On four species of the gobiid fishes of the genus Eleotris found in Japan

Abstract

There are four species of Eleotris found in Japan: E. oxycsephala TEMMINCK & SCHLEGEL, E. melanosoma BLEEKER, E. acanthopomus BLEEKER and E. fusca BLOCH & SCHNEIDER. Specimens of these four species and E. mauritianus BENNETT and E. sandwicensis VAILLANT & SALTVAGE which resemble them were compared in detail. The specimens counted and observed in this study are listed in table 1. Besides these, BLEEKER'S specimens from Rijksmuseum van Natuurli jke Historie were examined, and they are listed in table 14 with the results of the observation dealt with separately from the results of the observation in this study.
The fin counts and the characteristics of scales which show certain degrees of difference are shown in tables 2-9.
The characteristics of the gill rakers are clearly distinguishable among the species. The shapes of the outer gill rakers on the 1st arch of E. oxycephala are very distinct, being short pedestals covered with spines (fig. 14), while the outer gill rakers of others are rods with spines on their inner side (fig. 15). The number of outer gill rakers, the ratio between the length of the portion without gill rakers in the ceratobranchial against its length and the position of outer gill rakers in relation to inner gill rakers are different among the species (figs. 16-17, table 10-12). Together with these, the sizes of the membranes between the 1st gill arch and the pharynx are also different among them (figs. 18-21). The difference between E. melanosoma and E. acanthopomus which is not distinguishable on the scale counts is very conspicuous here. The patterns of pit organs on the head are the most distinguishable characteristics (figs. 1-12 and 22-25). The pit organs of E. oxycephala are very different from those of other species, having longitudinal rows of pit organs from the chin to the lower margin of the preopercle (fig. 9), instead of having transverse lines (fig. 10). And also E. oxycephala has a longitudinal line of pit organs immediately behind the eye (line 13 in fig. 5), instead of a transverse line (line 8 in figs. 6 and 7, and line 10 in fig. 8). E. melanosoma differs from the others in having more prominent lines of pit organs (fig. 23). The characteristic patterns of pit organs in the infraocular region of each species are shown in figures 5-8. In E. oxycephala, lines 3, 4 and 5 do not cross line 11 (fig. 5); in E. melanosoma, only line 5 does not cross line 13 (fig. 6); in E. acanthopomus, E. mauritianus and E. sandwicensis line 3 and 5 do not cross line 13 (fig. 7); in E.. fusca lines 3, 5 and 7 do not cross line 15 (fig. 8). Anomalous arrangements of pit organs are not found on both sides of any specimen examined except on a specimen of E. fusca which has seven lines instead of eight. The transverse lines of pit organs along the preopercle differ, too, among the species as shown in table 13. Pit organs on the opercle are of two types (figs. 11-12): the one in which line 1 and line 2 do, not meet is that of E. oxycephala, E. acanthopmus, E. mauritianus and E. sandwicensis; the other in which line 1 and line 2 meet and line 2 does not cross line 1 is that of E. melanosoma and E. fusca. The anal papilla of the female of E. oxycephala is also different from that of the others in that it has fringes on the posteriormargin (fig. 26), while the others have no fringes (fig. 27). E. oxycephala is different from the others in coloration, too, having two dark lines backward and downward from the eye, while the others have three, although it is often difficult to observe these lines. In most of the specimens of E. melanosoma these lines could not be seen. The clear difference in the characteristics of gill rakers and pit organs separating E. mauritianus and E. sandwicensis