Gametogenic cycle of the ponderous ark, Noetia ponderosa (Say, 1822), from Cedar Key, Florida.
Subject: Shellfish (Growth)
Shellfish (Study and teaching)
Shellfish (Behavior)
Fish populations (Study and teaching)
Authors: Power, Alan J.
Sturmer, Leslie
Lucas, Cheryl
Walker, Randal L.
Manley, Justin
Pub Date: 01/01/2005
Publication: Name: Journal of Shellfish Research Publisher: National Shellfisheries Association, Inc. Audience: Academic Format: Magazine/Journal Subject: Biological sciences; Zoology and wildlife conservation Copyright: COPYRIGHT 2005 National Shellfisheries Association, Inc. ISSN: 0730-8000
Issue: Date: Jan, 2005 Source Volume: 24 Source Issue: 1
Topic: Computer Subject: Company growth
Product: Product Code: 0913000 Shellfish NAICS Code: 114112 Shellfish Fishing SIC Code: 0913 Shellfish
Geographic: Geographic Scope: Florida Geographic Code: 1U5FL Florida
Accession Number: 133108158
Full Text: 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.

RESULTS

Water temperature and salinity at Cedar Key are given in Figure 1. Water temperature followed an annual cycle where maximum temperature occurred in July (29.8[degrees]C) to August (29.7[degrees]C) in 2001 and in August (29.6[degrees]C) to September (29.3[degrees]C) in 2002 and lowest temperatures occurred in January 2002 and 2003 (16.8[degrees]C and 14.3[degrees]C, respectively). Between the 2 y sampled, water temperatures broadly followed the same pattern, however a more rapid increase was observed during the spring of 2002, and temperatures later that year also reached lower than those previously recorded. Salinity remained high throughout the study period ranging from 22 ppt in June 2001 to 34 ppt in December 2001, however the summer months of 2002 experienced higher salinities than those recorded in the previous summer.

[FIGURE 1 OMITTED]

Arks ranged in shell length from a mean of 40.7 mm (April 2001) to 48.6 mm (January 2003). Mean total wet weight ranged from 23.9 g to 45.3 g. Of the 592 specimens histologically processed, 310 (52.4%) were male, 259 (43.8%) were female, and 23 (3.9%) were sexually indeterminate. The sex ratio was 0.84 females: 1.00 males and not significantly different was parity ([chi square] = 1.32). A small percentage of the animals (5.2%) were infested with one to two undescribed digenetic trematodes. No hermaphrodite arks were observed.

Noetia ponderosa appears to ripen rapidly during the spring and to spawn for a prolonged time period that peaks in the summer and fall months (Fig. 2). Spawning was however noted year round. In both years, 3 discrete peaks were noted in mean oocyte size (2001: April, August, November; 2002: May, August, December), again supporting an extended spawning cycle (Fig. 3). In the first year these peaks decreased successively, however this trend was not apparent in the second year (Fig. 3). On the whole, combined male and female gonadal indices and ovarian oocyte size increased in both years through the spring when early and late active arks dominated the reproductive stages (Fig. 2, Fig. 3). In 2001, mean oocyte diameter peaked in April (45.93 [micro]m) and remained high through the summer months (gonadal index peaking in July at 3.97) with varying levels of late active, ripe, and partially spent individuals then subsequently decreased through the fall (gonadal index dropping to 2.04 in August) to a mean of 21.94 [micro]m in December (Fig. 3). Through the winter months indices and oocytes remained small with spent and inactive individuals observed together with many partially spawned and some ripe individuals (Fig. 2). In the ensuing spring, oocytes matured and steadily increased to 39.86 [micro]m in May (gonadal index peak of 4.22) and remained large through the end of the year (41.51 [micro]m in December). During this period two minor decreases in oocyte size were observed (30.21 [micro]m in July and 33.40 [micro]m in September) prior to a substantial drop in January 2003 (21.87 [micro]m). Combined male and female gonadal indices for the same period exhibited a similar trend fluctuating, remaining until water temperatures begin to decrease in late summer and fall (Fig. 1) with indices dropping to 1.83 in July and 0.9 in November (Fig. 2).

[FIGURES 2-3 OMITTED]

DISCUSSION

Ponderous arks from Cedar Key started at an unknown age but were 40 mm in shell length. In Virginia, these would have been 4 y old (McGraw et al. 2001) and in Georgia about 3 y (Walker & Gates 2001). It is assumed that the natural recruitment of arks in newly planted hard clam bags in Cedar Key occurred when the pediveliger larvae metamorphosized to a postset benthos dweller. The first arks were collected from bags that were harvested after 12 to 20 mo, implying that growth rates on the Gulf coast of Florida may exceed those previously reported for Virginia and Georgia. Because monthly samples were randomly selected from arks pooled from harvested clam bags on an ongoing basis, we cannot be certain that we were dealing with a single cohort, and therefore growth data was not a focus of the present investigation and is being acquired by another aspect of this USDA CSREES funded project. However, raising these clams with hard clam gear and techniques does appear to be a viable option and growth to market size in these waters could potentially be reduced to half of what it may take in Virginia.

