DOCUMENTATION OF ANNUAL GROWTH LINES IN OCEAN QUAHOGS, ARCTICA ISLANDICA LINNE
JOHN W. ROPES.' DOUGLAS S. JONES,' STEVEN A. MURAWSKI.' FREDRIC M. SERCHUK,I AND AMBROSE JEARLD, JR.I
ABSTRACT
About 42.000ocean quahogs,Arctica islandica Linne. were marked and released ata deep (53 m) oceanic site off Long Island, New York, in 1978. Shells of live specimens recovered 1 and 2 years later were radially sec tioned, polished. and etched for preparationofacetate peels and examination byoptical microscopy ormicro projection; selected specimens were similarly prepared for examination by scanning electron microscopy. Specific growth line and growth increment microstructures are described and photographed. An annual periodicityofmicrost.ructure ia documented. providing a basis for accurate age analysesofthis commercially important species.
Numerous bivalve species form periodic growth lines of clearly separating such shell features was in their shells (Rhoads and Lutz 1980). Internal needed. growth lines found in the shells of ocean quahogs, Recent investigators of age phenomena in ocean Arctica islandica Linne, have stimulated interest in quahogs have microscopically examined the shells using these markings to determine age and growth and acetate peel images produced from sectioned. (Thompson et a1. 1980a, b), since fishery exploitation polished. and etched shells. This method greatly has increased significantly within the past decade aided separating the many crowded growth layers in 3 (Serchuk and Murawski 1980 ). the hinge plate and neal' the ventral valve margin of Documentation ofage and growth ofocean quahogs large. old specimens. Lutz and Rhoads (1977) found has been incomplete. Some studies included no alternating bands of aragonitic prisms and complex account of aging methodologies (Thorson in Turner crossed lamellar microstructures in the inner shell 1949; Jaeckel 1952; Loosanoff 1953; Skuladottir layer of ocean quahog shells that they believed were 1967); in others, concentric "rings" or "bands" related to periods of aerobic and anaerobic respira fOlmed in the periostracum ofsmall quahogs «ca. 60 tion. Thompson et a1. (1980a. b) reported that inter mm in shell length) were considered annuli, but nal growth bands corresponded toexternal checks on validation ofthe annual periodicity ofthesemarkings the valves and that the internal growth bands were was not provided (Loven 1929; Chandler 1965; formed by successive deposition of two repeating 4 Caddy et a1. 1974; Chene 1970 ; Meagher and Med growth layers or increments. Jones (1980) labelled 5 cof 1972 ). Microstructure of ocean quahog shells the growth increments (GI) as GI I and GI II, since has been studied. but the analyses did not each was microstructurally distinct. had thickness, specifical1y distinguish growth lines from growth and was formed within a time frame of several months. increments (Sorby 1879; B0ggild 1930: Taylor et a1. Forthese reasons. he considered the GI I layer to be 1969,1973; Lutz and Rhoads 1977, 1980). A means unlike minute "growth lines" or "striations" appear ing as subdaily deposits in the shells ofotherbivalves 'Northeast Fisheries Center Woods Hole Laboratory, National (Gordan and Carriker 1978); the GI II layer became Marine Fisheries Service, NOAA. Woods Hole, MA 02543. "Department of Geology. University of Florida, Gainesville, FL thinner and ill-defined from the GI I layer with 32611. ontogeny. 3Serchuk. F.M. and S.A. Murawski. 1980. Evaluation and status of ocean quahog. A rctWJ. is/andiro (Linnaeus) populations off the Since growth bands in ocean quahog shells seem to Middle Atlantic Coast of the United States. U.S. Dep. Commer.. lack microstructures of possible subannual peri NOAA, NMFS, Woods Hole Lab. Doc. 80-32, 4 p. odicities. the definitions of a growth line and growth Manuscript acceptedJuly 1983. FISHERY BULLETIN: VOL. 82. NO. I. 1984. FISHERY BULLETIN: VOL. 82. NO.1 ness or volume of tissue formed by accretionary METHODS growth between successive growth lines." In fact, Jones (1980:333) identified the layers as a "consist A commercial clam dredge vessel, the MY J)im!l' ently thin, dark gray, translucent increment" of pris Maria, was chartered for the marking operation dur matic microstructures which was "easily distin ing 25 July to 5 August 1978. The knife of the hy guished from" homogeneous and crossed micro draulic dredge was 2.54 m wide, and the cage was structural layers. lined with 12.7 mm square-mesh hardware cloth to Assessment research on ages of ocean quahogs retain small clams. Ocean quahogs for marking were requires accurate counts and measurements of collected within 9 km ofthe plantingsiteand released growth increments. Age observations are cus during a 10-d period (ca. 17,000 on 26 July, 3,000 on tomarily made of acetate peel images under optical 2 August. and 21.000 on 4 August 1978). Two 0.7 microscopes with transmitted light. An important mm thick carborundum discs. spaced 2 mm apart assessment requirement is that an annual increment and mounted in the mandrel of an electric grinder, has a distinct beginning and end. The concept of a produced distinctive parallel, shallow grooves from growth line forming between successive growth the ventral margin up onto the valve surface (Ropes increments fulfills that requirement. andMerrill 1970). Fouroperators