Reproductive pattern of the blood ark, anadara ovalis from the Northeast Coast of Florida.

ABSTRACT In blood arks, the sexes are separate; however, a low incidence of hermaphrodites was observed in the current study (2.17%). Mates occurred more frequently than females (m/f = 2.68). Ripe arks were present year round outside of the period from August to November 2002. A bimodal reproductive pattern was apparent with a peak in gonadal development during the late spring-early summer months (45% ripe in May), followed by a quiescent period in the late summer-fall, and followed in turn by a minor peak during the winter months (21% ripe in December). Spawning was apparent in all months with the exception of September, and individuals in this phase were most numerous in June (50%) and July (64.29%). Dribble spawning is likely used as a strategy to extend its spawning period and increase its reproductive success. A low incidence (3.33%) of parasitic infection by digenetic trematodes resulting in castration was also noted. The implications of these findings on the aquaculture potential of this species are discussed.

KEY WORDS: ark, gametogenesis, reproduction, sex ratio, spawning, Anadara ovalis

INTRODUCTION

Several ark species (Scapharca, Arca, and Anadara spp.) form the basis of economically significant molluscan fisheries and extensive culture operations (Baqueiro et al. 1982, Baqueiro 1989, Broom 1985, MacKenzie 2001, Manzi & Castagna 1989, Nie 1990, Umezawa 1992). A member of the ark shell family Arcidae, the blood ark, Anadara ovalis (Bruguiere 1789) occurs from Cape Cod, Massachusetts, to the West Indies and Brazil, at depths ranging from the low-tide line to >3 m (Abbott 1974, Anderson et al. 1984, Rehder 1981, Walker & Gates 2001). The species favors sandy deposits (Alexander 1993) and salinities above 15 ppt (Chanley & Andrews 1971). With a short lifespan (<5 y), early maturity (10-12 mm/-8 mo), rapid growth rates, and established markets in the Northeast, this bivalve has shown tremendous potential as an aquaculture species to diversify hard clam operations in Southeastern coastal waters (McGraw et al. 1996, McGraw et al. 1998, Power & Walker 2001, Power & Walker 2002, Walker 1998).

In Florida, A. ovalis occurs mainly on the eastern coast whereas the related ponderous ark (Noetia ponderosa, Say 1822) also inhabits the Gulf Coast. A concerted effort is currently underway among researchers and extension agents on both coasts of Florida and on the Georgia coast to investigate the hatchery-rearing protocol for both species. A systematic histologic examination of the gonads is a critical step providing information on the gametogenic cycle and sex ratios. By knowing the natural gametogenic cycle of a species, hatchery operators will know when to collect brood stock in the field while they are at their optimum ripe stage prior to their spawning. Histologic analysis of gonadal tissue also allows hatchery operators to know if animals front the natural population can be spawned more than once per year. This can save time and money in conditioning the bivalves to an optimum ripe stage within the hatchery. Knowledge of the sex ratio of a species determines the number of individuals that must be held for conditioning and spawning.

The reproductive biology of the blood ark has not previously been investigated for populations anywhere in Florida, In Virginia, blood arks are reported to spawn in the summer months when water temperatures reach above 17[degrees]C (Chanley & Andrews 1971, McGraw et al. 1998). In Georgia, blood arks spawn earlier, from late spring through summer (Power & Walker, 2002). In general, spawning periods and gametogenesis in marine bivalves start earlier and last longer in southern geographical areas than in northern ones (Eversole 1989, Thompson et al. 1996), and therefore the gametogenic cycles from Florida can be expected to be unique and clearly warrant investigation.

MATERIALS AND METHODS

Blood arks were collected from harvested clam bags during processing at certified shellfish wholesalers (Sturmer et al. 1995) and subsequently replanted in a commercial clam lease on the Matanzas River near "Marineland" south of St. Augustine, Florida, far holding and continued growout over the annual study period. The salinity and temperature of the river were monitored at the Whitney Laboratory floating dock every 30 min using a YSI 6600 data sonde.

