By Stacy Craft, M.A.
Sea Shell Protection
Sea shells are commonly picked up at beaches by children, collectors, and tourists. Little thought is put into collecting shells. Yet, shell loss on beaches can pose catastrophic effects to invertebrate organisms that live on and around beaches (Peura, Lovvorn, North, & Kolts, 2013). Invertebrate organisms utilize shells for protection. Hermit crabs utilize whole snail shells, and urchins and anemones utilize both whole shells and shell pieces to protect themselves (Kihia, Muthumbi, Okondo, Nthgia, & Njuguana, 2015; Oba & Goshima, 2004; Peura, Lovvorn, North, & Kolts, 2013; Turra & Leite, 2001). Without protection, hermit crabs would be without shelter, urchins and anemones would dry out in the sun, and many invertebrates would fall to predation (Kihia, Muthumbi, Okondo, Nthgia, & Njuguana, 2015; Oba & Goshima, 2004; Peura, Lovvorn, North, & Kolts, 2013; Turra & Leite, 2001). Although losing one organism does not cause the loss of a species, the loss of shells on beaches can adversely impact beach ecosystems.
Protection is crucial to invertebrate organisms for the survival of individuals and the survival of the species through reproduction. Without a means for protection, most invertebrate species are preyed upon, potentially resulting in species loss and ultimately habitat destruction (Kihia, Muthumbi, Okondo, Nthgia, & Njuguana, 2015). To combat such events from occurring, many governments create MPAs to protect the species and habitats from such destruction (Alexander & Gladstone, 2013; Ashworth, Ormond, & Sturrock, 2004; Barrett, Buxton, & Edgar, 2009; Bartlett et al., 2009; Ceccarelli, Beger, Kospartov, Richards, & Birrell, 2011; Lindholm, Auster & Valentine, 2004).
MPAs in San Diego
MPAs are a new concept to many other countries, which many individuals do not understand or value. This is because the first were being created in 1903, but most being created in the late 1900’s to early 2000’s (Tripp, 2014). It is important to determine if MPAs are valuable to our marine environments and the organisms that live within them. This is especially true as much research has been done, but none regarding southern California, specifically San Diego, CA MPAs (Alexander & Gladstone, 2013; Ashworth, Ormond, & Sturrock, 2004; Barrett, Buxton, & Edgar, 2009; Bartlett et al., 2009; Ceccarelli, Beger, Kospartov, Richards, & Birrell, 2011; Lindholm, Auster & Valentine, 2004).
I looked at the number of whole shells, the density of shell pieces, and the sizes of both whole shells and shell pieces within three sites. This was done to determine if MPAs contribute to the restoration of shell dense beaches. MPAs are expected to contain a statistically greater number of whole shells and shell pieces. Additionally, MPAs have statistically greater sizes of shells and shell pieces than a non-MPA to show that MPAs are necessary for marine ecosystems. This would aid in understanding that MPAs can allow for greater survival rates of invertebrate organisms.
Study Sites
This study was localized to San Diego County, at San Diego-Scripps Coastal State Marine Conservation Area (SDSCSMC), Matlahuayl State Marine Reserve (MSMR), and Del Mar Municipal Beach (DMMB). While there are a variety of MPAs within this region, the chosen fishing allowed MPA is SDSCSMC being located in La Jolla, CA. La Jolla is a popular tourist area and therefore susceptible to shell loss through tourist activity. To understand the effect MPAs have despite human interaction, it was important to choose locations open to the public. Furthermore, this location was within close proximity to the other two locations utilized for collection. Data was collected for this site on July 9, 2017 starting at 6:15 a.m., during low tide.
The non-fishing MPA study site utilized was MSMR which is located in La Jolla, CA directly adjacent to SDSCSMC. Similar SDSCSMC. It acquires a high volume of human traffic being located in a common tourist area. Additionally, it is in close proximity to the other two study site locations. Data was collected for this site on July 9, 2017 starting at 4:30 a.m., during low tide.
