TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 51 (No. 3), pp. 241-250, 2017

Stable carbon isotope compositions of foot tissue, conchiolin opercula, and organic matrix within the shells of two marine gastropods from a seagrass meadow in the Philippines

Makiko Ishikawa,1,2* Nanako O. Ogawa,3 Naohiko Ohkouchi,3 Daniel Edison M. Husana4 and Tomoki Kase5

1Faculty of Animal Health Technology, Yamazaki Gakuen University, 4-7-2 Minami-osawa, Hachiouji, Tokyo 192-0364, Japan
2Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
3Department of Biogeochemistry, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, Kanagawa 237-0061, Japan
4Institute of Biological Sciences, College of Arts and Sciences, University of the Philippines in Los Banos, College, Laguna, Philippines
5Department of Geology and Paleontology, National Museum of Nature and Science, 4-1-1 Amakubo, Tsukuba, Ibaraki 305-0005, Japan

(Received October 7, 2015; Accepted December 20, 2016)

Abstract: We determined the stable carbon and nitrogen isotope compositions of foot tissue, conchiolin opercula, and the shell organic matrix from the marine gastropods Cernina fluctuata (Ampullinidae) and Polinices mammilla (Naticidae) collected in March 2006 from a lower tidal flat on Cuyo Island, the Philippines, together with the seagrass Enhalus acoroides and the macroalga Padina australis. The data from the foot tissue samples confirmed that the food sources for C. fluctuata were autotrophs, such as macrobenthic marine plants and algae, whereas P. mammilla may derive carbon from epiphytes and phytoplankton via their bivalve prey. The δ13C and δ15N values for C. fluctuata foot tissues from specimens collected in 1995 were similar to those from specimens collected in 2006. The δ13C values for C. fluctuata shell organics collected in 2006 were approximately 3.6‰ higher than those for P. mammilla shells, which was a similar trend to the relationship for δ13C values observed in their foot tissues. However, the mean δ13C values for the shell organics from the specimens collected in 1995 showed no significant differences between C. fluctuata and P. mammilla. In all the gastropod specimens analyzed, we observed a large (at least 10‰) δ13C depletion in shell organics with respect to the foot tissues of C. fluctuata and P. mammilla, although the reason for this is unclear. The δ13C values for the P. mammilla conchiolin opercula were similar to those for the foot tissues, whereas there was a larger δ13C depletion in conchiolin opercula with respect to the foot tissues of C. fluctuata. Therefore, gastropod conchiolin opercula should not be used for short-term dietary analyses in some gastropod species, but they may rather reflect the integrated long-term dietary trend.
Key words: stable isotopes, shell organics, trophic interactions, naticid, Philippines

*Corresponding author E-mail: m_ishikawa@yamazaki.ac.jp

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