ofgrinders marked Counts of supposed annual growth bands seen in about 1,600 clams/h. Groups (ca. 3.000-8.000) of the shells of ocean quahogs by Thompson et a1. marked clams were released at loran-C coordinates (1980a, b) resulted in slow growth rates and an within a rectangular area of about 3 by 6 JlS. extreme longevity estimate of 150 yr. Slow growth Marked clam recoveries were made in conjunction rate and suspiciously long life for a bivalve seemed to with annual clam resource surveys. During recovery invalidate the thesis of only a single growth line and operations. a Northstar 6000fi loran-C unit and Epsco growth incrementbeingformed annually. Supportive loran-C plotteraided ina systematic search ofthe plant evidence included finding similar bands in surf ing site. Marked clam recoveries were highly variable. dams; finding a low number of bands formed during On 20 and 21 August 1979 and about 387 d afterthe the onset of sexual maturity that were not explained marking operation. 43 hydraulic dredge tows at the by less thananannual frequency; finding anexpected planting site captured 14.043 ocean quahogs and 74 number of bands in small specimens of known age; (0.5%) were marked; on 9 September 1980 and 773 d finding an expected number of bands formed after the marking operation. 1,899 ocean quahogs sequentially in samples taken frequently during 2 yr were captured in 2 dredge tows and 249 (13.1 %) were that had only an annual periodicity; finding a line marked. Some marked specimens were damaged, deposited during the fall-winter, a period coinciding but 67 recovered in 1979 and 200 recovered in 1980 with spawning; and finding ages determined by were alive and had intact paired valves. radiometric analyses that were comparable with Recaptured specimens were frozen to prevent band counting. The latterthreetypesofinvestigation periostracum loss from drying and to facilitate open have beenexpandedbyJones (1980) andTurekianet ing without shell damage. Microscopic examination a1. (1982) with the same results. As part ofthe study. of 267 notched shells was made to assess the effects I. Thompson (pel's. comm.) marked and released of marking and to obtain growth measurements. ocean quahogs in the natural environment. but none Shell measurements were made to the nearest 0.1 was recovered. Direct and readily comprehended mm by calipers: growth after marking was measured observations of shell growth after marking were con to the nearest 0.01 mm by an ocular micrometer in a sidered to be important additional evidence in sup dissecting microscope. Acetate peels of all marked port of the thesis of an annual periodicity of growth ocean quahogs were prepared hy the procedures de line and growth increment deposition. scrihed hy Ropes(19R2'). Briefly.radial sectionsfrom In 1978. the National Marine Fisheries Service the umbo to ventral margin were produced on left marked large numbers of ocean quahogs for release valves. orientedtoinclude the broadestsurfaceofthe and recovery at a site 53 m deep and 48 km south single prominenttoothin the hinge. This exposedthe southeastofShinnecockInlet. LongIsland. N.Y., (lat. internal growth lines in the valve and hinge tooth for 40"21'N.long.72 24'W). Detailsofthis project have later treatment. The paired notch marks in a valve been reported in Murawski et a1. (1982). Periodicity of growth line formation and shell accretion after 'Reference to trade names does not imply endorsement by the notching of recovered ocean quahogs and the micro National Marine Fisheries Service. NOAA. 'Rope•. ,I. W. 19R2. Procedure. for pl'eparinl( acetate ppels of structure of unmarked and marked shells are de embedded valves ofArctil'a islandica for aging. 1I.S. Dep. Commer.• scribed herein with photographic documentation. NOAA. NMFS. Woods Hole Lab.• Doc. 82-18. 8 p. 2 ROPES ET AL.: GROWTH LINES OF OCEAN QUAHOGS were not usually oriented in the above plane of radial was no evidence ofrepairby shell covering the cracks sections, so additional radial cuts were made to in quahogs recovered 2 yr after marking. expose growth lines in the notch area. Subsequently. shells were embedded in an epoxy resin, and the cut Sectioned Shells shell edge ground on wetable carborundum paper and polished. Acetate peels were produced after An interruption ofshell deposition from notching in etchingthe shell cut surfaces in a 1%HCl solution for soml' sectioned shells was visible without magnifica 1 min, followed by microscopic examination of the tion in the cut surfaces. Microscopic examination peels that were sandwiched between slides. revealed a depression that curved dorsally back into Preparation of polished and etched radial sections the shell from the external surface and became of marked and unmarked ocean quahog valves were increasingly attl'nuated until it was unrecognizable further examined by scanning electron microscopy from the usual shell features along the inner margin. (SEM)8. These examinations included vertical tran This type of interruption was greatest in shells of sectsfrom the periostracumto the shell's interiorand small quahogs. probably due to some mantle tissue specific sites affected by the marking operation. incision. Periostracum penetrated into the interrup Shell microstructure was diagnosed by using the tion to a depth of about 1 mm. The