Kozue Nishida

Ontogenetic changes in shell microstructures in the cold seep-associated bivalve, Conchocele bisecta (Bivalvia : Thyasiridae)

Abstract. We described the shell microstructure of the cold seep-associated bivalve Conchocele bisecta using the acetate peel method and scanning electron microscopy, and revealed complex microstructural changes with ontogeny. The shell of the bivalve has outer, middle, and inner layers that consist of aragonite. The outer layer consists of spherulites and spindles of various sizes and shapes; these microstructures are identified as spherulitic, planar spherulitic, spherulitic prismatic, and spindle-like structures. The middle layer is characterized by cone complex crossed lamellar structure in the outer part and crossed lamellar structure in the inner part. The inner layer is composed of cone complex crossed lamellar, fine complex crossed lamellar, and irregular prismatic structures. On the basis of the observations from the umbo to the ventral margin of each shell layer, we recognized two growth stages that are divided by microstructural distributions in the outer and inner layers and the positions of disturbance rings. The early growth stage is characterized by spherulitic structure in the outer layer, cone complex crossed lamellar structure in the inner layer, and scarce disturbance rings. The late growth stage, on the other hand, has specific microstructures (planar spherulitic, spherulitic prismatic, and spindle-like structures) that lie as sheeted layers within the spherulitic structure of the outer layer, and the disturbance rings are situated close to specific microstructures in the outer layer. In addition, alternations between areas that are cone complex crossed lamellar, thin-layered irregular prismatic, and fine complex crossed lamellar structures are observed in the inner layer. The characteristics of both growth stages indicate that shell growth rate is more variable in the late stage than in the early stage, and that growth rate decreases from the early to late stage, possibly caused by a physiological change. This study suggests that it is important to examine shell microstructural distribution to reveal shell growth and life history.

Thermal dependency of shell growth, microstructure, and stable isotopes in laboratory-reared Scapharca broughtonii (Mollusca: Bivalvia)

We experimentally examined the growth, microstructure, and chemistry of shells of the bloody clam, Scapharca broughtonii (Mollusca: Bivalvia), reared at five temperatures (13, 17, 21, 25, and 29°C) with a constant pCO2 condition (∼450 μatm). In this species, the exterior side of the shell is characterized by a composite prismatic structure; on the interior side, it has a crossed lamellar structure on the interior surface. We previously found a negative correlation between temperature and the relative thickness of the composite prismatic structure in field-collected specimens. In the reared specimens, the relationship curve between temperature and the growth increment of the composite prismatic structure was humped shaped, with a maximum at 17°C, which was compatible with the results obtained in the field-collected specimens. In contrast, the thickness of the crossed lamellar structure was constant over the temperature range tested. These results suggest that the composite prismatic structure principally accounts for the thermal dependency of shell growth, and this inference was supported by the finding that shell growth rates were significantly correlated with the thickness of the composite prismatic structure. We also found a negative relationship between the rearing temperature and δ18O of the shell margin, in close quantitative agreement with previous reports. The findings presented here will contribute to the improved age determination of fossil and recent clams based on seasonal microstructural records.

Skeletal oxygen and carbon isotope compositions of Acropora coral primary polyps experimentally cultured at different temperatures

We investigated temperature and growth-rate dependency of skeletal oxygen and carbon isotopes in primary polyps of Acropora digitifera (Scleractinia: Acroporidae) by culturing them at 20, 23, 27, or 31°C. Calcification was most rapid at 27 and 31°C. We obtained a δ18O-temperature relationship (−0.18‰ °C−1) consistent with reported ranges for Porites, indicating that juvenile Acropora polyps can be used for temperature reconstruction. A growth-rate dependency of skeletal isotopes was detected in the experimental polyps cultured at lower water temperatures, when the skeletal growth rate of these polyps was also low. The estimated upper calcification flux limit for a kinetic isotope effect to be observed in the δ18O-growth rate relationship (∼0.4–0.7 g CaCO3 cm−2 yr−1) was similar to the calcification flux in Porites corresponding to a linear extension rate of 5 mm yr−1, the maximum rate at which the kinetic isotope effect is evident. This result suggests that the calcification flux can be used as a measure of growth rate-related isotope fractionation, that is, the kinetic isotope effect, in corals of different genera and at different growth stages.

A new Paleocene species of Bentharca (Bivalvia; Arcidae) from eastern Hokkaido, with remarks on evolutionary adaptation of suspension feeders to the deep sea

Abstract. We describe a new deep-sea arcid species, Bentharca steffeni sp. nov., from the Paleocene Katsuhira Formation in Urahoro Town, eastern Hokkaido. This is the oldest certain record of this genus. Shell microstructure of this new species is similar to the Recent species, B. asperula (Dall, 1891) in having an outer layer composed of thin simple lamellar fibrous prismatic and crossed lamellar structures and an inner layer composed of irregular complex crossed lamellar and irregular prismatic structures. Of these, the simple lamellar fibrous prismatic and the irregular prismatic structures have never previously been recognized even in B. asperula. From the occurrence of the new species, it has been elucidated that the genus Bentharca adapted to the deep sea as a refuge at least in the Paleocene.

Geographical and Seasonal Variations of the Shell Microstructures in the Bivalve Scapharca broughtonii

Cyclical ontogenetic changes of shell microstructures have been observed in the subfamily Anadarinae (Mollusca: Bivalvia, Arcidae) including fossil taxa. The changes in the bloody clam Scapharca broughtonii are controlled by temperature, which fluctuates seasonally, and can be used to determine the age of the individuals and to reconstruct paleoenvironments. In this study, samples of S. broughtonii from eight localities covering broad geographical regions at various latitudes in Japan, Korea, and Russia were examined to assess the utility of time series variations in microstructures for paleoenvironmental and paleoecological studies. All specimens showed cyclical changes in the relative thickness of the composite prismatic and crossed lamellar structures in the outer layer with ontogenetic progression, and thus, this feature can be used as a proxy for water temperature of their habitats. Specimens from southern latitudes showed higher annual shell growth rates than northern specimens, suggesting that low temperatures arrest shell growth in S. broughtonii and play a key role in determining the longevity and body size in S. broughtonii. In long-lived individuals from the four northernmost localities, the relative thickness of the composite prismatic structure tended to decrease as the individuals aged, which may be a consequence of declining physiological activity, such as organic matrix secretion.