On the Gulf coast of Florida the ponderous ark appears to be a dribble spawner that releases gametes over most of the summer and fall and even into the winter months. Early active and late active stages tend to occur in the spring when water temperatures are recovering after the winter months. There appears to be a rapid change from late active to spawning stages. Female gonads were rarely observed so packed with eggs that eggs appear box shape, as is seen in the ripe stages of many marine bivalves (Loosanoff et al. 1966), however large oocytes were present in most samples. The transition from spent to early active also appears to be short as few inactive stages of reproduction were observed. In addition in some spent stage females, early active stages were observed in the same gonads.

The gametogenic cycle between consecutive years was not identical but quite similar and indicates a dribble spawning strategy, with spawning occurring over most of the year and peaking in the summer and fall months. The prolonged spawning period of Noetia in Cedar Key is similar to the pattern of spawning that was observed in Virginia (Chanley & Andrews 1971, McGraw et al. 1998). It was their conclusion that Noetia spawns about 8 mo of the year, with ripening occurring in spring when water temperatures increased above 17[degrees]C. The spawning pattern of the family Arcidae typically becomes broader with decreasing latitude, and many species including Anadara ovalis, Anadara similis (Adams, 1852), Anadara grandis (Broderip & Sowerby, 1829), and Anadara tuberculosa (Sowerby, 1883) have ripe individuals year round in the warm waters of Florida and Central America (Power et al. 2004, Cruz 1984a, Cruz 1984b, Cruz 1987). The advantage of a prolonged spawning period in terms of hatchery production is that operations can continue year round. However, it is worth examining the quality of gametes released after the initial major spawning in the summer/fall period. While three peaks in oocyte size were observed in 2001, each successive peak diminished as the year progressed (April 45.93, August 38.82, November 28.78). This trend was not apparent in the subsequent year with very little difference observed between peaks (May 39.86, August 42.92, December 41.51). It is worth noting that local waters warmed earlier and that salinities were quite different during the second year, and the mean monthly shell lengths observed in year two were larger and significantly different from the first (ANOVA, P = 0.03), possibly indicating differences in reproductive fecundity with age and size.

Members of the family Arcidae tend to have equal sex ratios and are dioecious: Anadara antiquata (Toral-Barza & Gomez 1985), Anadara granosa (Pathansali 1966, Broom 1983), Anadara senilis (Broom 1985), Anadara subcrenata (Broom 1985), Anadara trapezia (Broom, 1985), and Anadara tuberculosa (Cruz 1984a, Dzyuba & Maslennikova 1982). However males dominated in populations of Anadara ovalis in Florida (Power et al. 2004), Georgia (Power & Walker, 2002), and Virginia (McGraw et al., 1998). Females dominated in a population of Anadara transversa in Georgia (Walker & Power, in press) and in Anadara senilis in Africa (Yoloye 1974). Hermaphroditism occurs very rarely: 0.003% in Anadara granosa (Broom 1983), 0.004% in Anadara senilis (Broom 1985), and 2.17% in Anadara ovalis (Power et al. 2004). The ponderous ark appears to follow the typical pattern: an equal sex ratio and no hermaphrodites (this study; McGraw et al. 1998). Again this is favorable for hatchery production, reducing the number of broodstock required to attain adequate gametes from both sexes.

ACKNOWLEDGMENTS

The authors thank Ms. D. Thompson, Ms. Rebecca Green, and Ms. M. Mitchell for processing the histological samples, Mr. Justin Manley for determining oocyte diameters, and Ms. R. Varner and Mr. C. Taiani for collecting and holding ark clams in the field. This work was supported by the University of Georgia Marine Extension Service and the Institute of Food and Agricultural Sciences at the University of Florida under a USDA Grant No. 2002-34453-11946, entitled "Aquaculture, Florida Research Project," from the US Department of Agriculture's Cooperative State Research, Education, and Extension Service.

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ALAN J. POWER, (1), * LESLIE STURMER, (2) CHERYL LUCAS, (3) RANDAL L. WALKER (1), * AND JUSTIN MANLEY (3)

(1) Shellfish Research Laboratory University of Georgia, Marine Extension Service 20 Ocean Science Circle, Savannah, Georgia 31411; (2) University of Florida, Cooperative Extension Service P.O. Box 89, Cedar Key, Florida 32625; (3) Savannah State University 3219 College St., Savannah, Georgia 31404

* Corresponding author. Tel.: +1-912-598-2348; Fax: +1-912-598-2399; email: alanpower@arches.uga.edu
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