Shell Microstructure of the Patellid Gastropod Collisella Scabra (Gould): Ecological and Phylogenetic Implications

THE VELIGER The Veliger 48(4):235-242(January 25,2007) O CMS, Inc.,2005 Shell Microstructure of the Patellid Gastropod Collisella scabra (Gould): Ecological and Phylogenetic Implications SARAH E. GILMAN* Section of Evolution and Ecology, and Center for Population Biology, University of California, Davis, Davis, cA 95616-8755 Abstract. Shell microstructure has a long history of use in both taxonomic and ecological research on molluscs. I report here on a study of the microstructure of Collisella scabra, also known as Macclintockia scabra and Lottia scabra. I used a combination of SEM and light microscopy of acetate peels, whole shells, and shell fragments to examine the shell layers and microstructure. Regular growth bands were not present in most shells examined. Several shells showed multiple bands of myostracum, which indicate periods of extreme rates of size change, and may be evidence of abiotic stress. This study also suggests that shell layers previously described as "modified foliated" are "irregular complex crossed lamellar," with both fibrous and foliated second order structures. The presence of fibrous shell structures, in addition to other shared morphological characters noted by previous authors, suggests an affinity with the Lottiidae rather than the Nacellidae. INTRODUCTION scabra shells, and 2) to verify the earlier shell microstructure descriptions, including phylogenetic Shell microstructure has a long history of use in both implications. taxonomic and ecological research on molluscs. Shell growth bands are commonly used as records of individual growth (Frank, 1975; Hughes, 1986; Arnold MATERIALS AND METHODS et al., 1998) and for reconstructing environmental conditions (Rhoads &LuIz,l980; Jones, 1981;Kirby et Collection and Preparation of Shells al., 1998). Shell microstructure has also been used for taxonomic identification (Kennish et al., 1998) and as I collected 10-20 snails from each of the three field evidence of shared ancestry in systematic work (Kool, sites listed in Table 1. All three sites are rocky intertidal (sensu 1993; Schneider & Carter, 2001). In revising the benches on semi-protected outer coast Ricketts systematics of the patellogastropod genus Collisella, et &1., 1985). At each site, I collected snails from primarily Lindberg (1986) used shell microstructure characters wave sheltered areas, such as surge channels protected described by MacClintock (1967) to place Collisella or the wave sides of outcrops or ridges. I specifically scabra in a separate family from the rest of the species searched for snails with a minimum of shell erosion in the genus. Although Lindberg never formally and collected a range of sizes at each site. In the lab, I dissected each from its published a new taxonomy for this species (see snail shell. Lindberg, 1986), Kozloff (1987, 1996) contains Lind- berg's intended systematics and the name Macclin- Whole Shells tockia scabra has appeared in the published literature (e.g., Smith et a1., 1993). More recently, Fuchigami & To examine the muscle scar and adjacent layers on Sasaki (2005) re-examined the shell microstructure of the inner surface of the shell, I soaked several shells in 44 patellogastropod species, and updated several of 57a sodium hypochlorite for one hour and then rinsed MacClintock's (1967) original descriptions, including them under running water. I then examined the inner C. scabra.In this study I reexamined the microstructure surface of these shells under a dissecting microscope. of C. scabra with two goals: 1) to explore the use of To determine the mineral structure of the shell layers shell growth patterns as a way to age individual C. exposed on the inner surface, I treated two shells with Feigl Stain (Feigl, 1937; Freidman,1959), which stains * PresentAddress: Friday Harbor Laboratories, IJniversity of aragonitic areas black, but leaves calcitic areas un- Washington, Friday Harbor, WA 98250, e-mail: gilmans@ changed. I also used three of these bleached shells in u.washington.edu scanning electron microscopy (see below). 