DOI: 10.5061/dryad.ffbg79crw
title: Size-dependent predation and intraspecific inhibition of an estuarine snail feeding on oysters

description: Predator outbreaks have increased in the past two decades in many
 ecosystems and are predicted to become more common with climate change.
 During these outbreaks, predator densities increase rapidly, and can cause
 large reductions in prey populations or shifts in prey size structure.
 However, unexpected interactions may occur at high predator densities,
 necessitating a mechanistic understanding of how increased predator
 density affects predator-prey dynamics. In the northern Gulf of Mexico,
 outbreaks of southern oyster drill (Stramonita haemastoma, Linnaeus, 1767)
 occur during high salinity events, and can greatly reduce eastern oyster
 (Crassostrea virginica, Gmelin, 1791) populations. A large outbreak of
 drills occurred from 2013—2015 in Apalachicola Bay, FL which corresponded
 with an oyster fishery collapse in that bay. To improve our mechanistic
 understanding of predation during such a high-density outbreak, laboratory
 experiments based on field observations were used to quantify the prey
 size selection by drills and the drill functional response, as a function
 of drill abundance. Drills fed on medium-sized oysters (50-75 mm) more
 often than small and larger-sized oysters, and often formed aggregations
 during feeding events. However, despite this aggregative response, there
 was a negative relationship between per capita feeding rates and drill
 abundance. Indeed, the Crowley-Martin functional response model had the
 most parsimonious fit to the data, suggesting that predator-predator
 inhibition reduced attack rates and increased handling times. Due to an
 increase in regional drought conditions and water usage in the
 southeastern United States, drill outbreaks will likely increase in
 frequency and duration. A greater understanding of how predation rates
 change with predator densities during outbreaks will improve predictions
 of oyster mortality, and strengthen the scientific framework for oyster
 fishery decisions.
description: This experiment used drills and oysters that were collected from
 subtidal habitats in Apalachicola Bay, FL USA (both collections occurred
 around 29º 40’ 32.56” N, 84º 51’ 36.76” W and 29º 42’ 7.2” N, 84º 49’
 31.76” W), and was carried out at the University of South Florida, College
 of Marine Science in St. Petersburg, FL (USFCMS). At USFCMS three
 separate, closed-seawater systems, each comprising ten experimental tanks
 (47.3 l; 68 ´ 40 ´ 27.5 cm), were constructed for a total of 30 tanks. The
 base area of these tanks was similar to the first experiment, 0.27
 m<sup>2</sup>. Within each system, approximately 950 l of
 artificial seawater was recirculated through the experimental tanks and a
 large sump (633 l; 76 ´ 45.5 ´ 183 cm). In Experiment II at UFSCMS,
 salinity was maintained at 25 by mixing deionized water with Instant Ocean
 sea salt (Instant Ocean Spectrum Brands, Blacksburg, VA, USA), and checked
 daily with a YSI 85 (Xylem Inc, Yellow Springs, OH, USA). Water
 temperature was maintained between 20-22°C using Finnex TH-800 Plus,
 800-watt titanium heating rods (Finnex, Chicago IL, USA) with a Reef
 Octopus Heater Controller (Honya Co. Ltd, Shenzhen, China). The
 temperature and salinity were chosen based on conditions that occurred
 during an outbreak in the northern GOM (Menzel et al.,1966; FFWCC, 2013).
 During both experiments and the one week holding prior to Experiment II,
 oysters were fed Instant Algae Shellfish Diet 1800 (Reed Mariculture Inc,
 San Jose, CA) daily, following the manufacturer’s instructions of 3.6 ml
 per 100 g of oyster wet weight. In the seawater system
 at USFCMS, six drill abundances (2, 3, 4, 6, 8, and 12 drills per tank)
 were orthogonally crossed with five oyster abundances (3, 4, 6, 8, and 12
 per tank), yielding two replicates for each orthogonal cross. The wide
 range of abundances reflects a response-surface approach to the regression
 analysis, improving the chances of detecting interactions between
 experimental factors despite low replication at any one combination of
 treatments. The treatment abundances were equivalent to densities ranging
 from 7-44 m<sup>-2 </sup>(drills) and 16-65 m<sup>-2
 </sup>(oysters), both within the natural range observed for each
 species during and after outbreak conditions (See Supplemental Fig. 1). In
 this second experiment, drills ranged in size from 30-81 mm and were
 separated into small (30-55 mm) and large (56-81 mm) size classes. Oysters
 ranged in size from 25-99 mm and were separated into small (25-49 mm),
 medium (50-74 mm), and large (75-99 mm) size classes, as in Experiment I.
 Abundances of drills and oysters were split equally among each size class.