Each month between May 2002 and April 2003, approximately 30 specimens were shipped to the Shellfish Research Laboratory in Savannah, Georgia. Immediately upon arrival, each individual was measured for shell length, width, height, and total weight (McGraw et al. 1996), and a mid-lateral gonadal sample (ca. 1 [cm.sup.2]) was dissected for histologic analysis. Notes on the gonad color were recorded. Gonadal tissue was fixed in Davidson's solution, refrigerated for 48 h, washed with 50% ethanol, and preserved in 70% ethanol until processing. We processed tissues according to procedures outlined in Howard and Smith (1983). The examination of prepared gonadal slides was conducted with a Zeiss Standard 20 microscope (20x). Each animal was sexed and assigned to a developmental stage as described by Walker and Heffernan (1994) and Spruck et al. (1994). A staging criteria of 0 to 5 was used for early active (EA = 3), late active (LA = 4), ripe (R = 5), partially spawned (PS = 2), spent (SP = 1), and inactive (IA = 0). The determination of monthly gonadal index (G.I.) values was obtained by averaging the number of specimens ascribed to each category score. We tested sex ratios against a 1:1 ratio with chi-square ([chi square]) statistics (Elliott 1977).

RESULTS

Monthly mean water temperature and salinity data for the Matanzas River are shown in Figure 1. Water temperatures ranged from 12 to 28[degrees]C between the winter and summer months. Salinities showed less of an annual pattern, and values ranged from 22 to 33 ppt. Figure 2 displays the mean size parameters (length, width, height, and weight) of monthly samples. The average length of blood arks from all samples combined was 41.34 mm ([+ or -] 0.22 SE). The largest and smallest ark observed in terms of shell length, width, height, and weight measured 52.59 mm. 39.58 mm, 41.15 mm, 58.90 g, and 29.50 mm, 21.00 mm, 88.10 mm, and 13.10 g, respectively.

[FIGURES 1-2 OMITTED]

Of the 323 arks examined histologically, 73 (22.60%) were sexually indeterminate, 177 (54.80%) were male, 66 (20.43%) were female, and 7 (2.17%) were hermaphroditic (Fig. 3). Figure 4 reveals the size distribution of arks according to these categories. Both male and female arks were most abundant in the 42-mm shell-length class and represented in similar proportions in the smaller and larger size classes. Hermaphroditic arks occurred between 38 and 48 mm. Males dominated in all monthly samples with the exception of September 2002, where there were two with four females: however, a large percentage (75%) of individuals in this sample did not display any gonadal activity and therefore were sexually indeterminate (Fig. 5). The [chi square] test revealed that the overall male/female ratio of 2.68 was significantly different from parity ([chi square] = 7.89; P < 0.01). Histologic examination and visual observations of the gonads revealed that all orange-red colored gonads were females whereas those showing white coloration were typically males.

[FIGURES 3-5 OMITTED]

Ripe arks were present in most samples, with the exception of those taken from August to November 2002 (Fig. 6). Similarly, partially spawned individuals were observed throughout the year with the notable exception of September 2002, when all gonads were completely spent. While an extended spawning period from May to September was observed, a bimodal annual pattern was apparent in the G.I. plot. Gonadal development peaked during the late spring-early summer months (45% ripe in May), followed by a quiescent period in the late summer-fall, and followed in turn by a minor peak during the winter months (21% ripe in December). The largest coordinated spawning effort was in June and July, where 50% and 64.2% of individuals had partially spawned gonads. Partially spawned gonads often had smaller areas of ripe and late active follicles.

[FIGURE 6 OMITTED]

A low incidence (3.33%) of parasitic infection by digenetic trematodes resulting in castration was also noted. No seasonal pattern in parasitic infection was evident, with one specimen being infected in the months of May, June, July, September. November, December 2001, and March 2002, and two specimens in the months of January and February 2002. Spearman's rank correlation coefficients detected a significant correlation between mean water salinity levels and the mean monthly G.I. ([r.sub.s] = 0.67, t -2.07, df = 11, [alpha] = 0.05).

