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The Seasonal Abundance and Distribution of the Bivalve Lyonsia hyalina (Anomalodesmata: Pandoracea) on a Disturbed New England Mudflat
Kenneth A. Thomas and Mark D. Clements

Northeastern Naturalist, Volume 24, Issue 3 (2017): 300–316

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Northeastern Naturalist 300 K.A. Thomas and M.D. Clements 22001177 NORTHEASTERN NATURALIST 2V4(o3l). :2340,0 N–3o1. 63 The Seasonal Abundance and Distribution of the Bivalve Lyonsia hyalina (Anomalodesmata: Pandoracea) on a Disturbed New England Mudflat Kenneth A. Thomas1,* and Mark D. Clements1 Abstract – We investigated a casually recognized pattern of seasonal abundance exhibited by a population of the simultaneously hermaphroditic anomalodesmatid bivalve Lyonsia hyalina (Glassy Lysonia) at Bluff Hill Cove, Galilee, RI, and quantified the distribution of these intertidal to subtidal individuals from June 1993 through April 1994. Density in late spring averaged less than 2 individuals m-2 and was followed by a summer explosion of up to 200 individuals m-2 in localized areas, with an observed patchy distribution. A subsequent massive autumn mortality occurred, when the population returned to the previous low numbers. We propose several factors including predation, reduced temperature, and natural senescence as causes of the autumnal decline. Introduction Tidal flats have gained increasing attention as an endangered habitat and are commonly threatened by exploitation, pollution, and over-development (Reise 1985). Examination of the large numbers of organisms within the sediment indicates that these areas are important for many species of molluscs and polychaetes. They are an important food resource and shelter for a variety of temporary residents, act as nurseries for many fish and crustaceans, support numerous shellfisheries, and serve as a feeding ground for millions of shorebirds (Wolff 1987). Many tidal flats worldwide are threatened, such as those of the Yellow Sea where 50–80% of the tidal flats have declined over the past 50 years (Murray et al. 2015). Tidal flats and their biotic component thus merit serious attention and should be studied and conserved for future generations. The tidal flats of southern New England are subject to moderate tidal exchange with a large range of associated temperature changes, local disturbances, marked seasonal changes in weather, and bouts of seasonal predation. An organism's longterm survival in these areas depends on its ability to respond to these changes. One species that exhibits such adaptability is the bivalve mollusc Lyonsia hyalina (Conrad) (Glassy Lyonsia) that has been shown to rapidly colonize recently dredged habitats (Kaplan et al. 1975). This organism was previously reported to occur along the eastern North American Atlantic coast from Nova Scotia to South Carolina and inhabits sandy mud substrata, intertidally to a depth of 30 m (Abbott 1974), while others have reported distribution as far south as Florida and the Gulf of Mexico (Pimenta and Oliveira 2013). 1Department of Natural Sciences, Northern Essex Community College, 100 Elliott Street, Haverhill, MA, 01830-2399. *Corresponding author - kthomas@necc.mass.edu. Manuscript Editor: Melisa Wong Northeastern Naturalist Vol. 24, No. 3 K.A. Thomas and M.D. Clements 2017 301 The biology of the Glassy Lyonsia is known through the work of Chanley and Castagna (1966) and Chanley and Andrews (1971) where the development of larvae from fertilization until settlement is described. During spawning under laboratoryinduced conditions where water temperature was raised to 24–25 °C, Glassy Lyonsia released 8000–16,500 eggs during a single spawn (Chanley and Castagna 1966); this temperature is comparable to maximum summer field temperatures in southern Rhode Island, where the study described in this paper was conducted. Within 24 hours post-fertilization, larvae develop into the “straight hinge” stage, and later metamorphose over days 3–5 to lose the velum, gain gills, develop a ciliated foot, and become capable of attaching themselves to the substrate with a byssal thread (Chanley and Castagna 1966). These larvae resemble those of Pandora gouldiana (Dall) (Gould Pandora), but are shorter in length and probably have no need for an outside food source before settlement because of their short pelagic period, large egg yolk, and small size at metamorphosis (Chanley and Castagna 1966). Chanley and Castagna (1966) also showed that this species is capable of self-fertilization with no apparent developmental issues up to time of metamorphosis. Prezant (1977; 1979a, b) studied aspects of the