The reproductive cycle of the flame scallop, Ctenoides scaber , from the lower Florida Keys and its relationship with environmental conditions

Journal of Shellfisheries Research, August, 2005 by Angela K. Dukeman, Norman J. Blake, William S. Arnold

ABSTRACT The reproductive cycle, sex distribution, and gonadal characteristics of the flame scallop, Ctenoides scaber, formerly Lima scabra scabra (Born 1778), collected from Boca Chica Key, FL, were investigated over the 21-mo period from January 1998 through September 1999. Gametogenic cycles were examined using qualitative and quantitative methods, and the relationships between those observations and environmental conditions (e.g., water temperature, salinity and phytoplankton concentrations) were analyzed. The relationships between sex, gonad color and shell height were also examined. Gamete development in both sexes was initiated in winter and was associated with small oocytes and follicles, cool water temperatures and moderate concentrations of food. Growth of gametes occurred throughout spring, as temperature and chlorophyll-a concentrations increased. A partial synchronous spawn occurred in early summer but did not seem to be related to environmental conditions. Maximum gamete ripeness and size occurred in late summer, when water temperatures were at maximum values and food densities were increasing. Decreases in female gamete and follicle sizes and increases in occurrence of partially spawned, spent and early growth gonads in autumn were suggestive of synchronous spawning, which coincided with a rapid decrease in water temperature and maximum measured chlorophyll-a concentrations. Decreases in oocyte size in February coincided with annual water temperature minimums and may represent the conversion of energy from reproduction to survival, and not spawning. The presence throughout the year of juveniles, ripe and partially spawned flame scallops and chlorophyll-a concentrations sufficient to support gamete development suggest a reproductive strategy of continuous spawning, common in tropical marine invertebrates. No relationship was detected between salinity and gonad condition. Flame scallops collected for this study ranged in size from 21-68 mm shell height (SH) and those >25 mm SH had gamete development. The smallest animals were found principally in summer, which suggests a massive synchronous spawning event. Analyzing sex distribution by SH showed that flame scallops are protandric sequential hermaphrodites. Flame scallops <40 mm SH were predominantly male (83%), those [greater than or equal to] 40 mm SH were mostly female (71%), and 4% were in sexual transition near 40 mm SH. The sex ratio for the sampled population was 0.63M:1F. Histologic examination of fresh gonadal tissues revealed that female gonads were predominantly purple (96%) and male gonads were predominantly cream-colored (91%). Other gonad colors observed were not reliable indicators of sex nor was there a clearly defined association between color and those animals in sexual transition. Documenting the reproductive cycle of Ctenoides scaber in the existing Florida fishery is the first logical step to understanding its life history. Because flame scallops are important to the marine aquarium industry in the United States and are a potential food source for humans, it is important that we understand the reproductive cycle of this species to ensure proper management policies. This study provides basic information applicable to mariculture of flame scallops for commercial production to supplement the harvest of wild stock for the marine aquarium industry.

KEY WORDS: flame scallop, reproduction, sex ratio, Ctenoides scaber, spawning, environmental conditions, gonad color

INTRODUCTION

Tropical flame scallops, Ctenoides scaber, are commercially important for the marine aquarium trade in the United States and support a small-scale fishery in the Caribbean Basin (Gomez et al. 1990), but little research has been done on their life history. Because there has been no research on gametogenesis in Florida flame scallop populations, we used quantitative and qualitative methods to examine Ctenoides scaber's sex distribution and reproductive development over time as related to environmental conditions. We also examined the relationships between sex, gonad color (GC), and shell height (SH).

Ctenoides scaber range from North Carolina southward to Brazil (Abbott 1974). They are commonly found attached by byssal threads to rocks and coral rubble (Abbott 1974). The reproductive cycles of C. scaber have been studied in Venezuelan waters. These studies include those by Gomez et al. (1990, 1995), which examined the relationships between sex, SH, and biomass and their effects on flame scallop reproduction during March, May and July 1994. Another study by Lodeiros and Himmelman (1999) examined the relationship between C. scaber reproduction and environmental conditions. Animals collected in those studies ranged from 12.8-81.5 mm SH. The largest animals were collected in March, and the smallest in July (Gomez et al. 1995). Flame scallops are protandric, sequential hermaphrodites and sexual transition occurs as the animal nears 40-mm SH (Gomez et al. 1990, Gomez et al. 1995). Lodeiros and Himmelman (1999) calculated the sex ratio 1M:0.84F, which was not significantly different from the expected 1M:1F.

Gonadosomatic indices (GSIs) indicated that reproduction in Venezuelan flame scallops was highly variable throughout the year (Lodeiros & Himmelman 1999). Spawning events were characterized by a reduction of GSIs below 5%, and animals of all sizes were reproductively active (Lodeiros & Himmelman 1999). Gamete development was initiated in winter (GSIs were 2% to ~5%) during seasonal low water temperatures (23[degrees]C) and elevated chlorophyll-a (chl-a) concentrations (~0.5 to ~7.5 [micro]g/L) and increased steadily through early summer as water temperature increased and phytoplankton levels decreased (Lodeiros & Himmelman 1999). GSIs decreased sharply throughout June and early July and coincided with a 4[degrees]C decrease in water temperature and an increase in chl-a from ~0 to 3.0 [micro]g/L, which may have been evidence of a partial spawning event. Gomez et al. (1995) also detected reduced GSIs in May and June (6% to 8%). GSIs reached maximum annual values (~15%) in August (temperature 26[degrees]C; chl-a ~1 [micro]g/L). Two years of data showed marked decreases of GSIs to <5% in autumn, which also corresponded with temperature decreases and chl-a increases and indicated a synchronized spawning event (Lodeiros & Himmelman 1999). Only chl-a concentrations were statistically related to reproduction in Venezuelan flame scallops, which may suggest that C. scaber gamete development is stimulated by elevated chl-a, as is seen in other bivalves (Sastry 1979, Barber & Blake 1991). The increased recruitment of juveniles into the population in summer supports the hypothesis that massive synchronous spawning may have occurred (Gomez et al. 1995). In addition, highly variable developmental stages throughout the year suggest continuous spawning or partial synchronous spawning may contribute to the reproductive strategies of C. scaber (Lodeiros & Himmelman 1999).