DISCUSSION

In Long Island Sound, New York, the transverse ark, Anadara transversa (Say, 1822), is reported to spawn between May and August (Loosanoff & Davis 1963). Further south in the Chesapeake, this species spawns from May to September (Chanley & Andrews 1971). In Georgia, a more extended period is observed for the transverse ark with major spawning occurring between April and July, and some spawning also occurring in the winter months of December and January (Walker & Power, 2003). Similarly, for the related blood arks, a spawning period from May to September has been identified for the Chesapeake area (McGraw et al. 1998) whereas a more extended period over most of the year but peaking in the summer has been reported for Georgia (Power & Walker 2002). Further south in Cost Rica, ripe Anadara tuberculosa (Sowerby, 1883) are present throughout the year with a peak in spawning activity between May and September (Cruz 1984a). Additionally, Anadara similis (Adams, 1852) and Anadara grandis (Broderip & Sowerby, 1829) from these waters are also ripe year round but have peaks during the winter months (Cruz 1984b, Cruz 1987). From this synopsis, it would appear that the spawning pattern of the Arcidae family becomes less confined to a single narrow season with decreasing latitude and the associated reduction in temporal fluctuations in temperature and coastal phytoplankton abundance. Based on the current study, it is apparent that blood arks from the northeast of Florida can also remain in the ripe and spawning phases for most of the year and that salinity may be a controlling factor in the gametogenic cycle.

A prolonged spawning period has distinct advantages and disadvantages for producing the species in a commercial hatchery. During the microscopic analysis of histologic slides, partially spawned gonads with patches of ripe and late active follicles were frequently observed. It is therefore likely that this species achieves a long spawning season by maturing different sections of its gonad at different times. This would give rise to a "dribble" spawning reproductive strategy, wherein gametes are not released synchronously as a massive event within the population, but instead small amounts are periodically released over an extended time period. In a hatchery, such a strategy would reduce the number of viable gametes available through a single induced spawning effort. As a result, either more individuals or repeated spawning attempts during several weeks and months might be necessary to generate sufficient numbers of larvae.

A clear advantage is that broodstock can be obtained and spawned almost anytime outside of the fall months, allowing for multiple spawnings per year. Of course, the optimal time for spawning is the early summer based on the percentage of individuals synchronously in the desirable late active, ripe, and partially spent gonadal stages (>80%). During the winter reproductive peak, less than 50% of individuals display gonads at these stages. Therefore, spawning in the early summer months would reduce the number of individuals required for conditioning.

A reduction in the number of broodstock required is important because only one in three to four blood arks are likely to be female (M:F = 2.68:1.00). The male domination reported here is in general agreement with previously reported sex ratios for the species in Virginia (1.98:1.00 M:F; McGraw et al. 1998) and in Georgia (2.44:1.00 M:F; Power & Walker 2002). This is, however, the highest male to female ratio recorded for a member of the Arcidae family (see review in Power & Walker 2002). Hermaphroditism was observed for the species in the current study for the first time, albeit at a low incidence. One individual displayed a large area of mature oocytes sandwiched between two similarly sized areas of spermatozoa whereas the other six had only a few follicles of the extra gamete. Hemaphroditism is rare in the whole Arcidae family. A low incidence has been reported for Anadara granosa (0.003%) and Anadara senilis (0.004%) Broom (1983, 1985, respectively). It has been suggested that these might be protandric hermphrodites. Males typically dominate protandric bivalve species in the first year whereas older age classes are generally equal. However, the specimens examined in the current study were generally of the larger and older size classes and were still dominated by males. Males and females were also represented in similar ratios throughout the entire shell length size distribution analyzed (i.e.. 30-52 mm). In addition, one male ark maintained in a conditioning tank with 39 others (both sexes) at the Whitney Laboratory was induced to spawn female gametes 2 mo after having last released spermatozoa. This does not support a protandric hermaphroditic life cycle, and the observed phenomenon cannot be explained.