functional morphology and the ultrastructure of the arenophilic radial mantle glands, and Thomas (1993, 1996) conducted an investigation of the microanatomy and histology of the digestive system (Thomas 1993) and an ultrastructural analysis of gametogenesis (Thomas 1996). Glassy Lyonsia has been shown to be a first intermediate host of either or both of the bucephalid trematode fish parasites, Rhipidocotyle transversale (Chandler) and R. lintoni (Hopkins), where branching sporocysts and furcocercous cercariae have been observed in both the gonads and digestive glands (Stunkard 1976). Although these biological aspects of the Glassy Lyonsia are generally understood, the ecological dynamics of this organism remain generally unknown. The Glassy Lyonsia is an opportunistic species that will populate recently disturbed areas (Kaplan et al. 1975). This species has very short siphons and settles within the top layer of sandy sediment, often with the valves partially exposed (Virnstein 1979). This proximity to the sediment surface makes it vulnerable to bottom-feeding fish and crab predators such as Leiostomus xanthurus (Lacepède) (Spot Croaker) and Callinectes sapidus (Rathbun) (Blue Crab) in Chesapeake Bay (Virnstein 1977, 1979), Pogonias cromis (L.) (Black Drum) in Texas coastal bays (Cate and Evans 1994), and Pseudopleuronectes americanus (Walbaum) (Winter Flounder) in the Lower Hudson-Raritan Estuary (Steimle et al. 2000). It was anecdotally reported that during the summer months numerous Glassy Lyonsia were located on the tidal mudflat at Bluff Hill Cove, Galilee, RI. This population could be easily accessed during middle to low tide by simply walking out on the mudflat and sieving the sediment (R. Bullock, University of Rhode Island, Kingston, RI, 1988 pers. comm.). A repeating pattern of summer abundance was consistently observed from 1988 to 1992 (K.A. Thomas, pers. observ.). Specifically, an increase in abundance began in late spring to peak in the summer, followed by a period of mortality in September and October with a huge population decrease through November and December, to remain low overwinter until the cycle was Northeastern Naturalist 302 K.A. Thomas and M.D. Clements 2017 Vol. 24, No. 3 repeated the following year. Typically, in New England, this organism is found in subtidal areas and is regionally localized (R. Prezant, Southern Connecticut University, New Haven, CT, 1990 pers. comm.), so this readily accessible population provided us with the unique opportunity to easily examine the population dynamics in this species. This study was conducted in a temporally stratified manner to quantitatively describe the seasonal population shifts in abundance of this species at Bluff Hill Cove. This information contributes to the limited body of ecological knowledge available for members of the Anomalodesmata, and provides an important description of an organism well-suited to opportunistically colonize disturbed intertidal habitats. These data provide a basis for comparison to Lyonsia populations in similar but undisturbed habitats, which would aid in assessing the impact of human activity. Thus, the Glassy Lyonsia may be a potentially useful species in environmental assessment. Field-Site Description The study area at Bluff Hill Cove, Galilee, RI, is located at 41°38'N, 71°50'W (Fig. 1A–C). This ~26-ha mudflat is located in the southeastern corner of Point Judith Pond, a tidal inlet close to Block Island Sound, off the coast of southern Rhode Island (Fig 1A). It is a relatively shallow flat partially isolated by several islands to the north with northeastern access to Point Judith Pond, and a narrow western channel that leads to the south (oceanic) end of Point Judith Pond (Fig. 1B). The mudflat is bordered on the east by the mainland and south by a saltmarsh and is proximal to, though protected from, the seaward opening of its encompassing salt pond. As a result, this mudflat is regularly flushed with the semidiurnal tides. An access road atop a berm borders the southern portion of the mudflat, and several small outlets pass under this road to an isolated marshy region that serves as a bird sanctuary and borders a barrier beach 0.6 km to the south. The surrounding area is well-developed, and is a popular tourist destination that contains many summer residences. Before, during, and after this study, the mudflat was subject to frequent small-scale sediment disturbances caused by recreational clam fishing, especially during the summer months. During these events, the sediment was often raked, dug 10–30 cm, and piled into mounds by clammers, the result of the search for Mercenaria mercenaria (L.) (Quahog) and Mya