thicker and classification scheme of Carter (1980), wherein shell tougher periostracum of large clams was less easily microstructures are elucidated on the basis of their incised during marking and probably served to major (i.e., fIrst-order) structural arrangement, inde minimize incision of mantle tissue. pendent of genetic or optical crystallographic criteria. Acetate Peels of Sectioned Shells RESULTS Acetate peels enhanced detection of interruptions Whole Shells in shell deposition due to notching (Figs. 1d, 2d, 3d, 4d). In small clams «80 mm shell length), the Notch marks showed clearly on wet shells but interruption was immediately followed by a line periostracumobscuredthe ventralmost ends extend similarto a succession oflines formed throughout the ing well beyond the ventral valve margin on all valve before marking. No additional lines were evi specimens (Figs. 1-4). Cuts made in the shell-free dentthereafter to the marginal tip in shells examined periostracum beyond the ventral margin of some from the late August 1979 recovery. but in 34 shells large quahogs had not been repaired after 2 yr (Fig. of small clams recovered in early September 1980, a 4a); small individuals. however. had completely second line occurred about midway to the tip in all formed yellowish-brown periostracum (grayish white shells and a third line had formed very near the in thephotographs) overnew shell growth, which con marginal valve tip and along the inner margin in 47% trastl'd sharply with darker. parlier deposition (Fig. (Fig. 3d). These were all considered to be annual 3a). growth lines for reasons dicussed later. Shell deposi In some specimens, the mark formed V-shaped tion between growth lines in the outer layer had a notches at the marginal edge of the old shell, New granular appearance, which was sometimes broken shell deposition was obviously disrupted for quahogs by a faint line of uncertain origin (Figs. 1d, 3d). The with deep V-notches, since the marginal shell be interruption of shell deposition from marking was tween the notches was outlined in relief over new also evident in large ocean quahogs (Figs. 2d, 4d), shell (Fig. 3b, c). Faintpaired bulges were also found although an infiltration ofthe depressionwith perios on the ventral inner surface ofthe notched valve ofa tracum was not clearly evident. The separation of few shells and occasionally the notches extended shell deposits was more definite and extended part way onto the opposite valve. An occasional live deeper into the shell of large clams, sometimes to a quahog was found with a cracked valve caused from depth of 2 mm (Figs. 2d, 4d). Growth lines were very handling during the marking operation. The black closely spaced (ca. 100 #Lm) and the shell ened margins of the cracks, suggestive of reducing depositional texture in between lines appeared conditions, indicated that the cracks were old. There similar to that seen in smaller clams. In large ocean quahogs, new shell was formed laterally beyond the 'SEM work was performed on an ETEC Autoscan instrument at notch mark and was an indication that the notching the Dental Research Center. University of North Carolina. Chapel operation had little effect on shell deposition and Hill, N.C.; on the .JEOL.JSM-:35 of the BioloRV Department. Princeton University, Princeton, N.J.; and on the lSI 1200 of the Department growth (Figs. 5a, 6a). of Geology, University of Florida. Gainesville, Fla. None of the marked quahogs had as severe an 3 FISHERY BULLETIN: VOL. 82, NO. I a 10mm 5mm 1mm d FIGlTRE I.-la) Right valve of an 18-yr-oldocean quahog.Arrtica is/andira. 71.4 mm in shell length recovered on 21 August 1979. Estimated annual growth was 1.3 mm. The notch-mark ares is shown before (b) and after Ie) peeling off the periostracum. (d) Optical photomicrograph (microprojectorl of seven repetitive growth lines in an see tate peel of the valve margin. An arrow points to an internlption of growth and growth line formed at or soon after marking the clam in 1978. An intemalline is evident between the mark-induced line snd the valve edge.Thisline is the normally occurringage mark probably formed in late autumn-earlywinter. Scale bars of magnification are included. FIGURE 2.-(a) Left valve of an ocean quahog. An'lira is/andira. about 110 yr old and 100.5 mm in shell length recovered on 20 August 1979. Estimated annual growth in I yr was 0.1 mm. The notch-mark area is shown before (bl and after Ie) peeling off the periostracum. Id) Optical photomicrograph Irompound micro scope) of many repetitive growth lines in an acetate peel of the valve margin. An arrow points to an interruption of growth and growth line formed atorsoonaftermarking the clam. Scale bars ofmagnification are included. 4 ROPES ET AL.: GROWTH LINES OF OCEAN QUAHOGS a 10mm 5mm / d 5 FISHERY BULLETIN: VOL. 82. NO. I 10mm b 5mm .. : ~". - "--- . Imm d FIGURE 3.-(a) Left valve ofa 15-yr-old ocean quahog..4rctica islandica. 6 I.7 mm in shell length recovered on 9 Sep tember 1980. Estimated 2-yr growth increment was 3.2 mm. The notch-mark area is shown before (b) and after lel peeling off the periostracum. (dl Optical photomicrograph lmicroprojector) of five repetitive growth lines in an ace tate peel of the valve margin. An arrow points to an interruption of growth and growth line formed at or soon after marking the clam. Two additional lines. one midwa.v to the valve tip and one near the valve tip. were formed after marking the quahog. Scale bars of magnification are included. FIGURE 4.