Predation of Bay Scallops by Channeled and Knobbed Whelks
Busycon are known to prey upon bivalves with tightly closing shells (e.g. clams, oysters) while thinner-shelled Busycotyopus prefer to feed on bivalves with a larger shell gape, as well as carrion (Edwards and Harasewych 1988), but our observations confirm that channeled and knobbed whelks both prey upon live juvenile and adult bay scallops. The suggested preference of channeled whelks for larger scallops in lab experiments may be an artifact that reflects active swimming of juveniles at 25-30 mm (Tettelbach 1986), when peak levels of octopine dehydrogenase, the enzyme responsible for fueling bay scallop swimming, are seen (Garcia-Esquivel & Bricelj 1993) but this trend should be investigated further. Nevertheless, in spite of their swimming abilities, many juvenile scallops succumbed to predation <30 min after they were released into field plots.
The mechanics by which Busycotyopus and Busycon prey on scallops is still not clear. Most often, we observed whelks engulf the entire scallop shell with their foot, but on one occasion a channeled whelk inserted its shell lip between the valves of its prey. The latter is a common method employed by Busycon carica to open Mercenaria shells, but has rarely been observed with Busycotypus canaliculatus (Magalhaes 191). Prescott (1990) observed that knobbed whelks ate adult bay scallops (Argopecten irradians concentricus) in the laboratory, but did not observe any type of shell damage. In our lab experiments, most shells of scallops eaten by knobbed whelks were characteristically notched; this was never observed for Busycotypus. Although not observed directly, this type of damage may result from the ‘hammering’ method - by which pieces of Mercenaria shell margins are broken off prior to insertion of the proboscis (Magalhaes 1948).
Reduced rates of predation at lower water temperatures in our experiments parallel observations by baymen, who typically stop fishing for channeled whelks when they cease feeding in late November (P. Wenczel, pers. comm.). Relatively high rates of predation by whelks in the late August – early September 2013 trial were surprising, as this is the time of year when Busycotypus do not readily come to conch pots in the Peconic Bays (P. Wenczel, pers. comm), likely because they are mating/laying eggs instead of feeding (Edmundson 2016). Reduced catches of whelks in late August may also reflect higher water temperatures, as corroborated by the lack of predation in our 22-26 August 2012 trial.
Of 19,100 scallops planted in the field, 60% (11,471) were accounted for by the end of our experiments; 5.6% (1,067 of the total planted) were cluckers – which represents the minimum rate of predation. The loss of the other 40% of planted scallops may have been due to dispersal or predation. Relatively high rates of dispersal were illustrated by the transport of large numbers of cluckers outside the planted sectors (especially to the South) and by the rates of immigration of wild scallops into the grid; however, extensive searching beyond the grid perimeter, even out to a distance of ~15-20 m to the South (where the number of planted scallops and cluckers was very low), suggests that we recovered the majority of cluckers. Thus, much or perhaps all of the missing 40% of planted scallops (7,629) may have been lost to predation – whereby shells were crushed to smaller bits (i.e. to the point where they were not cluckers and could not be recognized as recently predated) or shells were removed from the area. Scup, Stenotomus chrysops, are abundant throughout the Peconic Bays and were observed within the planting area; since they swallow scallops whole (Weinstock 2010, Mladinich 2017) they (or other fishes) may be responsible for some of these scallop losses. If the missing 40% of planted scallops is added to the confirmed rate of predation (recovered cluckers = 5.6%), maximum cumulative predation amounts to 45.6% over the course of three field experiments: roughly 3-5% predation d-1. This aligns fairly well with empirical predation rates determined by Tettelbach (1986) for similar sizes of bay scallops planted in Connecticut over 1 week periods in the fall.
Since ~67% of recovered cluckers had shells that were cracked, chipped, pried or had hole punches, signatures most closely aligned with crustacean predation, we conclude that crabs were probably the most important cause of scallop mortality (Tettelbach 1986; Peterson et al. 1989; Prescott 1990). Cracked shells (36% of cluckers examined) are most likely the result of predation by large crabs – such as blue crabs, Callinectes sapidus, and large male spider crabs, Libinia emarginata; these are both common in the Peconic Bays and can consume scallops at high rates (Tettelbach 1986; Carroll et al. 2010).
