The Shell and the Locular Index of the Cuttle-Fishes, Sepia Esculenta Hoyle, Sepia Subaculeata Sasaki and Sepiella Maindroni De Rochebrune

Abstract

The locular index of the shell, which was introduced by HOYLE, W,E. (1886), is considered to be one of the most significant species diagnoses for the cuttle-fish. As for Sepia esculenta produced in Japan, Sasaki (1910, 1929) observed two different groups of it with different locular indexes. Later, Yamamoto (1945), Yasuda (1951) and Koito et al. (1956) infer, on the basis of difference in the locular index, the existence of local types of it. Hereupon, ToMI-YAMA (1957), ISHIKAWA and IWAI (1958) and Yagi (1960) made clear after more detailed examination on the form and growth of the shell that the locular index changes continuously with the growth of the cuttle-fish. The present author executed the hatching and growing of Sepia esculenta, Sepia suba-culeata and Sepiella maindroni, while studying on the form and locular index of the shell as against the growth. The results thus obtained are: 1) The shell of just-hatched Sepia esculenta has a length of 3.59 to 5.04 mm (4.33 mm in average), a width of 2.62 to 3.38mm (2.92mm in average) and 6 to 8 striped lines (7.0 lines in average). Characteristic of the shell is, as is compared with that of Sepia subacu-leata, the meek-developed inner cone with a circular trimming. The last loculus is found protruding at the stage of a shell length 6.6 to 7.1mm. No spine is found at the rear end of the shell immediately after the hatching; they are formed at the stage of a shell length 5.1 to 5.5mm. 2) Sepia subaculeata immediately after the hatching has a shell, 5.11 to 7.59mm long and 3.76 to 4.28mm wide, with 7 to 11 striped lines (8.1 lines in average). Compared with Sepia esculenta, the shell of this species has a much more developed inner cone with an acuteangled trimming. The spine at the rear end of the shell is recognizable even immediately after the hatching, and the last loculus appears at the stage of a shell length 30 to 35 mm. 3) The shell of just-hatched Sepiella maindroni is 3.04~3.31 mm long and 1.66~1.79mm wide with 7~9 striped lines. No spine is found at the rear end of the shell. Its morphological characteristic such as the well developed last loculus constitutes an easier distinction from Sepia esculenta or Sepia subaculeata. 4) The allometry expression applies well to the relation between shell-length and shell-width as well as to the relation between shell-lenth and shell-weight. On the curve of the relation of shell-length versus shell-weight, however, a slight inflection is observed at the point corresponding to the shell length of about 20 mm for all of the three species. © 1962, The Japanese Society of Fisheries Science. All rights reserved.