4 Page 236 The Veliger, Vol. 48, No. Iable I Location of samples collected for analysis. of snails Protocol Collection Site Location Collection Date Number l0 Acetate Fort Bragg, California (MFB) 39.282'N,123.803"W 05107l0l 0610610r l2 Feigl Stain. SEM l2 Acetate Shelter Cove. California (SCV) 40.022'N. 124.074"W 05/06/01 l5 Acetate, SEM Devil's Gate. California (DVG) 40.407"N.124.390"W 06109101 follow the definitions rn Acetate Peels describing shell structures Carter et al. (1990). Shells for acetate peels were embedded in clear epoxy The myostracum is a narrow band of irregular - resin (EPON 828 resin and DTA hardener, Miller simple prismatic structure (Figure 2)' Adjacent and the Stephenson, Danbury CT), and sectioned through exterior to the myostracum is the m + I layer apex along the midsagittal plane using an Isomet low (Figure 2), a very narrow band of branching crossed I ground and speed saw (Buehler Ltd., Lake Bluff, IL). lamellar structure. This layer is rarely visible in acetate Carter & etched the sections and made peels following peels, and much more visible in the SEMs or by direct mm thick acetate for the Ambrose (1989). I used 2 examination of the interior surface of bleached shells problems with curling and tearing peels, which reduces under a dissecting microscoPe. with much thinner acetate (Carter commonly reported The majority of the shell consists of the m + 2 and 1989). After 12-24 hr' I examined peels & Ambrose, m 1 layers, which contain a similar first order a compound microscope at 25x to 250x. under structure (Figures 2-5), best described as irregular Photographs of specimens were made with a 35 mm complex crossed lamellar (Carter et al., 1990). SEM body mounted onto the microscope. camera revealed the second order structure to be highly variable, consisting of long fibers (i.e., "fibrous Electron Microscopy prismatic," Figure 5) of varying width that grade into foliated sheets (i.e., "regularly foliated," Figure 4). I examined whole shells, shell fragments, and epoxy MacClintock (1967) indicated a possible m + 3 layer, embedded sections with scanning electron microscopy not describe it. In general, the shells examined (SEM). Whole shells and shell fragments were pre- but did viously bleached as described above and air dried for several days before use. Shell fragments were generated by gently tapping a whole shell with a hammer. Three APICAL from those used for embedded specimens were selected EXTERIOR the acetate peels. These were polished with aluminum oxide and re-etched in 57o HCI for 15 sec before SEM' All samples were prepared for SEM following Carter & Ambrose (1989). Once thoroughly air-dried, they were INTERIOR mounted onto specimen support stubs using silver m m-1 MARGINAL suspension paste (Ted Pella Inc., Redding' CA) and mf1 sputter-coated with gold (Pelco Model SC-7, Pelco International, Redding CA). Samples were viewed in ANTERIOR a Philips XL 30 Scanning Electron Microscope (FEI Co., Hilsboro OR) oPerated at 10 KV' RESULTS I identified four layers in the shell of C. scabra (Figure I ). Feigl staining revealed the muscle scar (myostracum) to be aragonite and the remaining shell POSTERIOR layers, calcite. Following MacClintock ( 1961), I iden- layers by their position relative to the tify these Figure l. Diagram of shell layers in C. sc'qbra. (A) Cross- are myostracum. Layers interior to the myostracum section of shell along sagittal axis, after MacClintock (1967' labeled with negative numbers, and shell layers exterior Figure l).(B) Shell layers as viewed on the ventral surface of to the myostracum are labeled with positive numbers the shell. after MacClintock (1967, Figure 82)' The shaded by "m" is the myostracum (muscle scar)' (Figure I ). Unless otherwise indicted all terms used in area indicated S. E. Gilnran.2005 Page 237 yi' FigLrre2. SEM of'sa-tlittalsection ctl-siutrplcDVG 2A. Thc sllell rvas cntbcc'lcleclin epoxy apcl sectic-rleclbcli re crossecllantellar. Scale bar is l0 micron for this study were too hi-ehlyeroclecl to cletermineif an DISCUSSION m+3layerexisted. Becauseof the irre_qularityof the The -soalsol- this str"rclywere two-fblcl: I ) to explore the nt + " und nt I layers, it is difflcLrlt to see consistent growth lines. luseof sl-rellgrowth patterns its erwuy to age individual Growtl-r discontinuities (i.e.. "growtl'r bands") are Colli.sallu.s't'uhru sl-rells. arnd 2) to verify earlier occasionally observed in acetate peels ( Fi-cLrre6). clescriptions and tlteir tuxonon-ric in-rplicatior-rs.Shell althougl-rthey were ilpparellt only in portions of shell growth berncisarc colllt-ron in other r-nollusksand are cross-sectionsand ltever consistently across the entire ofien usecl in flshcries research to er-qeindividuals shell.Other shellshad no banclsat al (e.g.. tri_uure7). (Rlroacls& Lutz. 1980).Banclin_s is occasictnallyevident The banding appears to be causeclby chan_eesin the in C. .st'tthru,but generally only in larger individr"rals orientation of tl-rehigher orcler structures (e.-q..m - | runclnot throughout the entire shell.Most shellsshowed layer of Figlrres 2 ancl 9). no growth banclsat all. The bands appear to be fbrmed Irregr-rlaritieswere also present in the myostrircLn-n. by chan-eesin tl-reorientatior-r of the tnd order-eler-nents In most shells. the myostralcLln-lcor-rsistecl of a sir-rgle of the m l ar-rclnt + t luyers. It is unclear how such band that expancleclwith the growth of thc- shell; orientartior-rclttrr-rgcs ntigltt relatc to annunl or seasonal

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