Signatures of predation attributed to whelks in the field and observed rates of predation in the lab were both relatively low – suggesting that the overall importance of whelk predation on planted scallops is of moderate importance. Whelk predation was inferred when cluckers recovered from field experiments exhibited notches (n = 101) or no shell damage (n = 245): 9.5% and 23% of cluckers, respectively. The only other known predator of adult and large juvenile Peconic bay scallops that leaves no trace of shell damage is the common sea star, Asterias forbesi. However, this species has not been observed in the central and eastern Peconic Bays over the last 15+ years (Tettelbach et al. 2015). As whelks sometimes also left other traces of predation in lab experiments (9% chipped, 9% cracked), we inferred that another 2.6% of cluckers (n = 28) recovered from the field resulted from whelk predation. Added together, the cluckers that can be attributed to whelk predation represent 4.3% of total presumed scallop losses (374/8709) or 2% of all planted scallops (374/19,100). This may somewhat underestimate overall whelk predation, as 32% of scallops eaten by both species in lab experiments had disarticulated valves and thus would not have been counted amongst cluckers in field surveys. If the rate of predation on scallops planted in the field is calculated on the basis of the numbers of acoustic tagged (n = 30) and observed wild whelks (total = 157), this works out to a predation rate of 0.13 scallops eaten whelk-1 day-1 (= 374 scallops eaten/157 whelks/18 days). This is comparable to observed predation rates by knobbed whelks in our laboratory experiments.
Busycon carica appears to represent a greater threat than Busycotypus canaliculatus to planted and natural bay scallop populations in the Peconic Bays because of its much higher abundance (~9-10x higher than channeled whelks: Udelson et al. in prep) and higher (24x) rate of predation on scallops in the lab (this study). The importance of predation by knobbed whelks is also suggested by our observations of high frequencies of notched scallops in the Peconic Bays: at one site, where this whelk species is particularly abundant (south of Cedar Island Lighthouse, in Northwest Harbor, NY: Tettelbach et al. 2015), notched scallops comprised 33.3% (7/21) and 39.1% (43/110) of adult and juvenile scallop cluckers, respectively, in dive surveys from Fall 2015 – Spring 2018.
Whelk Movement and Behavior in Response to Scallop Plantings
Intensive predation of scallops by channeled and knobbed whelks right after the first planting suggests that emergence of buried whelks and foraging activity were both stimulated; mucus trails confirmed that whelks also immigrated into the area. The orientation of most whelks into the direction of the prevailing current, along with a sweeping, side to side motion, were the same behaviors exhibited by Busycon carica in flume experiments (Ferner and Weissburg 2005). This strategy is likely advantageous in that scallops may not detect the odor plume of an approaching whelk until they are almost in physical contact, reducing the window of opportunity for scallops to escape.
While the lack of sudden attraction of other large predators (crabs, finfish) to the seeded scallops is similar to what we observed in previous plantings (Tettelbach et al. 2011), others have observed a marked attraction of crabs (Boulding and Hay 1984, Barbeau et al. 1996) or sea stars (Tettelbach and Wenczel 1993) to high density scallop plantings. These differences may certainly reflect predator/prey densities or environmental factors (e.g. water temperature), but a better understanding of potential differences in predator response is important within the context of restoration efforts - where decisions regarding planting density may be central to their success (Tettelbach and Wenczel 1993, Tettelbach et al. 2013, 2015).
The greater overall amount of time spent by tagged whelks in the grid after it was planted with scallops versus controls (no scallops) was expected, as were the shorter distances traveled and lower movement rates by whelks in these trials (The Nature Conservancy, 2018). These patterns likely reflected more directed searching for prey/lower encounter rates in the absence of scallops (The Nature Conservancy, 2018), but surprisingly there was a lack of concomitant differences in exploratory behavior probability exhibited by whelks in predation trials. These metrics may reflect the findings of Ferner and Weissburg (2005), who determined that Busycon carica were able to successfully locate prey odor plumes at distances of >1.5 m in flume experiments; searching behaviors (e.g. side to side scanning) were reduced at higher flow velocities and in the presence of obstructions, so that whelks reached the prey odor source more quickly.