-(a) Left valve of an ocean quahog, Arctira islandica. about 95 yr old and 91.7 mm in shell length recovered on 9 Septem ber 1980. Estimated 2-yr growth increment was 0.3 mm. The notch mark area is shown before (b) and after (c) peeling off the periostracum. (dl Optical photomicrograph (compound micro scope) of many repetitive growth lines in an acetate peel of the valve margin. An arrow points to an interruption ofgrowth and growth line formed ator soon aftermarking the clam. Scale barsofmagnification are included. 6 ROPES ET AL.: GROWTH LINES OF OCEAN QUAHOGS a lOmm c 5mm =O.lmm d 7 FISHERY BULLETIN: VOL. 82. NO.1 FIGURE 5.-la) Optical photomicrograph{compoundmicroscope) ofan acetate peelatanotch-markin theocean quahog. Arctica is/andica. shown in Figure 2 (scale bar = 100 /Lm). Note the single annual increment of growth formed laterally beyond the notch-mark. (b) SEM photomicrograph of the same shell specimen (scale bar = 100 /Lm). Abbreviatons of shell microstrncturaltecms in this and subsequent figures are expalined in this text. lcl Transitional CA-CL microstrnc- 8 ROPES ET AL.: GROWTH LINES OF OCEAN QUAHOGS ture interrupted by an ISP band that extends from the notch-line seen in (b) (scale bar = 10 ,.m). This photomicrograph was taken beyond the field ofview of (b). to the lowerleft and beyond the zone ofepoxy penetration. (d) Enlargement of epoxy penetration zone from (b) interrnpts nonnal shell microstrncture followed by a zone of cavernous. poorly organized. disrupted shell growth (scale bar = 10 ,.m). 9 FISHERY BULLETIN: VOL. 82. NO.1 FIGURE 6.-(a) Optical photomicrograph (compound microscope) ofan acetate peel at a notch-mark in a 100.3 mm long ocean quahog.Arctica islandica. about 110yroldrecovered on9 September 1980. Note two annual increments ofgrowth formed laterally beyond the notch-mark. estimated to be 0.1 mm (scale bar = 100 pm). (bl SEM photomicrograph ofthe same specimen (scale bar = 100 /Lm). The penetration of epoxy medium corresponds to the marking event. (I") Tran- 10 ROPES ET AL.: GROWTH LINES OF OCEAN QUAHOGS sitional CA-CL microstructure in the upperleftinterrupted by the penetrationofepoxy into the shell along the line cor responding with the notchingevent (scale bar = 10ILm). Beyond the zone ofepoxy penetrationthis line is recognized as an ISP band. Following this line is a zone of SphP disruption growth which gradually gives way to transitional CA-CL. This photomicrograph was taken off the field of view of (b), to the lower left. (d) Disruption growth of SphP micro structure following the line which corresponds to marking ofthe clam (scale bar = 50 ~). 11 FISHERY BULLETIN: VOL. 82. NO.1 interruption in shell deposition as did a 66.4 mm shape was only slightly atypical for ocean quahogs, shell-length specimen collected during a 1980 winter and no irregularity was found as an indication that an clam survey (Fig. 7 a, bl. A depression outlined the injury had oeeurred. The ratio between the greatest entire shape of the clam at the time of its formation shell height (21.4 mm) and shell length (25.9 mm) of and at 25.9 mm shell length. The shell formed before the smaller shell was 0.826. and for the entire shell the depression was raised laterally above thatformed (50.2 mm shell height; 66.4 mm shell length) was afterwards in a shinglelike fashion. Bothvalves ofthe 0.756. These values suggest that growth after the clam showed the interruption. The smaller shell depression departed from the more typieal. isomet- a 10mm O.lmm - FIGlIRE 7.-(a) Right valv~ of an OC~an quahog,Arc/ica is/andica. 23 yr old and 66.4 mm in sh~lll~ngth coll~ct~d n~ar th~ marking sit~ during 1980. An obvious int~rruption of growth (arrow) and radiating Iin~s in th~ an~rior half of th~ valv~ ar~ shown. (b) Optical photomicrograph (compound microscop~) of an ac~tat~ p~~l of th~ s~c tioned valv~ at th~ sit~ of growth int~rruption. Scale bars of magnification ar~ includ~d. 12 ROPES ET AL.: GROWTH LINES OF OCEAN QUAHOGS ric growth reported for ocean quahogs by Murawski with the second-order prisms radiating toward the et a1. (1982). Light radial lines extended from the depositional surface from a central, longitudinal axis. umbonal area to valve margin in the periostracum of Closer toward the inner shell layer the FP, SphP.and the anterior half of the shell formed after growth had CompP microstructures are gradually replaced by been interrupted, but their significance was not ISP bands. evident. The inner shell layer is characterized by growth lines composed of ISP which alternate with growth Microstructure of Unmarked Shells increment bands of FCCL microstructures. The hinge plate and tooth region have microstructures Theocean quahog shell is entirely aragonitic with an recognizable as distinct sublayers that are important inner and outer layer separated by an extremely thin for aging purposes. Here growth lines are construct prismatic pallial myostracum. The latter is com ed of narrow ISP (Fig. ge). These alternate with posed predominantly of irregular simple prisms growth increment bands that are composed of tran (ISP) and occasionally a few fibrous prisms (FP). sitional CA-CL. FCCL. and