Gametogenic cycle of the ponderous ark, Noetia ponderosa , from Cedar Key, Florida

Journal of Shellfisheries Research, Jan, 2005 by Alan J. Power, Leslie Sturmer, Cheryl Lucas, Randal L. Walker, Justin Manley

ABSTRACT The gametogenic cycle of the ponderous ark, Noetia ponderosa (Say, 1822), was studied in a Cedar Key, Florida population between March 2001 and January 2003. Ponderous arks are dioecious, and no hermaphrodites were found in this study (n = 592). The sex ratio of females to males was 0.84: 1.00, but was not significantly different from parity. On the Gulf coast of Florida, the ponderous ark dribble spawns over most of the year, peaking in the summer and fall months and with the least spawning activity occurring during the spring when gametes are maturing. A small percentage (5.2%) was found infested with an undescribed digenetic trematode. The Cedar Key area has an important commercial hard clam (Mercenaria mercenaria) aquaculture industry, and the implications of these findings on the potential for its diversification based on this species are discussed.

KEY WORDS: ark, gametogenesis, growth, reproduction, spawning, sex ratio

INTRODUCTION

A member of the ark shell family (Arcidae), the ponderous ark, Noetia ponderosa (Say, 1822), is common throughout the estuarine and nearshore waters of the southeastern Atlantic and Gulf of Mexico coastal areas of the United States (Abbott 1974, Anderson et al. 1984, Anderson & Eversole 1985, McGraw & Castagna 1994, Walker & Gates 2001). The species favor shelly substrates at depths from the low water line to 18 m and salinities above 17.5 ppt (Abbott 1974, Chanley & Andrews 1971, Rehder 1981, McGraw et al. 1996). Adult shells can attain a length of 7 cm (Rehder 1981). Other than a few reproductive and population dynamics studies from Virginia (Chanley 1966, Chanley & Andrews 1971, McGraw & Castagna 1994, McGraw et al. 1996, McGraw et al. 1998, McGraw et al. 2001) little is known about the species. Information is particularly scant on the arks biology and ecology in the Gulf of Mexico region.

At the northern geographical limit in the Chesapeake, ponderous arks are reported to spawn in late spring and summer, with additional activity albeit on a lesser scale in the fall and early winter (McGraw et al. 1998). Previously reported growth rates from Virginia are slow relative to the co-occurring blood ark, Anadara ovalis (Bruguiere, 1789), and the species can live for between 10 to 15 y (McGraw et al. 2001). Ponderous arks were commercially harvested from the Eastern Shore of Virginia during the 1990s to supply ethnic markets in the northeast region (McGraw & Castagna 1994). Given the species longevity and growth rates, it is not surprising that wild stocks were quickly overfished. In Georgia, an ark survey undertaken in the late 1990s found that most beds were limited in size and would not support a viable fishery (Walker & Gates 2001). Consequently, interest is mounting throughout the southeastern states in developing an ark based aquaculture industry (Power et al. 2004). This study, funded through the United States Department of Agriculture's Cooperative State Research, Education, and Extension Service (USDA CSREES), is part of an overall evaluation of this species to provide diversification to the hard clam aquaculture industry.

The culture of hard clams (Mercenaria mercenaria, Linnaeus, 1791) represents the fastest growing segment of the aquaculture industry in Florida, with an economic impact of approximately $34 million to the state (Philippakos et al. 2001). The industry supports over 400 growers, 14 hatcheries, 90 land-based nurseries, and 55 certified shellfish wholesalers (Ruth et al. 2003). During the early 1990s displaced workers from the commercial fishing industry on the Gulf coast of Florida were retrained in clam farming, and today the Cedar Key area has some of the most productive shellfish leases in the nation (Colson & Sturmer 2000). Ponderous arks in this area often naturally recruit into the clam culture bags, and they grow and survive well under these conditions (authors observations). It is believed that this species could represent an opportunity for diversification to small-scale hard clam culture enterprises using similar culture methods and thus improve farm incomes. The availability of ark seed is dependent on the development of hatchery protocol for this species. Because other ark species have displayed marked latitudinal differences in gametogenic patterns (e.g., Power et al. 2004, McGraw et al. 1998) a critical first step is to comprehend the reproductive cycle of the species at Cedar Key on the Gulf Coast of Florida.

MATERIALS AND METHODS

Arks were collected on an ongoing basis from harvested hard clam bags during processing at certified shellfish wholesalers (Sturmer et al. 1995) and subsequently replanted at a commercial clam lease in Cedar Key for holding during the study period. Stocking densities maintained were <538/[m.sup.2] (50/[ft.sup.2]). Bottom water temperature and salinity data were taken every 30 min by a YSI sonde (Model 6600) data recorder deployed on site at the clam lease area.

Each month between March 2001 and January 2003, excluding April 2002, 19 to 33 arks were randomly collected and shipped alive to the Shellfish Research Laboratory in Savannah, GA. Immediately upon arrival each individual was measured for shell length and total weight (McGraw et al. 1996), and a midlateral gonadal sample (ca. 1 [cm.sup.2]) was dissected for histologic analysis. Gonadal tissue was fixed in 10% formalin buffered with seawater for 48 h, washed with 50% ethanol, and preserved in 70% ethanol until processing. Tissues were processed according to procedures outlined in Howard and Smith (1983). The examination of prepared gonadal slides was conducted with a Zeiss Standard 20 microscope (x20). Each animal was sexed, and ratios were tested against a 1:1 ratio with [chi square] statistics (Elliott 1977). Individuals were assigned to 1 of 6 developmental stages: early active (EA), late active (LA), ripe (R), partially spawned (PS), spent (S), and inactive (IA). Stages in gonadal development are assigned based on gonadal appearance, the presence or absence of different sexual products, and evidence of gamete release. EA and LA are based upon descriptions of "early" and "later development" provided by Kennedy & Krantz (1982). Individuals at full maturity in their "later development" stage were treated as a separate stage, R for the present study. Kennedy and Krantz's "spawning" and "advanced spawning and regressing" describe our PS and S stages. IA is best described by Eversole & Minchener's (1980) "undifferentiated stage". A semiquantitative numerical assignment (0-5) was then used to rank these stages from 0 to 5: EA = 3; LA = 4; R = 5; PS = 2; SP = 1; IA = 0 (Walker & Heffernan 1994, Spruck et al. 1994). In reproductive studies, the expression of gonadal tissue as a percentage of somatic tissue is often used as an index of gonadal function. However, gonadal growth can be allometric, and therefore we selected the relative proportion of individuals in each gametogenic stage as our gonadal index (i.e., independent Of body size). The determination of monthly gonadal index (GI) values was obtained by multiplying the number of specimens ascribed to each ranked score by the respective score, summing all such values, and dividing this figure by the total number of males or females analyzed. Image analysis of ovarian sections was also carried out with a Motic digital microscope (B3 Professional Series) and the Motic Images 2000 Version 1.3 software. For each female, the mean of two oocyte diameter measures (taken at right angles to each other) was calculated for 30 randomly selected oocytes and used to calculate mean monthly oocyte size.