While tagged whelks spent more time encamped inside the grid when planted scallops were present, compared to control trials with no scallops, whelk typically exhibited a higher probability of an encamped behavioral state in sectors planted at lower (6.7 – 17.5 m-2) rather than higher (51.5 – 72.0 m-2) scallop densities - which is contrary to most animal movement studies. Encamped movement patterns are commonly characterized in landscapes where prey/resource abundance is high; thus, animal step lengths are short, with sharp turning angles, as animals perform fewer large-scale movements (Zollner and Lima 1999; Morales et al. 2004). Even at lower planting densities, which were ~4x those of the ambient wild scallop population, predator satiation and thus reduced foraging activity might have been expected on the basis of low daily rates of prey consumption in lab experiments (this study). The post-foraging lag time for appetite to return was determined to be 25-39 h for another temperate whelk species (Buccinum undatum) (Evans et al. 1996). While we did not examine scallop predation by whelks from a functional response standpoint, the unexpectedly higher amount of time spent by tagged individuals in grid sectors planted at lower scallop densities may reflect behavioral processes related to the modified Type III functional response described for sea stars, Asterias vulgaris, feeding on mobile sea scallops, Placopecten magellanicus (Wong et al 2006). These authors found that handling time and proportion of time spent searching for prey did not vary with prey density but suggested that the decreased foraging efficiency of sea stars observed at high prey densities may have resulted from conflicting stimuli from multiple nearby sea scallops (Wong et al. 2006). Interference competition from other predators (Gotelli 2008) within our high density sectors is another possible explanation for greater observed encampment at high versus low bay scallop densities. The potential presence of higher numbers of blue crabs (known predators of whelk and scallops) and spider crabs at higher scallop densities may have resulted in whelk burial/avoidance (Harding 2003; Cordero and Seitz 2014), contributing to more encamped behaviors for longer periods.
Mean overall movement rates of channeled whelk in field predation trials (average = 45.8 m d-1) and in control trials (223.5 m d-1) were much higher than those reported by Edmundson (2016) in Lake Tashmoo, a small embayment in Massachusetts, where she observed movement rates of 12 m d-1 from late October – early November for channeled whelks also tracked with Vemco acoustic tags. In other, longer term studies which employed traditional (non-acoustic) tags, daily movement rates were also lower than those for our short-term studies: 12 m d-1 for channeled whelks in Narragansett Bay, Rhode Island (Sisson 1972), and 18 and 0.7 – 7 m d-1, respectively, for knobbed whelks at Beaufort, North Carolina (Magalhaes 1948) and Wassaw Sound, Georgia (Shalack et al. 2007). The latter studies may have underestimated whelk movement rates because they were based on linear distances (start to end positions); however, because they encompassed periods of winter inactivity, annual rates of movement would expectedly be lower than those observed during warmer times of the year. In the study by Edmundson (2016), channeled whelks only moved an average of ~1 m d-1 over a 1 yr period. Nevertheless, total distances traveled by channeled whelks in other longer term (~10 mo) studies in larger bodies of water were considerable: up to 1.6 and 4.2 km, respectively, in Great South Bay, New York (Lynn 2018) and Narragansett, RI (Sisson 1972). These observations and suggestions of seasonality of channeled whelk movement, particularly in the spring and fall, that have come from many years of field observations by baymen (F. Sloup, P. Wenczel, pers. comm) deserve further study.
The above observations, and those from the present study, provide important insights into the timing and range of movement of mobile gastropods - which in turn may have management implications. For instance, Glazer et al. (2003) determined that the aggregated home range of queen conch (Strombus gigas) was twice as large as a designated no-take reef in the Florida Keys, resulting in incomplete protection. An acoustic telemetry study of giant triton snails (Charonia tritonis) revealed their average daily movement rate (234.2 m d-1) was 23x the maximum daily movement rate of their prey, the crown-of-thorns starfish (Acanthaster planci) - suggesting tritons could successfully control population outbreaks of the latter species in sections of the Great Barrier Reef (Schlaff et al. 2020). Additional data on the movement rates of channeled and knobbed whelks for longer durations than our trials and across seasons would provide a more comprehensive picture of spatio-temporal variability in whelk-scallop interactions, movement and behavior (Sperry et al. 2008) and thus might help inform the timing of scallop plantings to improve survival and hence success of restoration programs.
