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The Impact of Crown Conch on Intertidal Oyster
Populations in Mosquito Lagoon

By: Casey Craig, Courtney Buck, Chelsea Landau, and Jordan Filipponi
Mentor: Dr. Linda Walters

Materials and Methods

Garvis and colleagues (2015) have documented that there are three intertidal oyster reef types in Mosquito Lagoon: functional natural reefs, dead reefs, and functional restored reefs. Natural oyster reefs are reefs with large numbers of live adult oysters, are submerged at high tide, and have minimal, if any, dead margins (bleached white piles of disarticulated oyster shells) on their seaward edges (Garvis et al. 2015). Many oyster reefs near boating channels have accumulated significant dead margins and have since undergone restoration efforts making them, once again, functional reefs (Garvis et al. 2015). Dead reefs are comprised of only disarticulated shell (Campbell 2015; Garvis et al. 2015).

In our field research in CANA, we examined: 1) natural reefs adjacent to boating channels, 2) natural reefs not adjacent to boating channels, and 3) restored oyster reefs. Restored reefs were only located along boating channels as boat wakes cause the development of dead margins (Grizzle et al. 2002; Campbell 2015). We used natural reefs adjacent to and away from boating channels, since intense boating activity (40+ boats per hour) may directly impact M. corona.

Abundance and Distribution

We administered detailed surveys to estimate the population of M. corona on intertidal oyster reefs in CANA, six of each reef type, for a total of 18 reefs. Teams of observers completely covered each reef type at low tide. Specifically, observers stood one meter apart spanning laterally across the reef starting at one edge. Observers walked in straight lines across the reef. This was repeated until the entire reef was traversed, enabling 100% cover of each reef. To account for any M. corona just off the reef footprint, observers also surveyed one meter beyond the edges of all reefs. All large gastropods were identified to species, measured for shell length, and returned live to their original location on the reef. To determine if a shell was empty, observers poked into each shell with a wooden rod or left the shell out of water for a minimum of five minutes to determine if an occupant would become visible. The number of all gastropod egg cases on and within one meter of all reefs was also recorded.

In addition, we conducted our surveys seasonally to determine any variation in population sizes and shell lengths of M. corona. The winter survey was conducted in February/March 2014, the early summer survey took place on 17 May 2014, and the fall survey occurred between the 6 and 7 September 2014. To analyze variation of conch shell size among seasons, we conducted a one-way ANOVA.

Feeding Trials

To determine the relative feeding preferences of M. corona for live oysters of different sizes, we also conducted an in-situ cage experiment in CANA waters. The goal was to examine the relationship between shell lengths of M. corona and the sizes of the oysters they can consume. Twelve 0.5 x 0.25 x 0.25 meter rectangular mesh cages (mesh diameter: 1.9 cm) were placed in Mosquito Lagoon at a depth that ranged from 0.5 to 1.0 meter below water, depending on the tide. Concrete irrigation weights were used to secure the cages in place, and one M. corona and one oyster were placed in each cage. Three cages contained one small oyster (2.5 – 4.4 cm in length), three contained one medium-sized oyster (4.5 – 6.4 cm in length), and three contained one large oyster (6.5 – 8.4 cm) (Table 1). We randomized the shell length of each M. corona relative to cage number and conducted three trials between 1 and 30 June 2014, each having a duration of 72 hours. At the end of each trial, oysters were recorded as fully consumed (hinged oyster shell separated, no soft tissue of oyster remaining), partially consumed (hinged oyster shell intact but open, some soft tissue removed, oyster dead), or not consumed (oyster shell intact, oyster alive). Each M. corona was only used in one feeding trial and then was released on an oyster reef after the trial ended. Using JMP Pro 11 software, we conducted a regression analysis using a linear fit model to determine if there was a significant correlation between conch shell length and consumed oyster shell length.

Conch Movements

Lastly, we conducted a tracking experiment to determine the distance and speed at which M. corona travels on and between reefs. Specifically, we conducted three replicate trials on intertidal oyster reefs at high tide when conch are most active. A trial consisted of five M. corona on each of the three reef types. Each M. corona within a trial was considered a single replicate, and each trial lasted 60 minutes with observations made at six 10–minute intervals. Melongena corona were individually identified with tags created from plastic disks (diameter: 0.5 cm) that were attached to the top (dorsal region) of the shell with super glue. In preliminary trials where individual movements were tracked before and after tag placement, we determined that the tags did not interfere with locomotion after they were attached.

At the beginning of each trial, each M. corona was placed at the center of a reef. No con–specific individuals or hermit crabs were nearby to avoid any responses to chemical cues. We placed flags attached to wire stakes adjacent to M. corona to mark the start location of an individual, and we marked the subsequent location at each 10–minute interval. Flags did not obviously impact conch movements, nor did the presence of an observer stationed a minimum of four meters away. The linear distance between flags was measured with metric transect tape measures. The distance moved in each 10–minute interval was averaged per individual conch and then by reef type. After averaging the distance traveled per 10 minutes for each reef type, the values were scaled up by a factor of 6 to acquire an hourly locomotion rate, and then by 24 to determine how far M. corona are capable of traveling in one day. Individuals were then released at the end of each trial.

Results >>