arenaria (L.) (Soft-Shell Clam). During the late fall, winter, and early spring, the area had little fishing activity . Methods This study of abundance of the Glassy Lyonsia was conducted from June 1993 to April 1994. We established 3 parallel NNE/SSW transects (Fig. 1C) 35 m apart on the mudflat along the tidal gradient (labelled I, II, and III from west to east). Along each transect, we sited four 15 m x 15 m quadrats. Transect I contained quadrats 1–4, transect II contained quadrats 5–8, and transect III contained quadrats 9–12. Three quadrats high on the tidal gradient and closest to the shoreline —numbers Northeastern Naturalist Vol. 24, No. 3 K.A. Thomas and M.D. Clements 2017 303 Figure 1. Study area at Bluff Hill Cove, Galilee, RI (41°38'N, 71°50'W). (A) Map showing study site in relation to Massachusetts, Connecticut, Rhode Island (dark gray), and New York. (B) Lower Point Judith Pond; study site indicated by arrow. (C) Mudflat and sandbar at Bluff Hill Cove, illustrating exposure at mean low water. Transects I–III were established along a NNE/SSW line, with 4 quadrats per transect. Quadrats 1, 5, and 9 were abandoned early in the study due to excessive disturbances caused by recreational clam fishing. During mean high tide, all areas shown except “Saltmarsh” became inundated with seawater. On the flood tide, water flow was generally west to east, and opposite that on the ebb tide. At low water, flow became more tortuous; during spring low tides, quadrats 3, 7, and 11 often became an isolated shallow pool. 1, 5, and 9—were abandoned early in the study because of excessive human disturbance due to local clamming activity and a subsequent lack of intact specimens in these areas. Quadrats 2, 6, and 10 were established adjacent to one another and located in the mid-tidal region at the 250-m mark on each transect. Quadrats 3, 7, and 11 were situated in a subtidal channel that ran diagonally across the transects and located at the 305-m, 312.5-m, and 322.5-m marks, respectively. Quadrats 4, Northeastern Naturalist 304 K.A. Thomas and M.D. Clements 2017 Vol. 24, No. 3 8, and 12 were located at the 373.3-m mark of each transect, adjacent to each other on a sandbar. This area became exposed unevenly at ebb tide beginning 30–45 minutes previous to quadrats 2, 6, and 10, exposing quadrat 4 first, then quadrat 8, and finally quadrat 12. Water flow was generally perpendicular to the transects during flooding and ebbing, moving west to east on the incoming tide (transects I to III) and east to west (transects III to I) on the outgoing tide. The area surrounding quadrats 3, 7, and 11 (in the subtidal channel) sometimes becomes a large isolated pool during spring low tides; however, during lesser tides, flow through this channel continued during low tide, often in a tortuous manner. We marked quadrats at each corner with a 1-m length of PVC pipe (1.25 cm diameter) submerged into the substratum until 4–5 cm remained exposed. Many of the wooden stakes (2 cm x 2 cm x 1 m long) we deployed outside each corner for quick quadrat identification were lost during the study period; however, all PVC pipes were retained and allowed for exact quadrat location. We subdivided quadrats into 900 subsequently numbered 0.5 m x 0.5 m sample plots. On sampling days, we used a random number generator to select four 0.5 m2 plots for each quadrat and sampled without replication during the study. Note that 3 plots/quadrat were sampled on 27 June 1993, but we increased the sample size to 4 plots/quadrat thereafter due to initially low collection numbers. For each quadrat, an area of 19.75 m2 was surveyed during the study over the course of 20 separate sampling days and covered 8.78% of the total area within each quadrat. With all quadrats combined over the entire study, a total area of 177.75 m2 was sampled. We estimated that this mudflat covers an area of 260,000 m2 (using Google Earth’s polygon area tool); thus, we examined approximately 0.068% of the entire mudflat. Samples (including live organisms and shell fragments) were collected in a temporally stratified manner, so that the most extensive sampling was performed during the known period of peak abundance (i.e., weekly in summer with a gradual tapering in frequency during autumn, until a monthly sampling period was established in November and maintained until March, for the remainder of the study). We recorded water temperature using an alcohol-based field thermometer in an area of flowing water near the study transects on 17 sampling days (water temperature was also recorded for several periods prior to, and after this study). We surveyed the plots with the use of a 0.5 m x 0.5 m (internal