DMMB was the chosen study site for a non-MPA location. While DMMB covers a vast area of land, the beach between 15th and 18th street was utilized for this study. This space was chosen because it had the greatest amount of human traffic making it comparable to the other two MPA beach locations. Data was collected for this site on July 5, 2017 starting at 10:30 a.m., during low tide.
Data Collection
Each study site was measured in length and 450 meters was designated from each site as the study area. A separate transect was made within each study site. Then, a 1 meter by 1-meter quadrat was placed along the transect line as the data collection space. Data was collected every 5 meters for a total of 90 collections along each transect.
With each collection, the number of whole shells and the number of shell pieces were counted. Then, each whole shell and piece of shell was measured at its longest length in centimetres to determine size. The information collected was of exposed whole shells and shell pieces only. All collections were done during low tide, which varied in time of day by collection site.
Statistical Analysis
Means were determined for the number of whole shells, the number of shell pieces, and sizes of whole shells and shell pieces. With these means, ANOVA tests were performed to obtain p-values. An ANOVA was chosen because three means (one for each study site) were being compared for shell size and density.
San Diego-Scripps Coastal State Marine Conservation Area
Ninety samples were taken from SDSCSMCA study site, resulting in 428 whole shell measurements and 483 shell piece measurements. The whole shell means for this study site was 4.8 (SD=4.2) and the shell piece mean was 5.4 (SD=6.2) (Figure 1). With the lengths in centimetres from the 911 whole shells and shell pieces determined, the mean length (in cm) was 1.3cm (SD=0.6cm) (Figure 2).
Matlahuayl State Marine Reserve
Ninety samples were taken from the MSMR study site, which resulted in measurements for 1490 whole shells and 79 shell pieces. The mean sample size of whole shells was 17 (SD=35) and the mean sample size of shell pieces was 0.88 (SD=1.8) (Figure 1). Length in centimetres was calculated for the 1490 whole shells and 79 shell pieces resulting in a mean length of 1.01cm (SD=0.21cm) (Figure 2).
Del Mar Municipal Beach
The third set of ninety samples were taken from the DMMB study site. Within this set of ninety samples, two whole shells and 6 shell pieces were measured. Mean sample size in whole shells was 0.02 (SD=0.21) and mean sample size of shell pieces was 0.06 (SD=0.29) (Figure 1). The calculated mean lengths in centimetres of the whole shells and shell pieces were determined to be 1.7cm (SD=0.65cm) (Figure 2).
Study Site Comparisons
To determine statistical significance for the number of whole shells between the three study sites, a one-way ANOVA was used. It resulted in a p-value which explains a significant difference in mean number of whole shells found between the three sites (Table 1). A p-value deduced for shell pieces indicated a significant difference in the number of shell pieces between the three sites (Table 1). Finally, a p-value that shows a significant difference in shell size between the three sites was deduced (Table 1).
Study Site Whole Shell and Shell Pieces Mean Density Comparison
The data obtained from DMMB was consistent with Bartlett, et al. (2009). Therefore, the quantity of whole shells and shell pieces was lower due to lack of government protection. The data collected from SDSCSMC and MSMR showed similarities to Lindholm, et al.’s (2004) study. It showed that the number of shells would be greater in unfished areas due to higher risk of predation by fish compared to unfished areas. While Lindholm, et al.’s (2004) results were consistent with those of the researcher’s, they contradicted the evidence found in multiple other studies (Ashworth, Ormond, & Sturrock, 2004; Barrett, Buxton, & Edgar, 2009). There were higher numbers of shells available in fished areas because more gastropods were able to survive and reproduce with lower predation from fish (Ashworth, Ormond, & Sturrock, 2004; Barrett, Buxton, & Edgar, 2009). Despite the discrepancies, this study deduced a significant difference in number of both whole shells and shell pieces found at each study site (Ashworth, Ormond, & Sturrock, 2004; Barrett, Buxton, & Edgar, 2009; Bartlett, et al., 2009; Lindholm, Auster, & Valentine, 2004).