HOM microstructures Both principal shell layers contain two growth sub (Fig. ge). layers: The thin annual growth line and the wider In summary. ocean quahog shells are composed annual growth increment. Significantvariations were largely of HOM. CA-CL, and minor amounts of found in the microstructure of each during FCCL microstructures with prismatic bands of local examinations by SEM. importance. The latter constitute the growth line The distribution of microstructures in a typical layer; the former the growth increment layer. Figure ocean quahog shell may be seen by considering a 10 is a diagrammatic sketch of the distribution of transect from the exterior to interior depositional microstructures in the two principal layers of the surfaces. The thick. dark brown or black perios valve of a typical ocean quahog. tracum is an obvious exterior surface covering. but it is intimately associated with the shell. Some Microstructure ofMarked Shells aragonitic shell material is invariably removed when peeling off the periostracum and granules of Variations in the shell microstructure associated aragonite were found embedded in it during with notching and subsequent shell growth of ocean examination of unetched thin sections under the quahog specimens were studied by examining the crossed nicols of a polarizing microscope. The ventral margins of six quahogs. The same basic pat aragonite is dissolved by etching the polished sur tern described for unmarked shells was observed in faces of sectioned shells. leaving cavities, some with these specimens. Optical and scanning electron angular faces in the periostracum (Fig. 8a). photomicrographs of two shells illustrate the salient Important microstructures for aging purposes are features (Figs. 5.6). found mostly in the outer shell layer. The dominant The notching event in both shells was accompanied growth increment sublayer beneath the perios by a disruption of the normal growth pattern and a tracum exhibits a granular homogeneous (HOM) resumption ofshell growth at a new orientation. This microstructure which is very cavernous and has is seen in Figures 5a and 6a as a prominent flattened bleblike isolated crystalmorphotypes (ICM) (Fig. 8b, surface in the exterior shell surface from the marking d. These microstructures typically grade into operation followed by a lateral extension of the shell incipient ISP (Fig. 8d). Below the prisms is a layer of margin out beyond the notch mark and old shell sur crossed microstructures which appear to be tran face. The extension represents renewed growth at a sitional between simple crossed lamellar (CL) and new orientation. Retraction of the mantle during the crossed acicular (CA) structures (Fig. 8d). The latter marking process and resumption of shell growth at a predominates in the middle portion ofthe outer shell slightly new orientation resulted in a zone of either layer with occasional occurrences of fine complex loosely calcified or uncalcified shell paralleling the crossed lamellar (FCCL) microstructure. Tran shell margin and extending from the notch inward sitional CA-CL microstructures are also seen in toward the depositional surface. This zone is filled Figure 8e. with epoxy medium during the embedding process In the thin growth lines of the outer shell layer, FP and is seen in Figures 5band 6b. c, and d as the resis near the external surface soon give way to very dis tant, unetched material penetrating several milli tinctive spherultic prisms (SphP) (Fig. 9a-d). These meters into the outer shell "layer. The penetration SphP themselves grade into composite prisms zone disappeared shortly beyond the field ofview in (CompP) which are comprised of first-order prisms Figures 5b and 6b. Where this happened (Fig. 5c), a 13 FISHERY BlILLETIN: VOL. 82. NO. I FiGlIRE8.-Microstructuralvariation in the shell ofa 97.5 mm long, 92-yr-old ocean quahog,Arctica islandica. The central photograph is ofan acetate peel from a radial, polished, and etched section through the valve at the ventral margin. Black lines locate SEM photomicrographic enlargements of specific polished and etched areas in the section valve (scale bar = 1 mm). (a) Periostracum t"P") with angular cavities and HOM microstructure (scale bar = 10 pm). (b) Cavities in HOM microstructure of outermost valve surface. White rectangle encloses are enlarged intc) (scale bar= 10 pm). (c) Cavities showing bleblike ICM structures (scale bar= 1 pm). (d) HOM microstructure attoptrendingto ISP, then totransitional CA-CL toward the bottom. White rectangle encloses the area enlarged in (e) tscale bar= 10pm). (e) Transitional CA CL microstructure at higher magnification (scale bar = 10 p.m). 14 ROPES ET AL.: GROWTH LINES OF OCEAN QUAHOGS FIGURE 9.-Microstructuralvariation in the shell ofocean quahog,Arc/ira islandica, continued from Figure 8. (Central photograph scale bar= 1/Lm). (a) Annual growth line sublayer (region between upperand lower-most dark bands) in outershell layer, located beneath the CA-CL mi crostructure ofFigure 8e. White rectangle encloses the area enlarged in (b) (scale bar= 10/Lm). (b) Thecenterofthe growth line sublayershow ing FP.SphP•and CompP microstructures (scale bar= 10 /Lm). (cl Transitional CA-CL microstructure ofthe growth incrementsublayerbelow (b) (scale bar = 10 /Lm). (dl ISP fonning growth line sublayer within FCCL microstructure. Photomicrograph from area closer to the depositional surface thanprevious photos (scale bar= 10/Lm). Ie) HOMmicrostructure interrupted by two prominentISP bandsfrom asection through the hinge plate (scale bar = 10 /Lm). 