dimensions) PVC square, placed on the sediment surface. The top 5–6 cm deep layer was collected and passed through a 2.5-mm–mesh sieve, outside and down-current from the quadrat to minimize sediment disturbance within the quadrats. Retained fractions were bagged and preserved with a 5% formalin/seawater mixture. Sampling was tidally coordinated over the study area so that on sampling days we usually took collections when the quadrats were covered with 0.25 to 0.5 m of water. Care was taken to create minimal disturbance when measuring and sampling within the quadrats. Selected sample plots were occasionally subject to recent human disturbance (most frequently during the su mmer months), in which case we substituted the nearest undisturbed plot in place of the disturbed plot. Samples were brought into the laboratory where we sorted them under a dissecting microscope to isolate whole Glassy Lyonsia and any shell fragments. Northeastern Naturalist Vol. 24, No. 3 K.A. Thomas and M.D. Clements 2017 305 We recorded the location, number, and overall length (measured to the nearest 1 mm using an ocular micrometer and dissecting microscope) of individuals in all plots. Collected individuals were stored in 70% ethanol following the initial fixation in 5% formalin/seawater, usually within 1–2 weeks. We noted the presence and number of shell fragments collected simply as a gross estimation of mortality and counted equally all fragments, regardless of size. Some fragments were nearly complete shells and occasionally attached to the complementary shell, while others represented only a small portion of the individual from which it came. It is unknown if multiple fragments collected in the same sample were from the same individual, but it was assumed this would not have readily occurred, because Lyonsia shells are fragile and likely break down quickly into smaller, less noticeable fragments in situ that would not be retained during collection. We consider the naturally occurring presence of noticeable shell fragments in the field samples to be direct evidence of recent mortality. We tabulated specimen size and relative age (assuming individual Glassy Lyonsia grow at equal rates, and that larger individuals are older) using 4 separate size classes marked as categories 1–4 as follows: category 1 = 3–4 mm long, category 2 = 5–7 mm in length, category 3 = 8–10 mm long, and category 4 were Glassy Lyonsia ≥ 11 mm in length. Results Overall size frequency distribution Overall, we collected a total of 3284 individual specimens of Glassy Lyonsia, varying from 3–19 mm in length, with a mean size of 6.99 ± 0.04 mm (all means are reported with ± 1 SE) (Fig. 2). Of all individuals collected, 65.23% were ≤7 mm in size, with approximately equal numbers of 5 mm (21.22%) and 6 mm (20.1%) individuals, and fewer 7 mm (14.83%) individuals. Larger individuals were less common (8 mm = 11.88% of total; 9 mm = 8.80% of total; 10 mm = 6.15% of total; 11 mm = 3.53% of total; 12 mm = 1.95% of total; 13 mm = 1.00 % of total). Glassy Lyonsia >13 mm were rare and comprised 1.46% of total observations. Mean size vs. density Examination of the mean size vs. density of Glassy Lyonsia (Fig. 3) shows that in late June average size was large (10.78 ± 1.16 mm) while density was low (2.07 m-2). As density began to increase in mid-July (8.22 m-2), average size declined (5.57 ± 0.21 mm). Density then sharply increased over summer and peaked in August at 59.44 m-2, with localized densities as high as 200 m-2, while size increased slowly to an average of 6.80 ± 0.09 mm. A sharp decline in density followed in September (24.33 m-2), concurrent with continued average size increase (8.02 ± 0.18 mm), followed by a sharp decline (6.50 ± 0.15 mm). Density continued to decline steadily during fall while average size increased (9.69 ± 0.66 mm). This density decline continued (2.89 m-2 in mid-December) with a winter low in February (0.78 m-2, 7.00 ± 1.27 mm) and March (1.33 m-2, 9.33 ± 1.14 mm), with sizes approaching that of individuals observed the previous spring. Northeastern Naturalist 306 K.A. Thomas and M.D. Clements 2017 Vol. 24, No. 3 Water temperature Water temperature (Fig. 4) warmed during the month of June, ranged from 22 °C to 26 °C during July–September, and droppped through the fall and early winter to reach a minimum of 2 °C from January through March. Water temperature then began an annual spring increase during the month of April. Shell-fragment analysis Analysis of Glassy Lyonsia shell fragments (Fig. 5) shows that the lowest density occurred in late June (10.50 ± 2.84 fragments m-2). This number increased during the next sampling period (20.78 ± 4.12 fragments