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, TOMIYAMA (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 escutenta, Sepia subaculeata and Sepielta 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 escutenta has a length of 3.59 to 5.04mm (4.33mm 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 subaculeata, 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 35mm.
3) The shell of just-hatched Sepiella maindroni is 3.04-3.31mm long and 1.66-1.79mm wide with 7-9 striped lines. No spine is found at the rear end of the shell. Its morphologi-cal 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 20mm for all of the three species.
5) The locular index of the shell for the artificially reared individuals is 46-23 for Sepia esculenta (shell length 4-90mm), 41-25 for Sepia subaculeata (shell length 5-155mm), and 66-44 for Sepiella maindroni (shell lengtn 3-105mm). The index is greater in the youngling stage and decreases with the growtn.
6) The shell locular index for Sepia esculenta varies a great deal with the growing condition. The index is large for the well-grown (42.3-37.9) and small for the undergrown (36.9-34.3).
7) Though the locular index falls within a certain domain peculiar to each of the species and thus forms a species character, it is altogether a variable and, therefore, it involves a risk to judge species or presume the existence of local types just out of the index values.