Vol. 424: 133–143, 2011 MARINE ECOLOGY PROGRESS SERIES Published March 1 doi: 10.3354/meps08993 Mar Ecol Prog Ser Latitude and coastline shape correlate with age-structure of Chlorostoma (Tegula) funebralis populations Erin E. Cooper*, Alan L. Shanks University of Oregon, Oregon Institute of Marine Biology, PO Box 5389, Charleston, Oregon 97420, USA ABSTRACT: Between 2005 and 2008, size-frequency distributions from 22 populations of the inter- tidal snail Chlorostoma (Tegula) funebralis were studied to determine the effects of latitudinal region and coastal topography. Upwelling, which may affect larval recruitment, varies along a latitudinal gradient, while coastal topography may mitigate the effects of latitude. Sites were categorized by lat- itude: Northern (Oregon, 15 sites), and Southern (California, 5 sites; Baja California, 2 sites); and by coastal topography: protected (bays and coves, 7 sites in north, 3 in south), or exposed to the open ocean (7 sites in north, 5 sites in south). In Oregon, population structures at protected and exposed sites were significantly different (p = 0.001). At protected sites, a large percentage of individuals were pre-reproductive juveniles. At exposed sites, the population was weighted towards large indi- viduals capable of high reproductive output with few juveniles present in the population. This pat- tern may reflect inter-annual variation in recruitment in Oregon, with protected areas experiencing more constant recruitment. In California and Baja California all populations were predominately composed of juveniles, regardless of coastal topography (p = 0.336). Differences in age-structure of the population create significant differences in relative reproductive potential. The potential repro- ductive output of a population was calculated by quantifying the eggs produced by snails covering the size range of reproductive individuals. Populations in the north at exposed sites produce eggs at a rate up to 2 orders of magnitude higher than populations in the south (p = 0.004). These variations in size-structure of C. funebralis populations with coastal topography and latitude may have impor- tant impacts for the population distribution. KEY WORDS: Population dynamics · Age structure · Chlorostoma funebralis · Recruitment Resale or republication not permitted without written consent of the publisher INTRODUCTION capacity, their movement may largely be determined by the movement of water masses, which can vary lat- The size-frequency distributions and age structures itudinally as well as locally. Frank (1975) and Wright of many marine invertebrates vary between popula- (1975) collected data on the size distribution of tions (Ebert & Russell 1988, McQuaid & Phillips 2006). Chlorostoma (Tegula) funebralis along the west coast At times, these attributes vary along a latitudinal gra- of North America. These distributions showed a latitu- dient (Frank 1975, Ebert 1983, Menge et al. 2004). dinal gradient in the number of adult individuals, with These differences may be the result of larval supply the largest number of adults in the northern popula- and recruitment, since years of poor recruitment will tions and more juveniles in southern populations. result in gaps in the size-frequency distribution as the However, populations for these studies were geo- population ages (Ebert & Russell 1988, Roughgarden et graphically disparate and coastal topography was not al. 1988, Menge et al. 2004). Differences in recruit- incorporated into the experimental design. To distin- ment, in turn, may be driven by hydrodynamics and guish between latitudinal gradients that might be dri- coastal topography. As larvae have limited swimming ven by changes in coastal hydrodynamics and the *Email: [email protected] © Inter-Research 2011 · www.int-res.com 134 Mar Ecol Prog Ser 424: 133–143, 2011 effects of coastal topography, more sampling of more Cape Blanco to Point Conception (Roughgarden et al. populations over a larger latitudinal range and differ- 1988). Roughgarden et al. (1988) hypothesized that the ent coastal topographies is necessary. intermittent upwelling in central and northern Oregon The intertidal snail Chlorostoma (Tegula) funebralis should transport pelagic larvae to the rocky intertidal, is an ideal organism to study variation in size- reducing larval mortality and causing constant, annual frequency distributions because it is common and often larval supply. More constant upwelling in central and abundant in the rocky intertidal from Baja, California northern California should transport larvae offshore, to British Columbia in both high and low energy envi- resulting in delivery of larvae to the shore only under ronments (Morris et al. 1980). The duration of the pela- rare conditions (Menge