m-2) and remained steady for several weeks. A rapid increase followed over the next 8 weeks, to peak at 162.56 ± 15.68 fragments m-2 during mid-September, following the peak in live individual density by 1 month. Density of shell fragments declined steeply, then steadily, to 11.89 ± 1.68 fragments m-2 in March. Frequency distribution by quadrat The number of individuals varied across quadrats (n = 190–593; Table 1) with a patchy distribution and highest relative abundances in quadrats 11 (n = 593) and 12 Figure 2. Frequency distribution by size. In this study, Lyonsia hyalina (Glassy Lysonia) varied from 3 mm to 19 mm in length, and over half collected were 5–7 mm. The abundance of animals in the 3–4 mm range probably are an underrepresentation due to a lack of sieve retention, with actual numbers likely exceeding those in the 5–6 mm range. Northeastern Naturalist Vol. 24, No. 3 K.A. Thomas and M.D. Clements 2017 307 (n = 551), both located on transect III (Fig. 1C). A large number of individuals were also observed in quadrats 2 (transect I; n = 420) and 7 (transect II; n = 396) with lower relative abundances on either side of this patch. On the sandbar, abundance fell dramatically toward quadrat 4 (quadrat 4: n = 190, quadrat 8: n = 324) where aerial exposure was the greatest. Figure 3. Average size vs. density of Lyonsia hyalina (Glassy Lysonia). At the beginning of this study, size (average length) was highest, while density was very low. Density abruptly increased during the summer months, then dropped sharply in late summer. During this initial increase in density, size dropped quickly, indicating that a recruitment event took place. Table 1. Number of Lyonsia hyalina (Glassy Lysonia) a per quadrat. Some quadrats had many fewer organisms than others. The abundance was highest in quadrats 11 and 12, showing the patchy nature of this species. Quadrat n 2 420 3 226 4 190 6 292 7 396 8 324 10 292 11 593 12 551 Northeastern Naturalist 308 K.A. Thomas and M.D. Clements 2017 Vol. 24, No. 3 Figure 4. Water temperature at Bluff Hill Cove during the study period (June 1993–March 1994). Water temperature the month prior to the beginning of the study was 19 °C (11 June 1993), followed by a warming trend over the summer, then a slow steady cool down during autumn to reach winter lows in January. The beginning of the subsequent spring warming period can be seen at the end of the study. Figure 5. The number of Lyonsia hyalina (Glassy Lysonia) shell fragments per m2. Fragments were initially low in the spring and mirrored the increase in live animal abundance, but ~1 month later, likely to be a reflection of major mortality of this species which increased in summer and peaked in late September. Northeastern Naturalist Vol. 24, No. 3 K.A. Thomas and M.D. Clements 2017 309 Frequency distribution by size class By individual quadrat, Category 1 Glassy Lyonsia (3–4 mm) averaged 9.01% (± 0.44%) of the total Glassy Lyonsia collected (Table 2), with a narrow range, and were observed most often in quadrats 11, 2, 7, and 12. Category 2 (5–7 mm) was the most abundant size class and these Glassy Lysonia averaged 55.82% (± 3.32%) of total found per quadrat, with a wide range, and were seen most often in quadrats 11, 2, 12 and 7. Category 3 individuals (8–10 mm) collected averaged 26.94% (± 2.08%) of total, with a moderate range, and were found most often along rransect III in quadrats 11, 12 and 10. Category 4 individuals (≥11 mm) averaged 8.23% (± 1.75%) of total, also with a moderate range, and were collected in greatest numbers from quadrats 12, 8, 11, and 4. Overall temporal frequency distribution by size class The overall incidence of category 1 individuals across the study area gradually increased during the summer (Fig. 6) with a distinct initial peak demonstrated in the sample taken on 14 August 1993 (n = 46), and a second spike 1 month later (14 September 1993, n = 23). There was a dramatic increase in the abundance in category 2 individuals (starting in early July to spike on 14 August 1993 (n = 320). A rapid drop in abundance ensued over the next several weeks, followed by a second smaller spike in abundance (14 September 1993, n = 138) and further drop, marking the beginning of a slow steady decline to reach winter lows. Category 3 Glassy Lyonsia began to increase 2–3 weeks after the category 2 increase, to spike on 27 August 1993 (n = 180), followed by a rapid decline over the next 3 weeks (n = 33 on 14 September 1993). Numbers fluctuated for 2 months, then subsequently dropped to reach winter lows. A less dramatic increase began in category 4 individuals 3–4 weeks after the category 3 onset and 4–5 weeks after the category 