ACKNOWLEDGMENTS

This work was supported by congressional funds from the State of Florida (Grant No. USDA Grant # 2002 34453-11946). The authors thank Mr. Phil Cubbedge for providing and holding the arks on his lease and Mr. Micah Alo for his assistance in the field. Thanks to Ms. Rebecca Green. Ms. Dodie Thompson, and Ms. Mary Sweeney-Reeves for dissecting the samples and to Ms. Lisa Calvo and Prof. Gene Burreson at the Virginia Institute of Marine Science Department of Environmental and Aquatic Animal Health for diagnosing the trematode infections.

LITERATURE CITED

Abbott, R. T. 1974. American seashells, 2nd ed. New York: Van Nostrand Reinhold. 663 pp.

Alexander, R. R. 1993. Correlation of shape and habit with sediment grain size for selected species of the bivalve Anadara. Lethaia 26(2):153-162.

Anderson, W. D., W. H. Lacey III & A. G. Eversole. 1984. An investigation concerning the feasibility of harvesting and marketing underutilized Anadara off the coasts of South Carolina and Georgia. Report prepared for the Gulf and South Atlantic Fisheries Development Foundation under Grant Contact No. CASAFDI 22-18-17320

Baqueiro, E. C. 1989. Clam culture in Mexico: past, present and future. In: J. J. Manzi & M. A. Castagna, editors. Clam mariculture in North America. New York: Elsevier. pp. 383-394.

Baqueiro, E. C., M. D. Mucino & R. M. Merino. 1982. Analisis una poblacion de Pata de Mula Anadara tuberculosa sujeta a explotacion intensiva en la Bahia de la Paz, Baja California Sur, Mexico. Ciencia Pesquera Instito Nacional de la Pesca. Mexico 3:75-82.

Broom, M. J. 1983. Gonad development and spawning in Anadara granosa (L.) (Bivalvia: Arcidae). Aquaculture 30:211-219.

Broom, M. J. 1985. The biology and culture of marine bivalve molluscs of the genus Anadara. ICLARM, International Center far Living Aquatic Resources Management, Manila, Philippines. ICLARM Studies and Reviews 12, Contribution No. 263.

Chanley, P. & J. D. Andrews. 1971. Aids for identification of bivalve larvae of Virginia. Malacologia 1:45-119.

Cruz, R. A. 1984a. Algunos aspectos de la reproduccion en Anadara tuberculosa (Pelecypoda: Arcidae) de Punta Morales, Puntarenas, Costa Rica. Rev. Biol. Trop. 32(1):45-50.

Cruz, R, A. 1984b. Algunos aspectos reproductivos y variacion mensual del indice de condicion de Anadara similis (Pelecypoda: Arcidae) de Jicaral, Puntarenas, Costa Rica. Brenesia 22:95-105.

Cruz, R. A. 1987. The reproductive cycle of the mangrove cockle. Anadara grandis (Bivalvia: Arcidae) in Costa Rica, Brenesia 27:1-8.

Elliott, J. M. 1977. Some methods for the statistical analysis of samples of benthic invertebrates. Freshwater Biological Association Scientific Publication 25. Titus Wilson & Son LTD, Kendal, U.K.

Eversole, A. G. 1989. Gametogenesis and spawning in North American clam populations: implications for culture. In: J. J. Manzi & M. A. Castagna, editors. Clam mariculture in North America. New York: Elsevier. pp. 75-109.

Howard, D. W. & C. S. Smith. 1983. Histological techniques for marine bivalve mollusks, NOAA Technical Memorandum NMFS- F/NEC-25, 97 pp.

Loosanoff. V. L. & H. C. Davis. 1963. Rearing of bivalve mollusks. Advances in Marine Biology 1:1-136.

MacKenzie. C. L. 2001. The fisheries for mangrove cockles. Anadara spp., from Mexico to Peru, with descriptions of their habitats and biology, the fisherman's lives and the effects of shrimp farming. Marine Fisheries Review 63(1):1-39.

Manzi, J. J. & M. A. Castagna, editors. 1989. Clam mariculture in North America. New York: Elsevier.