et al. 2004). gic lecithotrophic larval stage is between 5 and 8 d Coastal topography has also been shown to affect (Moran 1997, Guzmán del Próo et al. 2006). Individuals larval dispersal and settlement. As many types of lar- live up to 30 yr in the northern portion of the range vae have limited swimming capability, local, smaller- (Darby 1964) and 6 to 7 yr in the southern portion of the scale oceanographic features may play a role in retain- range (Frank 1975). Growth rates have been calculated ing larvae or moving them offshore. Ebert & Russell throughout the species range (Frank 1975, Wright (1988) and Morgan et al. (2000) found a correlation 1975). C. funebralis grow throughout their lifespan, so between coastal headlands and larval recruitment. it is possible, using size, to age individuals and use Sites with low recruitment were near coastal head- size-frequency distributions as an indication of age lands, where regular coastal upwelling jets are hypo- structure of populations (Frank 1975). The differences thesized to move larvae offshore. In South Africa, pro- in the number of adults in populations of C. funebralis tected bays and exposed headlands have different along a latitudinal gradient (Frank 1975, Wright 1975) population structures of mussels, possibly related to present an opportunity to study the mechanisms of recruitment patterns. Higher recruitment in bays in population structure. comparison to nearby exposed populations may be due Larval supply can drive patterns in recruitment and to larval retention within the bay and larval advection adult distributions (Ebert & Russell 1988, Roughgarden from exposed sites (von der Meden et al. 2008). The et al. 1988, Menge et al. 2004). Populations with low flushing times of bays is correlated with retention of larval supply have been shown to have low population cyprid larvae (Gaines & Bertness 1992). In one study, densities. This relationship between larval supply and the presence of oceanographic fronts at the mouths of population structure has been seen, for example, in ur- bays and coves was correlated with upwelling, and the chins (Ebert & Russell 1988), mussels (McQuaid & fronts acted as a barrier to the shoreward movement of Phillips 2006) and barnacles (Menge et al. 2004). The larvae into the bay (McCulloch & Shanks 2003). recruitment of many marine invertebrate larvae to The distribution of adult marine invertebrates is adult populations is driven by hydrodynamics both the cumulative effect of recruitment success, juvenile large-scale and local. Species with a planktonic larval mortality, and adult mortality. Each of these factors stage have a dispersal distance determined by hydro- may vary between populations independently of each dynamics, larval duration, and larval swimming be- other. Previous studies have shown that large Chloros- havior, primarily vertical migration (Shanks et al. 2003, toma funebralis are absent in the southern portion of Shanks 2009). Upwelling, which moves surface waters the species range when compared to the northern por- offshore, is hypothesized to move larvae away from tion of the species range (Frank 1975, Wright 1975). benthic habitats where they must ultimately settle This absence may be a reflection of a latitudinal gradi- (Roughgarden et al. 1988). When upwelling is relaxed, ent in adult mortality, but not enough populations have surface waters move back towards the shore, carrying been studied to determine the potential effects of other larvae back to the adult habitat. Larval capacity for factors. The presence of large adults in the population vertical migration may be a confounding factor, en- may have a significant impact on the reproductive abling larvae to stay nearshore even during upwelling capacity of the population (McQuaid & Phillips 2006) events (Poulin et al. 2002, Shanks & Brink 2005, Mor- and so is important to understand. gan et al. 2009a,b, Shanks & Shearman 2009). Larvae Previous studies of Chlorostoma funebralis did not may also have a preference for depths below the sample enough sites within each oceanographic region Ekman transport layer, which would minimize move- to separate the confounding effects of upwelling and ment offshore during upwelling (Shanks & Brink 2005, coastal topography on population structure. Frank (1975) Morgan et al. 2009a,b, Shanks & Shearman 2009). sampled 6 sites in the northern portion of the range and On the west coast of North America, coastal up- 5 in the south. In the present study, 22 sites covering a welling varies on a latitudinal gradient. Summer wind- wide latitudinal range and 2 types of coastal topo- driven upwelling is intermittent on the Oregon Coast graphies were sampled for the size-frequency distribu- south to Cape Blanco, and relatively constant