if not properly curated. Thus, one must be aware of these possible sources of variation when using otolith characters such as those in Table 2. Most authors have GAGO:SYMBOLOPHORUS OTOLITHMORPHOLOGY 959 previously used length: depth ratios as taxonomic characters to discriminate among otoliths of different groups. However, the use of regression analysis (model I or II) may provide a better characterization of the relationship between such data. The use of otolith width (thickness) as a morphometric character is doubtful. Such a small measurement, increases the amount of error and might not be feasible to use with groups that have highly arched otoliths (Hiirkonen, 1986). However, the shape of the otolith in a medio-Iateral axis could be important in certain groups of fishes. The present study indicates some of the problems in the taxonomy of the genus Symbolophorus. Further analysis of the variation in otolith morphology among the species of Symbolophorus not covered in this study is necessary and may prove helpful in the much needed revision of this genus.

ACKNOWLEDGMENTS

I express my appreciation to S. L. Bower, J. E. Craddock, R. E. Feeney, C. Klepadlo, R. J. Lavenberg, G. E. McGowen and H. J. Walker, Jr. for their help in gathering data and their encouragement throughout the study. I also thank A. Thompson for her technical assistance with the electron microscopy of the otoliths. For their helpful discussions and comments on earlier drafts of the manuscript I thank J. E. Craddock, R. E. Feeney, R. J. Lavenberg, G. E. McGowen, and M. A. Neighbors. For their support I acknowledge: The Natural History Museum of Los Angeles County, and the Department of Biological Sciences, University of Southern California.

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of Captain Sir James Clark Ross, R.N" F,R.S. In the zoology of the voyage of the H. M. S. EREBUS and TERROR1839-43. 2: 1-139. Schwarzhans, W. 1980. Die tertiiire Teleosteer-Fauna Neuseelands, rekonstruiert anhand von Oto- lithen. Berliner geowiss. Abh. (A) 26: 1-211. Sokal, R. R. and F. J. Rohlf. 1981. Biometry. The principles and practice of statistics in biological research, 2nd ed. W. H. Freeman and Co, San Francisco. 776 pp. Hilling, A. V. 1928. Synopsis of scopelids in the North Atlantic. Vidensk. Medd. fra Dansk. Naturh. Foren. 86: 49-69. ---. 1932. Notes on scopelids from the Dana Co\lections I. Vidensk. Medd. fra Dansk Naturh. Foren. 94: 125-146. Whitley, G. P. 1953. Studies in ichthyology. No. 16. Rec. Aust. Mus. 23(3): 133-138. Wisner, R. L. 1976. The taxonomy and distribution of lanternfishes (family Myctophidae) of the eastern Pacific Ocean. Navy Ocean Research and Development Activity Rep. 3: 1-229. Zar, J. H. 1984. Biostatistical analysis, 2nd ed. Prentice-Hall, Englewood Cliffs, New Jersey. 718 pp.

DATEACCEPTED: July 29, 1992.

ADDRESS: Section of Fishes. Natural History Museum of Los Angeles County, 900 Exposition Blvd., Los Angeles, California 90007-4000.