Marine Biology (1995) 122:355-360 9 Springer-Verlag 1995 R. V. Arellano 9 O. Hamerlynck 9 M. Vincx 9 J. Mees K. Hostens 9 W. Gijselinck Changes in the ratio of the sulcus acusticus area to the sagitta area of Pomatoschistus minutus and P. Iozanoi (Pisces, Gobiidae) Received: 29 November 1994 / Accepted: 11 January 1995 Abstract The Pomatoschistus minutus (Pallas) and P. lo- zanoi (de Buen) stock of the ebb-tidal delta of the former Grevelingen estuary was sampled monthly from Septem- ber 1992 to October 1993. Sagittae were extracted and the areas of the sagitta and sulcus acusticus were measured with a digitizer. The frequency response and auditory threshold of these two sympatric goby species were deter- mined from the ratio of the sulcus acusticus area to the sa- gitta area (S:O ratio). The S:O ratio ofP. minutus increased while in P. lozanoi the S:O ratio was constant with increas- ing standard length. The average S:O ratio of P. lozanoi was significantly higher than that of P. minutus of the same mean length, which makes the former more sensitive to sound frequency. The S:O ratios of these two sympatric fishes were lower than the S:O ratios of demersal and pe- lagic fishes available from the literature. An interspecific growth difference of sagitta and sulcus acusticus was ob- served. The sagitta and sulcus acusticus of P. minutus are larger than those of P. lozanoi of the same length. Introduction Otoliths are useful sources of information on age (Cam- pana and Neilson 1985; Gauldie and Nelson 1990; Jones 1992), daily growth (Pannella 1980; Campana and Neil- son 1985), biological records of environmental changes (Pannella 19~0; Radtke 1984), equilibrium and accelera- tion (Stevenson and Campana 1992), and sound transduc- Communicated by O. Kinne, Oldendorf/Luhe R. V. Arellano ([]) 9 O. Hamerlynck 9 M. Vincx - J. Mees K. Hostens 9 W. Gijselinck Marine Biology Section, Zoology Institute, University of Gent, K.L. Ledegankstraat 35, B-9000 Gent, Belgium O. Hamerlynck - J. Mees - K. Hostens Centre for Estuarine and Coastal Ecology, Vierstraat 28, 4401 EA Yerseke, The Netherlands R. V. Arellano Cagayan State University, Aparri, Cagayan, Philippines tion of teleost fishes (Parker 1908; Morris and Kittleman 1967; Popper 1977; Fay 1980; Schuijf 1981; Fay and Pop- per 1985; Gauldie 1988). They are also used for stock iden- tification (Parrish and Sharman 1959; Jones 1992), as a taxonomic tool in fossil fish species (Gaemers 1984). The sagittae, one of the three pairs of otoliths, which are located in the saccular vestibule of the pars inferior of the vestibular apparatus (Fay and Olsho 1979; Secor et al. 1991), are peripheral mechanoreceptors (Schuij f 1981, Se- cor et al. 1991). The sagitta acts as a transmitter of me- chanical stimuli to the cilia of the macula inserted in the sulcus acusticus (Morris and Kittleman 1967). Gauldie (1988) postulated that the ratio of the macula area to the otolith area (M:O ratio), and subsequently the ratio of the sulcus acusticus area to the otolith area (S:O ratio), are re- lated to the frequency response and auditory threshold of the sagitta (see Table 1). The aim of the present paper is to determine whether there are interspecific and ontogenic differences in the fre- quency response and auditory threshold of two sympatric gobies Pomatoschistus minutus and P. lozanoi. The S:O ra- tios of these species were compared to the S:O ratios of other species available from the literature. Materials and methods The right and left sagittae were extracted from Pomatoschistus min- utus and P. lozanoi collected on a more or ]ess monthly basis from the ebb-tidal delta of the Grevelingen area (southwest Netherlands) from September 1992 to October 1993. The standard length (SL: from the tip of the lower jaw to the end of the notochord) of the pre- served fish were measured to the nearest mm and converted to fresh SL (Arellano unpublished data). The areas of the right and left sagittae and the sulcus acusticus were drawn with the aid of a drawing tube and a binocular micro- scope at 50 x magnification. The sagittae were air-dried to facilitate distinguishing the sulcus acusticus area (Fig. 1). The two areas, expressed in mm 2, were measured with a digitizer (Hipad Plus, Tbasic). The presence of extreme-outliers was detected by means of a case- wise plot of residuals. Observations falling outside the mean _+ 3 x SD were considered as outliers. Log-transformation of the data, e.g. pow-