Cohort Structure and Habitat Association of the Pinna Nobilis Population in Northern Corsica at STARESO Abstract Introduction
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Cohort structure and habitat association of the Pinna nobilis population in Northern Corsica at STARESO Kenan Chan & Emma Chiaroni Abstract The Mediterranean fan mussel, Pinna nobilis can grow to be over a meter in length and are typically found half submerged in the sand within Posidonia oceanica. In recent years, P. nobilis has been under increasing pressure due to collection and habitat destruction. Previous studies have examined P. nobilis population structure elsewhere in the Mediterranean, however, our study investigated the specific demographics of the population at STARESO Marine Research Station, located on the Northern coastline of the French island of Corsica. We created an equation that enabled non-lethal age determination based on the maximum width of the individual. Our study was based on principles of the intermediate disturbance hypothesis and aimed to quantify environmental disturbances. We looked at depth and substrate as an indicator of disturbance, with shallower regions being more exposed to high wave action and composed of rocky substrate. We found evidence that suggest that P. nobilis associates with the environmentally stable P. oceanica meadows at STARESO and that there exists distinctive cohorts within the population. Introduction anthropogenic activity. In order to develop conservation strategies there must be scientific knowledge on the effects of The intermediate disturbance environmental disturbances on species hypothesis (IDH) predicts that levels of assemblage and diversity (Richardson et al. species diversity will be maximal in 2004). environments experiencing intermediate The Mediterranean fan mussel, levels of disturbance (Aronson et al 1995, Pinna nobilis, can grow to be over a meter Connell 1978, Connell 1979, Sousa 1979). in length and is typically found half Elevated disturbance environments favor submerged in the sand within Mediterranean species with high reproductive capacity and seagrass, Posidonia oceanica, (Katsanevakis low competitive ability, whereas low et al. 2008). Fundamental knowledge of P. disturbance environments allow for some nobilis biology is limited, however, studies species to become competitively dominant, have revealed a depth-related size causing the formation of a climax segregation (Garcia-March et al. 2007 A, B). community (Connell 1979). Studies performed off the Mediterranean Although the IDH was based off coast of Spain suggest that smaller P. nobilis terrestrial ecological succession, it can be were found in sandy sheltered areas and applied to the marine environment. rocks at shallow depths, while large Environmental disturbances in the marine individuals were are found to be associated environment manifest in the form of with P. oceanica meadows at deeper levels hydrodynamic forces, shifting substrate and (Garcia-March et al. 2007 A). There is also 1 little knowledge about the recruitment of P. protection of this species. This study nobilis, however evidence suggests that investigated the role of environmental settlers recruit over a depth gradient. The disturbance, by using depth as a proxy for majority of settlement occurs in late autumn disturbance, with the age related segregation and winter (Garcia-March et al. 2007 A, of individuals within the P. nobilis population Richardson et al. 2004). on the Northern coast of Corsica at the Habitat destruction, pollutants, and STARESO Marine Research Station. Our collection have caused P. nobilis to be listed study looked at four main hypotheses: (1) as an endangered species in the Does the STARESO Pinna nobilis population Mediterranean (Richardson et al. 1999, show cohort structure; (2) Do P. nobilis show Richardson et al. 2004). P. nobilis used to be a substrate preference with respect to cobble, widely distributed within the shallow coastal boulder, sand, and P. oceanica; (3) Do P. nobilis show an association for more stable waters of the Mediterranean, but in recent communities (3a) Is there a positive years, increases in anthropogenic activity, association with deeper depths; (3b) Does the including the development of resorts and the age vary between the different subregions at destruction of P. oceanica meadows, have STARESO; North, Harbor and South; and (4) caused P. nobilis populations to decline. Is there a shared trend in orientation between Expanding on the limited scientific individuals? knowledge of P. nobilis ecology will aid in making informed decisions regarding the METHODS Study Area to replicate those established in 2012 STARESO Marine Research Station (Elsmore and McHugh 2012). The Harbor is located on the Northern coastline of the transect extended from the ladder within the French island of Corsica. The station is a harbor (N42.58026, E8.72418), to a relatively protected site allowing for the permanent PVC pipe (N42.57988, observation of species that inhabit the E8.72449) 50 meters out at a heading of 150 prolific sea grass meadows. There is a small degrees. The North transect extended 85 breakwater that creates a shallow sheltered meters from the North PVC pipe boat harbor. To the North and South of the (N42.58004, E8.72499) inshore (N42.58075, station are steep cliffs that line the coastline, E8.72503) at a total distance of 85 meters. creating unique geological formations The South transect ran between the South including cobble fields and vertical walls. PVC pipe offshore (N42.57951, E8.72475) We used SCUBA for all aspects of and continued for 60 meters (N42.57934, the field data collection. In order to find E8.72410) inshore. The shallowest end of individual P. noblis previously found in the each transect was assigned meter mark 0. 