Study Site Whole Shell and Shell Piece Mean Size Comparison
The largest whole shell and shell piece mean was found at DMMB (1.7, SD=0.65), which is consistent with Alexander and Gladstone (2013), but contradicts Bartlett et al. (2009). While this information is credible, Bartlett et al. (2009) also found that periodically harvested and permanently closed coral reef reserves provided similar mean sizes. This contradicts SDSCMCA obtaining the smallest whole shell and shell piece mean at 1.0cm (SD=0.21) and MSMRs being 1.3 (SD=0.62). The mean whole shell and shell piece mean sizes were found to be significantly different from one another relating more closely to the results of Alexander and Gladstone (2013). DMMB may have had a greater mean size because shells collected were from individuals that lived longer.
Potential Confounding Variables
The data from each study site was done during low tide on the day of collection. While this was done intentionally, the rate at which the tide rose was different depending on the study site. This could skew results as shells were possibly washed away at DMMB where the low tide came in quickest. Another factor regarding tide lines was that collection for SDSCMCA was the same as MSMR. Collection occurred for SDSCMCA before MSMR, likely affecting how low the tide was when collection occurred. Data was also taken on different days, which could have affected the data collection. The collection for both SDSCMCA and MSMR was performed on July 9, 2017, while data collection for DMMB was on July 5, 2017.
All study site locations were of similar popularity to tourists and local pedestrians and allowed for children and leashed dogs. SDSCMCA and MSMR were in close proximity compared to DMMB. This could have affected the possible marine gastropods and therefore ultimately affect the shell availability.
A potential, but not likely, confounding variable is beach grooming. While San Diego, CA beaches are groomed, they are only groomed above high tide during grunion season (Beach Grooming, 2015). Grunion season is when a species of fish commonly known as grunion come onto the beach to spawn, occurring in March and June through September (Grunion Run Schedule, 2017).
Looking Ahead
MPAs provided necessary protection to local marine species in order for those species to reproduce and survive. Without the protection of MPAs, many species would not be able to survive or would have critically low population sizes. Gastropods are preyed upon at a higher rate in MPAs than non-MPAs due to the decreased pressure of fishing on the local fish species. This results in more shells from preyed upon gastropods in MPAs.
The results demonstrated a difference in density of both whole shells and shell pieces found on beaches of contrasting levels of government protection. The size of whole shells and shell pieces among these three study sites were significantly different with DMMB having the greatest sizes and SDSCMCA demonstrating the smallest sized pieces. These conclusions justify the understanding that MPAs provide protection for many species to survive compared to non-MPAs. However, they do not support gastropods surviving longer, therefore growing to larger sizes, correlating with breeding age in several species. It is important to further investigate the length of time gastropods are able to survive in areas of different government protection based on shell size. This is due to discrepancies between previous studies, common thought, and the results of this study.
The PDF file is available HERE. See also an article written by Stacy Craft HERE
Stacy Craft, M.A.
Instructional Aide, T.E.R.I., Inc., San Marcos, CA
Instructor, Helen Woodward Animal Center, Rancho Santa Fe, CA
Correspondence can be sent to Stacy Craft B.S. by email at craftsl@miamioh.edu or by phone to (909) 342-3995.
Acknowledgement: Stacy Craft, M.A. completed this project as a part of her graduate work with Project Dragonfly at Miami University in Oxford, Ohio in conjunction with San Diego Zoo Global. She would like to thank Project Dragonfly students and staff that helped with editing her work, with special thanks to Emily Craft for support throughout this process.
Marine Protected Areas (MPAs) are government-protected marine locations that reduce the strain humans have on the environment. Marine invertebrates, which are regularly found in MPAs, require a source of protection in order for survival. Commonly protection is found in gastropod shells. Therefore, it is important to determine if MPAs provide greater protection to invertebrates than non-MPAs based on shell availability.
References
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