15 FISHERY BULLETIN: VOL. 82. NO. ! ~~,...-Periostracum Outer Shell Layer Inner Shell Layer 1 ISP FIGURE IO.-Idealized. partial. radial cross section through the shell ofa typical ocean quahog,Arctica isltlfldica. showing the distribution of shell microstructures. Ventral margin is toward the left. Section is located inside the pallial line. Legend ofacron.vms: ICM = isolated crystal morphotypes; FP = fibrous prisms; SphP = spherulitic prisms; CompP = composite prisms; ISP = irregular simple prisms; HOM = granular homogeneous; CA = crossed acicular; CL = crossed lamellar; FCCL = fine complex crossed lamellar. growth line of ISP continued parallel to the earlier 60 mm shell length) are often visible macroscopically growth lines. onthe external surfaces ofwhole valves andinthe cut Thetypicaloutershell layerstructure formed by the surfaces of radial sections. However, macro- or mi time ofmarking is noted in Figures 5b and 6b. These croscopic examinations of large ocean quahog figures clearly show the alternation ofthe growth line valves are consistently frustrated by a lack of clear and growth increment sublayers. However. im differentiation of the same growth phenomena, Pre mediately following the marking event all specimens paration of acetate peels of shell cross sections. as showed a disruptionin microstructural development, has been described and photographically docu especially out near the shell surface. This coincided mented. greatly enhances discrimination of the lines with the presence of loosely organized SphP and increments of growth throughout the range of immediately following the marking event line (see shell sizes. particularly Fig. 6c. d). The disruption zone con Past investigators of the microstructure of ocean sisted of cavernous, poorly organized. microstruc quahog shells described some of the basic com ture (Fig. 5d). ponents. but did not clearly elucidate differences Inall six shells examined. the growth line associated between the lines and increments of growth. Sorby with the marking event continued inward toward the (1879: 62) appears to have given the first description shell interior. even beyond the zone ofepoxy penetra of the structure of the Arctica (= C-,prina) islandica tion (Figs. 5b, 6b). When this line was traced inward. shell: "InCyprina islandica we have another extreme itwas indistinguishablefrom the many earlierformed case, in which thefibres perpendiculartothe plane of growth lines. Such a view is seen in Figure 5c located growth are so short as to appear like granules, though well off the field of view Figure 5b. to the bottom left. the optic axes are still definitely oriented in the nor Here the growth line consistedofa diagonal ISP band mal manner." B4>ggild (1930:286) !'epOl'ted that he bounded on both sides by transitional CA-CL was unable to confirm Sorby's observations. Instead microstructure. he stated thatArctica islandica belongs to a group of species within the Arcticidae (= Cyprinidae) having DISCUSSION the least visible structure among all the bivalves. He termsthis structure homogeneous butsuggeststhere The layering and separation between growth lines are small traces of other structures in the shell, and growth increments ofsmall ocean quahogs « ca BlIlggild (1930) goes on to point out that the lower 16 ROPESET AI..: GROWTH LINES OF OCEAN QUAHOGS part of the shell (inner layer) is perhaps more the valves for examination by SEM, he found "a thin ..... representative of the common, complex struc discrete calcareous layer continuous over the outer ture ... and ... there are alternating layers of more surface of the valves between the periostrarum and transparent layers and finely grained ones." More the outermost shell layer." The layer is called mosaio recently Tayloretal. (1969, 1973) examined the shell stracum. The shell mio:rostructure in the growth incre microstructure ofArctica i.~/alldica. which they adopt ment sublayer beneath the periostracum is HOM, as ed as their "type species" to illustrate homogeneous Bl6ggild (1930) and Taylor et al. (1969, 1973) re shell microstructure. Basically. the general picture ported. The " ... minute, il"egular, rounded gran by Bl6ggild (1930) agrees with that of Taylor et al. ules ... have highly irregular contacts ..... (Taylor (1969). who used electron microscopy in their inves et al. 1969:51) that are particularly well exposed in tigation. However. they disagreed sharply with fracture sections. An abundant transitional CA Bf/lggild thatthe inner shell layer was "representative CL microstructure was found in the middle portion of the common complex structure." After examining of the outer shell layer and growth increment sub unetched fractured sections and polished and etched layer. This study confirmed its presence in ocean sections of both shell layers. Taylor et al. (1969, quahogs as reported by Carter (1980). The growth 1973) concluded that both shell layers in Arctica line sublayer of the outer shell layer had four islandica are built of minute, irregular rounded grades of prismatic structure (FP, SphP. ComP, and