 To quantify oyster mortality in the absence of drills, each level of
 oyster abundance was paired with a corresponding control treatment (n=2)
 that lacked an oyster drill; no oysters died in these controls. After
 collection from the field, drills were held for one week and allowed to
 feed <i>ad libitum</i> on oysters before the start of the
 experiment to reduce any transportation effect to USFCMS from Apalachicola
 Bay, FL. The experiment began with a 5-day starvation period, based on the
 feeding rate of individual-housed drills from Experiment I, to standardize
 predator hunger. During feeding trials, tanks were checked twice daily for
 dead oysters, and the number of drills on each oyster was counted. Any
 dead oysters were replaced with a live oyster from the same size class to
 preserve a constant prey density, as assumed in predator functional
 response models that quantify the instantaneous feeding rate as opposed to
 the integrated feeding rate. After the 15-day experimental period, both
 the per capita feeding rate and the aggregate feeding rate (oysters
 consumed per replicate) were calculated.
description: Data descriptions for Functional Response Data from Pusack et al.
 2018 JEMBE (501) 74-82 File: "drill
 aggregation feeding_ Table 1 Fig 5.csv"
 Columns and descriptions:<br> round: the experiment was
 conducted in three rounds due to logistical constraints of the number of
 tanks available, not all treatments were done in each round so blocks were
 incomplete<br> tank: Tank ID per round for each treatment
 combination of drill and oyster abundance<br> drill.abundance: the
 number of drills placed in each tank<br> oyster.abundnace: the
 number of oysters maintained throughout the experiment. When an oyster was
 found dead, it was replaced with one of similar size.<br>
 total.no.oysters.consumed: over the entire experimental period, the total
 number of oysters consumed per tank<br> days: total length of the
 experimental period<br> oyster.per.drill: the number of oysters
 consumed per drill, which was calculated as =
 total.no.oyster.consumed/drill.abundnace<br> group.feeding.rate: the
 daily feeding rate for each tank, i.e. the group feeding rate (no. of
 oysters consumed/tank/day), which was calculated as =
 total.no.oysters.consumed/days<br> per.cap.feeding.rate: the per
 capita feeding rate for each tank (no. of oysters consumed/day/drill
 abundance of the tank), which was calculated as =
 total.no.oysters.consumed/days/drill.abundance
 File: "drill size feeding Fig
 1.csv" Columns and
 descriptions:<br>   <b><i>column
 heading</i></b> <b><i>description</i></b> round Experiment number.  There were 2 round of this experiment block The experiment extended across several water tables.  These were considered blocks water.depth Whether the experiment took place in shallow or deep water cage.no There were several cages within each block. drill.no Drill identification.  There were 63 individual drills used in this experiment drill.size.mm Size of drill in mm. drill.group Drill size in small or large category (&gt;59.5 is large, &lt;59.5 is small) date Date the oyster was found dead. day Day of the experiment the oyster was found dead. tday Number of days the experiment lasted. oys.size.mm Size of the oyster in mm. oys.type Whether the oyster was present at the start of the experiment (original) or if it was replacement for a dead oyster during the middle of the experiment (replacement) oys.status Whether the oyster was a gaper or it was alive gaper whether the oyster was a gaper (1) or not (0). drill.on.oyster How many drills were found on the oyster? oys.group The sizes of oysters were split into groups (0-49=1, 50-74=2, 75+=3). notes Any extra notes.  Ex. was the oyster replaced? Were drill holes present? File: "aggregation size and feeding Fig 2.csv" Columns and descriptions:<br> drill.ttt: Number of drills in that tank n: number of oysters observed over the course of the experiment total: the total daily checks for each oyster when a feeding even could have occured sub.total: a subtotal that limits the data to when a feeding event of at least 1 drill feeding on an oyster occured oys.group: the number of drills in the feeding group on a single oyster Values: whether the numbers are expected (exp) based on a poisson distribution or from observed (obs) data prop.total: The proportion of the daily checks for each oyster with the number of drills (oys.group) feeding on an oyster. Zero, “0” is included in this total, i.e. no feeding. prop.subtotal: The proportion of the specific oyster aggregation when a feeding event occurred (“0” or no feeding was excluded for this proporation). values: the actual number observed of the feeding event of a specific drill aggregation size. File: " drill aggregation feeding_ Fig 3 Fig 4.csv" Columns and descriptions: <b><i>column heading</i></b> <b><i>description</i></b> tank There were 3 water tables each with 10 tanks (B3 refers to the third tank in the second water table. drill.no.ttt Number of drills in that tank drill.den.m2 Density per meters^2 that this drill density in this tank would correspond to in nature oys.no.ttt Number of oysters in that tank drill.oys.ttt Combining the drill density and oyster density into a code for that tank oys.den.m2 Density per meters^2 that the oyster density in this tank would correspond to in nature mean.size Mean size of the drills in that tank sem.size Standard error of the mean of the drills in that tank tot.biomass biomass in g of the drills in that tank mean.biomass mean biomass of the drills in that tank (tot.biomass/drill.no.ttt) sem.biomass standard error of the mean of biomass of drills in that tank total Total number of oysters eaten during the experiment days Number of days the experiment ran per.ind number of oysters eaten per drill (total/drill.no.ttt) feed.rate number of oysters eaten per day in that tank (total/days) oys.day.ind number of oysters eaten per day per drill in that tank (feed rate/drill.no.ttt)