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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, Nanako O. Ogawa, Naohiko Ohkouchi, Daniel Edison M. Husana, Tomoki Kase
Geochemical Journal, Vol. 51, No. 3, P. 241-250, 2017

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.

KEYWORDS

stable isotopes, shell organics, trophic interactions, naticid, Philippines

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