2 onset, with a 27 August peak (n = 59). As with the other categories, there was a sharp drop off in abundance followed by low numbers in the late fall that tapered in January to reach winter lows. Table 2. Overall frequency of Lyonsia hyalina (Glassy Lysonia) by size per quadrat. Category 1 Category 2 Category 3 Category 4 Quadrat ≤4 mm % 5–7 mm % 8–10 mm % ≥11 mm % 2 44 10.48 277 65.95 84 20.00 15 3.57 3 22 9.73 152 67.26 42 18.58 10 4.42 4 16 8.42 78 41.05 66 34.74 30 15.79 6 21 7.19 185 63.36 71 24.32 15 5.14 7 42 10.61 258 65.15 83 20.96 13 3.28 8 25 7.72 150 46.30 97 29.94 52 16.05 10 29 9.93 139 47.60 103 35.27 21 7.19 11 57 9.61 333 56.16 170 28.67 33 5.56 12 41 7.44 273 49.55 165 29.95 72 13.07 Mean 9.01 55.82 26.94 8.23 SE 0.44 3.32 2.08 1.75 Median 9.61 56.16 28.67 5.56 Northeastern Naturalist 310 K.A. Thomas and M.D. Clements 2017 Vol. 24, No. 3 Temporal frequency distribution by size class within individual quadrats In the temporal analysis of size classes within individual quadrats (Fig. 7), differences in category 1 are difficult to resolve because of low abundances. However, in all quadrats there was a clear initial spike in category 2 individuals, usually between July and mid-August, often with a subsequent (and sometimes third) peak, and in some quadrats (2, 4, 6, 8, and 12) this delayed spike exceeded the magnitude of the first spike in category 2. In quadrats 3, 7, and 11 (all of which lie within the subtidal channel), this second spike was delayed until mid-September. In all quadrats, abundance of category 3 individuals increased 1–4 weeks after the initial spike for category 2 Glassy Lysonia. Category 4 individuals show a clear abundance spike in only 4 quadrats (4, 8, 11, and 12) in late summer, always after the spike of category 3 Glassy Lysonia. All quadrats show tapering of abundance in every size class through the fall and into the winter. Discussion This study quantifies a pattern similar to that observed casually over the previous 4 years, and one described anecdotally for several years prior (R. Bullock, 1988 Figure 6 . Relative abundance by size of Lyonsia hyalina (Glassy Lysonia). The number of individuals collected in each of the size classes (categories 1–4) are shown over the study period. All categories showed a large increase during the summer which culminated in peak abundance during middle to late August. Abundance in all size classes decreased to reach winter lows during November or December. Northeastern Naturalist Vol. 24, No. 3 K.A. Thomas and M.D. Clements 2017 311 pers. comm.). During the spring, there was an initially low abundance of large-sized Glassy Lyonsia, followed by a water temperature increase and population explosion during the summer, indicating massive recruitment, with a subsequent crash in the fall. Over the summer, average individual size slowly increased (Fig. 3), and size-category peaks shifted sequentially (Fig. 6), suggesting that survivors systematically grew into the next size class. Although average size of individuals dropped during July and August, the dramatic increase in population likely attracted benthic-feeding predators such as Blue Crab, which has demonstrated optimalforaging type behavior (i.e., marginal value; Charnov 1976) during predation of the bivalve Macoma balthica L. (Baltic Macoma; Clark et al. 2000). As individuals grew over the summer, predation pressure, along with intra- and inter-specific competition and natural senescence, may have caused the sharp mortality increase that was observed in August and peaked in September. The declining numbers of Figure 7. Relative abundance of Lyonsia hyalina (Glassy Lysonia) by quadrat. This series of graphs illustrates the number of individuals collected temporally in each of the size categories (1–4) per quadrat over the study period. Northeastern Naturalist 312 K.A. Thomas and M.D. Clements 2017 Vol. 24, No. 3 individuals and presence of shell fragments indicate that a slow but steady die off continued during the late fall and tapered as winter arrived. During this time, the average size of individuals increased, leading to a comparatively small population of larger, overwintering adults that helped repopulate stocks the following spring. Although most individuals in the Bluff Hill Cove population appear to survive for only a few months, an annual growth line was observed in the shells of some category 4 individuals, which Prezant (1977; pers. comm.) used to estimate a 2–3 year lifespan in this species. In September, secondary spikes in abundance of category 1 and 2 individuals suggest that another recruitment event occurred in quadrats 2, 3, 7, and 12. Examination of chlorophyll-a