McGraw, K. A., M. Castagna & S. D. Dennis. 1996. Population structure of the arkshell clams Noetia ponderosa and Anadara ovalis in the oceanside lagoons and tidal creeks of Virginia and implications for fisheries management. Final Report submitted to Saltonstall-Kennedy Program Office of the National Oceanic and Atmospheric Administration National Marine Fisheries Service. NOAA Grant No. NA46FD0339.

McGraw, K. A., M. Castagna & S. D. Dennis. 1998. The arkshell clams Noetia ponderosa and Anadara ovalis in the Oceanside lagoon system of Virginia: a study of predation, reproductive biology and condition index. Final Report submitted to Saltonstall-Kennedy Program Office of the National Oceanic and Atmospheric Administration. National Marine Fisheries Service, NOAA Grant No. NA66FD0010

Nie, Z-Q. 1990. The culture of marine bivalve mollusks in China. In: R. W. Menzel, editor. Estuarine and marine bivalve culture. Boston. Ann Arbor & Boca Raton: CRC Press. pp. 261-276.

Power, A. J. & R. L. Walker. 2001. Growth and survival of the blood ark Anadara ovalis (Bruguiere, 1789) cultured in mesh bags on soft-bottom sediments in the coastal waters of Georgia, Journal of the World Aquaculture Society 32(3):269-277.

Power, A. J. & R. L. Walker. 2002. Growth and gametogenic cycle of the blood ark Anadara ovalis (Bruguiere, 1798) in coastal Georgia. J. Shellfish Res. 21(1):157-162.

Rehder, H. A. 1981. National Audubon Society field guide to North American seashells. New York: Alfred A. Knopf.

Spruck, C., R. L. Walker, M. Sweeney & D. Hurley. 1994. Gametogenic cycle in the non-native Atlantic surfclam, Spisula solidissima (Dillwyn, 1817), cultured in the coastal waters of Georgia. Gulf Research Reports 9:131-137.

Sturmer, L. N., E. Quesenberry, J. Scarpa & D. E. Vaughan. 1995. Development of a shellfish aquaculture industry on the west coast of Florida. World Aquaculture Society '95, Book of Abstracts, San Diego, CA, p. 279.

Thompson, R. J., R. I. E. Newell, V. S. Kennedy & R. Mann. 1996. Reproductive processes and early development. In: V.S. Kennedy, R. I. E. Newell & A. F. Able, editors. The Eastern oyster Crassostrea virginica. College Park, MD: Maryland Sea Grant College, University of Maryland System. pp. 335-370.

Umezawa, S. 1992. Experimentation to improve recruitment of blood ark shell, Scapharca broughtonii, in the Seto Inland Sea. NOAA Technical Report National Marine Fisheries Service 106:105-114. Walker, R. L. 1998, Growth and survival of the blood ark, Anadara ovalis (Bruguiere, 1789), in coastal Georgia. Georgia Journal of Science 56: 181-200.

Walker, R. L. & A. J. Power. 2003, Volume 18. Growth and gametogenic cycle of the transverse ark, Anadara transversa (Say, 1822) in coastal Georgia. American Malacological Bulletin.

Walker, R. L. & K. W. Gates. 2001. Survey of ark, Anadara ovalis, Anadara brasiliana, and Noetia ponderosa, populations in coastal Georgia. Athens, GA: School of Marine Programs, Marine Technical Report Number 01-2. 34 pp.

Walker, R. L. & P. B. Heffernan. 1994. Temporal and spatial effects of intertidal exposure on the gametogenic cycle of the northern, Mercenaria mercenaria (Linnaeus, 1758), in coastal Georgia. J. Shellfish Res. 13:479-486.

ALAN J. POWER, (1) JOSE NUNEZ, (2) MARCY MITCHELL, (1) RANDAL L. WALKER (1) AND LESLIE STURMER (2) (1) Shellfish Research Laboratory, University of Georgia Marine Extension Service, Savannah, Georgia 31411-1011; (2) Whitney Laboratory, University of Florida, St. Augustine, Florida 32080