Microscopic observations of larval Ostrea circumpicta in brood chambers

Journal of Shellfisheries Research, August, 2004 by Do-Hyung Kang, Se-Jae Kim, Kwang-Sik Choi

ABSTRACT This study is the first report of the microscopic features of larvae and gonads of Ostrea circumpicta collected in Jeju, Korea. Female oysters brooded larvae at the early veliger stage (115-135 [micro]m) in the infrabranchial chamber. Larvae in this chamber displayed a well-developed velum covered with long and short cilia: the adductor muscle was incompletely developed. The digestive system of the larvae, including the mouth, esophagus, stomach, digestive gland, and intestine, was identifiable in histologic preparations, indicating that the veliger larvae in the brood chamber were capable of feeding using the velum and cilia. Diameters of ripe eggs in the ovary ranged from 67-120 [micro]m. Most female oysters collected in June 2001 were spawning or absorbing whereas most of male oysters were spent or sexually inactive.

KEY WORDS: ostreidae, Ostrea circumpicta, reproduction, larval morphology, brood oyster, Korea

INTRODUCTION

Oysters of the genus Ostrea are brooders, releasing larvae instead of gametes during spawning (Buroker 1985, O Foighil & Taylor 2000). Gametes of brood oysters are fertilized at the ctenidia, and the embryos are released into the branchial chamber of females. In this chamber, the larvae undergo pelagic development, including trochopore and veliger stages (Strathmann & Strathmann 1982, Mackie 1984, O Foighil & Taylor 2000). Veliger larvae in the chamber are characterized by velum covered with cilia that are used for locomotion and feeding (Elston 1999, Chaparro et al. 1999 & 2002). Swimming veliger larvae are often observed in suprabrachial or infrabranchial chambers (Chaparro et al. 1993). However, brooding oyster larvae are often kept in interlamellar spaces or brood sacs and are immobile (Mackie 1984, Tankersley & Dimock 1992).

Microscopic examination of larvae provides valuable information on feeding type, developmental mode, planktonic period, and larval settlement (Strathmann & Leise 1979, Pechenik 1986, Strathmann et al. 1993, Chaparro et al. 1999). Histology has been widely and extensively used to examine the larval structure and development of marine bivalves (Howard & Smith 1983, Elston 1999). In addition, video cameras and endoscopes are often used in microscopic observations of marine bivalve larvae (Ward et al. 1991, Strathmann et al. 1993, Tankersley & Dimock 1993, Bosch & Slattery 1999, Elston 1980, Chaparro et al. 1999&2002).

Ostrea circumpicta is abundant on rocky intertidal to subtidal zones of Jeju Island (Kwon et al. 1993), Korea, where salinity remains high and stable all year (Lee et al. 2000). Like other oysters of the genus Ostrea, this species is a brooder and is widely distributed in southern Japan, China, and Korea (Hirase 1930, Bernard et al. 1993, Kwon et al. 1993). Despite its abundance, few studies have investigated the ecology and reproductive biology of O. circumpicta (Bae & Bae 1972, Yang 1999&2001). Here, we provide the first report of the microscopic features of the gonads and larvae of O. circumpicta in sexually mature individuals.

MATERIALS AND METHODS

Munseom, an uninhabited island located off the southern coast of Jeju (33 [degrees]13' 25"N, 126 [degrees]33' 58"E), is well known for its high species diversity and richness of marine biota (Je et al. 2002, Fig. 1). The island characterized by volcanic rocky intertidal and subtidal zones that are subjected to strong wave action. The rocky substrata are enriched with sessile fauna such as oysters, barnacles, and numerous species of anthozoans. In particular. O. circumpicta heavily encrusts the subtidal cliff at depths between 3 and 6 m. Surface water temperatures and salinity in this area vary from 16 [degrees]C to 22 [degrees]C and from 32.2 34.4 psu annually (Choa & Lee 2000, Lee et al. 2000). Due to the influence of the warm Kuroshio current, surface water temperatures during winter are several degrees higher than in northern Jeju.

[FIGURE 1 OMITTED]

O. circumpicta were collected in June 2003 by SCUBA diving at depths between 3 and 5 m. Oyster tissues were fixed in Davison's fixative, and longitudinal sections were made along the middle of the body of fixed specimens (Fig. 2A) using standard histologic methods. Microscopic images of oyster eggs and larvae in brood chambers were converted to digital image using a digital camera installed on the microscope. The size of eggs and larvae were then measured from the images using image analysis software (Kang et al. 2003).

[FIGURE 2 OMITTED]

RESULTS

Figure 2 shows a female O. circumpicta collected in June 2003. This species has a relatively large adductor muscle and gills compared with nonbrooding oysters. The mantle is composed of two layers with thick mantle lobes. We found no hermaphroditic individuals in this study. Figure 3 shows numerous early veliger larvae in the infrabranchial chamber. The visceral masses of the larvae were filled with coelomic fluid in the coelomic cavity. The larvae were approximately 115-135 [micro]m long, and we observed no physical connections between female oysters and larvae. No obvious structural changes for holding larvae were found in the gills. Figure 4 shows intact larvae isolated from the chamber. The upper part of the velum was covered with two types of cilia; long outer preoral cilia (i.e., OPC, Chaparro et al. 1999), 20-40 [micro]m long and short inner preoral cilia (IPC), 5-10 [micro]m long.