granules, quite variable in size (1.5-3 JLm across). ISP). Lutz and Rhoads (1977) examinl'd the inner having highly irregular contacts with their neighbors shell layernearthe umbo ofocean quahogs andfound and being poorly stacked. Taylor et aJ. (1969:51) bands of simple aragonitic prisms alternating with further reported: "In peels and sections ofthe inner complex-crossed lamellar and homogeneous struc layer, within the pallial line there is a marked colour tures. We found similar microstructures in the inner banding, in greys and browns. The only fine structure shell layer of the valve of ocean quahogs. Our that can be resolved is a suggestion of minute grains. analyses identified distinct microstructures, not which are most conspicuous in the translucent, unlike those found in the valve for the growthline and grey-colourless parts of the shell. These grains are growth increment layers in the hinge plate. arranged in sheets parallel to the shell interior. In the Growth line deposition more nearly approximates outer layer grains can also be resolved, but are an annual event than any shorter or longer interval. arranged in sheets parallel to the margin of the shell Marked clams recovered in late August 1979 had and growth lines." They also noted that these formed only one growth line other than the mark features are more clearly seen in the umbonal region induced check soon after the notching operation in where the orientation of grains normal to layering is 1978. They had been free about 22 d longer than a very conspicuous. Taylor et al. (1969) suggested that calendar year. Those recovered in early September the layering is a reflection of repeated (?diurnal) 1980 all had formed the growth line soon after the deposition of carbonate (a prospect deemed very notching operation. like those recovered in 1979,and unlikely by Thompson et al. 1980a). Also in the a second line appeared midway to the ventral valve umbonal region are thin (2-3 JLm) prismatic bands edge. which in all probability had been formed after which parallel the layering. Outside the pallial line. the late August 1979 recovery effort. These clams Taylor et al. (1969) reported the outer shell layer to were free about33 d more than 2 calendaryears since be very dense and opaque. with the most obvious the notching operation. A feature of the specimens structural features being fine grains arranged in recovered in 1980 was that about half had formed a sheets giving the layer a finely banded appearance. third line very near the ventral valve edge and along Analyses under SEM of oriented fractured, and the inner margin. All ofthe narrow growth lines were polished and etched sections ofocean quahog shells separated by relatively even. broad areas of growth revealed that microstructural variation is more com increment deposits suggestive ofno more orless than plex than had been proposed by B0ggild (1930) or an annual interval for the deposition of growth lines. Taylor et al. (1969, 1973). Thin sections of isolated even though the time of formation of such lines may periostracal fragments examined under crossed not correspond to an exact number of calendar days. nicols confirmed the presence of embedded These observations confirm similar conclusions ofan aragonite granules in the periostracum of ocean annual periodicity of growth line formation by quahogs reported for other recent bivalves (Carter Thompson and Jones (1977), Thompson et at and Aller 1975). These granules probably form a (1980a, b). and Jones (1980). layer like that described for the blue mussel.1\(ytilus Radiometric techniques for aging bivalve shells edulis. by Carriker (1979). After special treatment of have recently been applied to ocean quahogs. 17 FISHERY BVLLETIN: VOL. 82. NO. I Thompson et a1. (1980a) reported that the predicted growth line formation in marked ocean quahogs and radiometric age of an ocean quahog having 22 bands analyses of growth in the same specimens by corresponded exactly to 22 yr when aged using mRa. Murawski etal. (1982) presentsignificant supporting Turekian et al. (1982) concluded that age deter evidence for the hypothesis ofslow growth and a long minations of ocean quahogs from radiometric life span in the species. Ocean quahogs apparently analyses are compatible with counts ofbands formed live longer than any other bivalve known to man. annually. Thus. radiometric studies support the con tention of an annual periodicity of growth lines in ACKNOWLEDGMENTS ocean quahogs. Various environmental disturbances have been We thank Brenda Figuerido and John Lamont for implicated in the formation of shell abnormalities their assistance in preparing the art work and and atypical growth lines in other bivalve species photographs, andIdaThompson. UniversityofEdin (Weymouth et al. 1925; Shuster 1957; Merrill et al. burgh. Department of Geology, King's Building, 1966; Clark 1968; Palmer 1980). It is therefore. con Edinburgh EH 9 3JW, Scotland, for encouragement ceivable that the stress imposed by dredging. mark in undertaking the study and helpful comments on Ing. and returning the ocean quahogs to the ocean the manuscript. floor and their burrowing activities hastened the for mation of a growth line in 1978. Thereafter. natural events affecting the metabolism of shell deposition LITERATURE CITED are more likely stimuli. Such