2012 BIOE 159 course, permanent transect lines were set up in three locations (Map); (1) the STARESO harbor (2) North of the STARESO harbor and (3) South of the STARESO harbor. All transects were set up 2 Map: Map of STARESO with 3 permanent transects Figure 1: Total length, maximum exposed length and (marked in red), North individuals (blue) Banana submerged length shown. individuals (purple), Harbor individuals (yellow) and South individuals (green) plotted in Google Maps. orientation, arc thickness, maximum width Each of the three study locations had (figure 2B), distance from transect and their own distinguishing features. The meter-mark on the permanent transect for Harbor’s substrate included cobble and each found individual. Depth was measured boulders, man-made jacks, and P. oceanica by placing a dive computer where the patches. Depth ranged from 0 to about 10m. mussel met the substrate (depth The North location was lined with large measurements were taken using the Imperial boulders and cobble mounds that dropped system due to our dive computers settings). off quickly to a P. oceanica dominated landscape. The North location also included a few unique formations dubbed the Bananas for their sickle, banana-like shape. The Bananas were regions where P. oceanica receded away from bare sand due to erosion. The South was composed of steep rocky walls that met contiguous P. oceanica meadows. Both the North and A. South study sites typically sloped from 5 to 20 m offshore. General Data Collection To test whether P. nobilis abundance and age are higher in areas of low disturbance, we recorded the depth, B. maximum exposed length, submerged length (figure 1), Figure 2A/B: Figure 2A depicts the measurement of the arc width while Figure 2B shows the maximum width. The maximum exposed length and maximum width were measured with rulers while the submerged length (figure 1) was 3 measured with a PVC pipe that was inserted run a Chi-Squared test in to test for an into the substrate until it met resistance association. We set our critical p-value to (submersion technique), where the byssal .05. threads of the P. nobilis meet the rhizomes (Richardson et al 1999). Orientation was determined with a compass, using reciprocal headings along the margin where the two valves meet. The thickness of the valves was measured with calipers from the top of the P. nobilis valves (figure 2A). A meter tape was used to measure the distance from the permanent Figure 3: 1m² with 9 UPC points. transect line to the mussel. We used this data to run a Binomial Demographics: Pinna nobilis age Probability test with a critical p-value of distribution and cohort structure at 0.05 to determine if a difference existed STARESO between shallow and deep depth zones. To determine the average age of the mussels at In order to relate age to a each location, we ran an ANOVA test using morphometric measurement, we established a critical p-value of 0.05 in JMP Pro 11 a total length to maximum width (JMP Pro 11 was used in all subsequent relationship using measurements collected statistical tests). To test for a shared from dead P. nobilis. Shells collected for orientation between individuals at this purpose ranged from 11.8 cm to 69.5 STARESO, we used a Chi-Squared test with cm. Total length, maximum width, shell a critical p-value of 0.05. thickness, arc length, shell volume, adductor Once the mussels were sized and scar length, and adductor scar ring (figure 4) their positions noted, GPS coordinates were number were recorded. taken for each individual via a float and surface support. We then plotted GPS coordinates using Google Earth and Google Maps to visualize the distribution of the mussels. Habitat Association In order to quantify P. nobilis habitat association with certain substrates, we carried out Uniform Point Contact (UPC) surveys at each mussel using 9 total points Figure 4: Adductor mussel scars on the internal side of a P. on a 1 square meter PVC quadrat (figure 3) nobilis valve. Photo: Kenan Chan (note: center mark placeholder was always P. nobilis individual). This data was then By graphing measurements of total length 1 used with a wider spread UPC data set to against maximum width from the dead individuals, the following linear relationship 1 An expansive UPC data set was conducted by J.