Mean rates of movement by whelks in our control trials (223.5 m d-1) were considerably higher than in predation trials. This may have reflected greater searching in the absence of scallop prey, as discussed above. However, in the first control trial, greater dispersion and directed movement of tagged whelks to deeper offshore waters (to the northeast), perhaps to avoid increased wave energy and exposure, may well have been precipitated by a gale that blew in right at the time of planting. Thus, wind direction and wind speed were well-supported predictors of whelk movement in the best models for control trials, but they were not in experimental trials.
Effects of atmospheric phenomena on animal behavior have been described for a variety of taxa but are much more well-known for vertebrates than invertebrates (Massie et al. 2019; Strickland et al. 2020). For example, rock blackfish (Girella elevata) in shallow subtidal habitats responded sharply to increased wind speed by moving to deeper depths - likely to avoid greater wave height (Stocks et al. 2015). Blacktip sharks were capable of anticipating incoming hurricanes along the Florida Gulf Coast with as little as a 5mb drop in barometric pressure, without large changes in wind speed, and temporarily migrated to deeper waters to avoid storm exposure (Heupel et al. 2003). Other fish, such as summer flounder, responded to declines in average atmospheric pressure of 4 mbar/week by increasing emigration from estuaries to deeper waters on the continental shelf off New Jersey (Sackett et al. 2007). Amongst insects, rapid changes in barometric pressure (30 mbar h-1) resulted in reduced flight initiation frequency in polyphagous wasps (Trichogramma spp.) (Fournier et al. 2005). Pellegrino et al. (2013) found that curcurbit beetles (Diabrotica speciosa) showed reduced locomotive activity when exposed to decreasing barometric pressure at rates as low as 0.4 mbar h-1.
Atmospheric pressure was not a strong predictor of whelk behavioral switching in any trial, which was surprising considering that whelks in the control trials (1, 5) experienced average ranges of 20.5 mb: 2x higher than those during predation trials (2, 3, 4). However, it is important to note that average distance traveled was nearly 4x higher in control trials when atmospheric pressure was declining, indicating that whelk locomotive activity may be partially responsive to changes in barometric pressure. Although we do not know the specifics of how whelks sense atmospheric pressure, it is known that both marine invertebrates and vertebrates without swim bladders detect small changes (5-10 mbar) in hydrostatic pressure via vestibular hair cells (Fraser and MacDonald 1994; Fraser et al. 2003). Our novel results strongly suggest that gastropods respond to weather events; this certainly deserves further study.
The importance of time of day in explaining channeled whelk movement patterns, as supported in HMMs for both control and treatment trials, with highest observed activity in early evening/twilight hours, corroborates the findings Magalhaes (1948). While we cannot pinpoint the mechanism(s) behind the crepuscular activity pattern within the confines of our dataset, predator avoidance (Lima and Dill 1990) is a plausible explanation. Blue crabs (see above) are most likely to exhibit agonistic and defensive behaviors during this time (Clark et al. 1999). Diurnal patterns in dissolved oxygen (DO) levels may also potentially influence whelk activity patterns. Greater locomotion of another large gastropod, queen conch (Lobatus gigas), is suggested to coincide with higher DO concentrations in late afternoon/evening (Dujon et al. 2019).
Pots used to commercially fish for whelks in New York are typically spaced at distances of >50 m (F. Sloup, P. Wenczel, pers. comm); however, distances from which whelks travel to baited pots are not precisely known. Our work suggests that both channeled and knobbed whelks may be drawn to high density scallop plantings. Given this, and the fact that both whelk species can consume juvenile as well as adult scallops, these behaviors should be factored into decisions concerning site selection and other potential strategies (e.g. predator removal) employed in bay scallop restoration.
Examining the behavioral responses of predators to external surroundings and prey density is crucial for identifying both management and ecological implications (Schmitz and Barton 2014). We have provided novel insight into predator-prey interactions among three commercially important marine invertebrate species in both the field and lab. Despite the relatively low scallop predation rate observed for both channeled and knobbed whelks in the lab, field experiments revealed that whelks were probably responsible for 4.5% of scallop mortality and that 45% of all planted scallops were lost after 14 days in the field - plausibly the result of predation. Therefore, additional field studies are needed to robustly quantify the impact of channeled and, especially, knobbed whelks on wild bay scallop populations and assess their impact on planted stocks in the context of ongoing scallop restoration efforts.