data from nearby Station 2, West Passage, Narragansett Bay, RI (Narragansett Bay Phytoplankton Time Series, 1959–1997; NarrBay.org 2004) shows that chlorophyll-a spiked three weeks prior, probably due to an increase in phytoplankton, and may have contributed to these events. Hunt et al. (2009) tested predictions that bedload transport could dramatically affect bivalve distribution up to several kilometers over 1 month. They found their model to be consistent for the Soft-Shell Clam and a pooled group of bivalve species, including Glassy Lyonsia, in the Navesink River estuary, NJ. Their model, however, was inconsistent in the case of Gemma gemma (Totten) (Amethyst Gemclam), and they suggested that sandy sediment selection and lack of larval stages in this species influence its distribution (Hunt et al. 2009). Infaunal bivalves such as Soft-Shell Clams 3–10 mm in length are known to sometimes become entrained and moved around via bedload transport, so this may also occur with similarly sized Glassy Lyonsia (H. Hunt, University of New Brunswick, St. John, NB, Canada, 2017 pers. comm.). Here, localized bedload displacement of some individuals may have occurred, but the isolated nature of the study site makes it unlikely that this mechanism brought in a new cohort of individuals from outside the mudflat (H. Hunt, 2017 pers. comm.). Thus, we believe that the secondary abundance spikes observed were related to secondary larval recruitment and that the sequential spikes in size-category abundances reflect growth of individuals from smaller sizes, rather than entrainment from outside sources. The disturbed nature and physical characteristics of the mudflat at Bluff Hill Cove makes this habitat well-suited for Glassy Lyonsia. Interestingly, Prezant et al. (2008) found that another anomalodesmatan, Laternula rostrata (Sowerby), (corrected from L. truncata L.) maintained refugia in mangrove roots but were uncommon in open areas subjected to heavy shellfish harvesting. During the Glassy Lyonsia density peak in August, there were an average of 59 individuals m-2, with 1 localized, single plot density of 200 m-2, but by January average density had fallen to 2 m-2 and then to below 1 m-2 in February. The largest individual collected during the sampling period was 19 mm (n = 1), but the average size was much smaller (6.98 ± 0.04 mm). Prezant (1977) observed specimens as long as 9.8 mm in a Massachusetts Bay population of Glassy Lyonsia (sampled from depths of 15–18 m), although average size in that study was only 3.6–5.6 mm. Shells that washed up in Zostera marina L. (Eelgrass) mats from a Martha’s Vineyard, MA, salt pond were Northeastern Naturalist Vol. 24, No. 3 K.A. Thomas and M.D. Clements 2017 313 generally twice this size and up to 22.8 mm in length (Prezant 1977). Individuals as long as 24 mm were observed by Virnstein (1979) during fish-predator–exclusion experiments, and Prezant et al. (2002) reported a “giant” individual over 25 mm long in coastal Virginia. A “scarce to abundant in patches” distribution pattern for this species was described in a Virginia population by Chanley and Andrews (1971), and data from this study indicates that patchiness may have a size-dependent component. Smaller Glassy Lyonsia (3–7 mm) were found in greatest densities diagonally across the study area and account for more than 65% of all individuals in this study (Table 2, Fig. 7). These smaller individuals were least abundant in quadrat 4, which had the longest period of aerial exposure as compared with all other quadrats, suggesting that extended exposure decreases the survivability in juvenile and/or newly settled individuals. Glassy Lyonsia was previously described as capable of reproducing multiple times during its life (i.e., iteroparous) with 1 annual spawn (Chanley and Andrews 1971, Mackie 1984). Spawning is known to occur in March and April and possibly continues through May, June, and even July in coastal Virginia populations (Chanley and Andrews 1971). In coastal Massachusetts, spawning occurs in July and August (Prezant 1977), and evidence from this study indicates a major and minor recruitment event occurred in Rhode Island. This organism’s high fecundity, ability to self-fertilize, and short time as pelagic larvae prior to settlement may allow for a small number of gravid adults to reseed extensive seasonal populations like that observed on the mudflats at Bluff Hill Cove. Post-settlement growth data on this species appears lacking and growth time from larval size (150–180 μm) to category 1 size (3–4 mm in length) is previously unreported. Virnstein (1979) showed that Glassy Lyonsia flourished during enclosure experiments, and grew to sizes larger than normally found in unenclosed populations (up to 24 mm in 6 months or less). Assuming that growth was continuous over 6 months, then the average daily growth rate was 0.13 mm d-1. Using that rate, we extrapolate that it takes ~ 23 days for post-settled larvae to grow to 3 mm length. The mid-July drop in average size of Glassy Lysonia observed in this study (Fig. 3) indicates that a recruitment event had occurred and suggests that newly settled clams grew quickly over 2–3 weeks. Previously undetected individuals would have increased in size from 3.5 mm or less (smaller than sieve mesh size) to ~5.6 mm (average size in mid-July). Conservatively, growth calculated using these data show a rate of 0.10–0.15 mm d-1. However, peak abundance data for category 2 and 3 (Fig. 6) suggest a growth rate of 0.23 mm d-1, where individuals grew 3 mm over 13 days. The growth rate of Glassy Lyonsia appears comparable to that of Soft-Shell Clams, reported as approximately 0.13–0.20 mm d-1 for individuals 10–24 mm length (Chalfoun et al. 1994), but much greater than those of Corbicula fluminea (Müller) (Asian Clam) which has been reported as 0.03 mm d-1 in individuals less than 10 mm in length (Cataldo et al. 2001). It is known that some bivalve species will grow at reduced rates in response to the activity of predators. Quahogs, for instance, exhibit reduced growth rate in Northeastern Naturalist 314 K.A. Thomas and M.D. Clements 2017 Vol. 24, No. 3 response to the presence of the predatory snail Busycon carica (Gmelin) (Knobbed Whelk; Nakaoka 2000), and increased nutrient load will accelerate growth rate in Quahogs and Soft-Shell Clams (Chalfoun et al. 1994). If marine phytoplankton become more abundant, or if predator populations increase, Glassy Lyonsia may adjust their growth rate accordingly. Predation pressures on this species (from avian, molluscan, fish predators, etc.) likely occur at Bluff Hill Cove; however, future studies are needed to elucidate specific predatory patterns and if such predation reduces growth rate. Currently we lack clear understanding of the source of recruitment into Bluff Hill Cove. Based upon geographic proximity, individuals that make up this summer population explosion could be recruited from 3 sources: first, individuals that overwinter on the mudflat at Bluff Hill Cove; second, adjacent subtidal areas within Bluff Hill cove, but not on the mudflat (and hence are never exposed to direct atmospheric conditions); and third, a stock population(s) that overwinters in areas outside of Bluff Hill Cove. We know little about the hydrodynamics of this area, and such understanding would help determine source populations, perhaps indicating whether or not this population is genetically heterogeneous. Genetic analyses would help determine whether members of the Bluff Hill Cove mudflat population are interrelated and indicate if there is a seed population of overwintering individuals. Such studies may also help to identify if self-fertilization is a natural strategy employed by this species and should be included in future investigations. This study begins to describe what is apparently a complex system. Numerous ecological questions are generated from these observations regarding this organism’s recruitment, population dynamics, and role in the food chain. Future studies should be directed toward these questions including hydrodynamic analyses through the tidal cycle, quantification of planktonic and newly settled Glassy Lyonsia larvae, predator exclusion and mark/recapture experiments, and identification of nearby populations with comparative genetic analysis to the Bluff Hill Cove population. Acknowledgments We thank the following people for their assistance during field collection and help with specimen sorting: Jane Fieldhouse-Thomas, Joseph Grenier, Julie Hammer, Neil Hurley, Alex Mooza, Bradley Peterson, and Steve O’Neil. Thanks also to Bradley Peterson, Robert Prezant, and 1 anonymous reviewer for their critical comments of this manuscript. Heather Hunt provided insight into bivalve juvenile transport, and David Borkman supplied access to Theodore Smayda’s extensive Narragansett Bay phytoplankton data. Special thanks to the Rhode Island Department of Environmental Management for their continued encouragement and support of this research. This work was financially supported by the National Capital Shell Club’s Carl I. Aslakson Scholarship, the Western Society of Malacologist’s Student Research Grant in Malacology, the University of Rhode Island, and Northern Essex Community College. Northeastern Naturalist Vol. 24, No. 3 K.A. 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