events apparently did B0GGILD. O. B. not occur during the period after the formation ofthe 1930. The shell structure of mollusks. K. Dan. Vidensk. growth line in 1978 and recovery of clams in late Selsk. SK (Copenhagen) 2:231-325. August 1979. Instead a growth line that in all prob CARRIKER. M. R. ability had formed in 1979 was found in the shells of 1979. Ultrastructure of the mosaicostracal layer in the shell clams recovered on 9 September 1980. Its formation of the bivalve Mytilus eduli." Veliger 21:411-414. CADDY. J. F .. R. A. CHANDLER. AND D. G. WILDER. may have occurred in late August 1979. but the third 1974. Biology and commercial potential of several underex line found in halfofthe clams recovered on9 Septem ploited molluscs and crustaceans on the Atlantic coast of ber 1980 suggests the possibility of its formation in Canada. Proceeding of a Symposium on the Industrial early September 1980. By inference. then. growth Development Branch of Environmental Canada, Mon treal. Feb. 5-7. 1974. III p. (Prepared at Fish. Res. line formation in 1979 and 1980 occurred in Board St. Andrews BioI. Stn.. N.B.) September. CARTER••J. G. The reported life span (150 yr. Thompson et a1. 1980. Guide to bivalve shell microstructures. In D. C. 1980a) ofocean quahogs surpasses similar estimates Rhoads and R. A. Lutz (editors). Skeletal growth of for other bivalves. Age and growth of the far east aquatic organisms. p. 645-674. Plenum Press. N. Y. CARTER. J. G.. AND R. C. ALLER. mussel, Crenomytilus grayanus, have been deter 1975. Calcification in the bivalve periostracum. Lethaia mined from examinations of shell stl1lcture, an 8:315-320. oxygen-isotope method. and notching experiments CHANDLER. R. A. (Zolotarev 1974; Zolotarev and Ignat'ev 1977; 1965. Ocean quahog resources of Southeastern Northum berland Strait. Fish Res. Board Can.. Manuscr. Rep. Zolotarev and Selin 1979). These investigations (BioI.) 828. 9 p. indicated that longevity of the mussel may exceed CLARK. G. R.. II. 100 yr. Turekian et a1. (1975) proposed a longevity of 1968. Mollusk shell: Daily growth lines. Science (Wash.. about 100 yrfor a deep-sea nucoloid, Tindaria callis D.C.) 161:800-802. tl/ormis. after determining ages by radiometric 1974a. Growth lines in invertebrate skeletons. Annu. Rev. Earth Planet. Sci. 2:77-99. means and counting regularly spaced bands in the 1974b. The Paleoperiodicity Newsletter. Vol. 1. p. 1-2. shell of one of the largest (8.4 mm in shell length). It GORDON. J .• AND M. R. CARRIKER. seems likely that longevity of ocean quahogs may 1978. Growth lines in a bivalve mollusk: Subdaily patterns exceed 150 yr. Murawski and Serchuk (1979) report and dissolution of the shell. Science (Wash.. D.C.) 202:519-521. ed a maximum shell length of 131 mm for ocean JAECKEL. S., JR. quahogs in extensive collections takenfrom the Mid 1952. Zur Oekologie der Molluskenfauna in der westlichen dle Atlantic Bight. A specimen of this size is half Ostsee. Schr. Naturwiss. Ver. Schleswig-Holstein 26:18 again as large as the 88 mm example of a 149-yr-old 50. ocean quahog reported by Thompson et al. JONES. D. S. 1980. Annual cycle of shell growth increment formation in (1980a). two continental shelf bivalves and its paleoecologic In conclusion. the foregoing description of annual significance Paleobiology 6:331-340. 18 ROPES ET AL.: GROWTH LINES OF OCEAN QlIAHOGS Lo08ANOFF. V. L. Introduction. Nuculacea-Trigonacea. Bull. Br. Mus. 1953. Reproductive cycle in Cyprinn islnndica. BioI. Bull. (Nat. Hist.) Zool. Suppl. 3:4-125. (Woods Holel 104:146-155. 1973. The shell structure and mineralogy of the Bivalvia. II. LOVEN,P.M. Lucinacea - Clavagellacea. Condusions. Bull. Br. Mus. 19:!!!. Hietnie ~ur Kenlllnis del' {:\'I"'illll is/rl/ldi('a L. in Ore (Nat.. Hist.) Zool. 22:255-294. sund (Contributions to the knowledge oft'yprina islandica THOMPSON, I.. AND D. S. JONES. L. in the Oresund). K. Fysiogr. SiiUsk. Lund Handl. N.F. 1977: The ocean quahog. Arc/ica islandica. "tree" of the 41: 1-38. {Transl. by Lang. Servo Div.. Off. Int. Fish.. North Atlantic shelf. IAbstr.] Geol. Soc. Am. 9:1199. NMFS. U.S. Dep. Commer.. Wash.• D.C.] THOMPSON. I.. D. S. JONES, AND D. DREIBELBIS. LUTZ. R. A.• AND D. C. RHOADS. 1980a. Annual internal growth banding and life history of the 1977. Anaerobiosis and a theory of growth line for ocean quahog Arc/ica islandica (Mollusca: Bivalvia). mation. Science (Wash.. D.C.) 198:1222-1227. Mar. BioI. (Berl.) 57:25-34. 1980. Chapter6. Growth patterns within the molluscan shell. THOMPSON. I.. D. S. JONES. ANDJ. W. ROPES. An overview In D. C. Rhoads and R. A. Lutz (editors), 1980b. Advanced age for sexual maturityin theocean quahog Skeletal growth of aquatic organisms. p. 203-254. Arc/ica islandica (Mollusca: Bivalvia). Mar. BioI. (Berl.) Plenum Press. N.Y. 57:35-39. MERRILL. A. S.• J. A. POSGAY. AND F. E. NICHY. TlIREKIAN, K. K.. J. K. COCHRAN. D. P. KHARKAR. R. M. CERRATO. 1966. Annual marks on shell and ligament of sea scallop J. R. VAII':NYS. H. L. SANDERS. J. F. GRASSLE. AND J. A. Plarop"r/en magellanirus. U.S. Fish Wildl. Serv.• Fish. ALLEN. . Bull. 65:299-311. 1975. Slow growth rate of a deep-sea clam determined by MURAWSKI. S. A.. J. W. ROPES. AND F. M. SERCHUK. "IRa chronology. Proc. Natl. Acad. Sci. 72:2829-2832. 1982. Growth of the ocean quahog. Arc/ira islandira, in the TlIREKIAN. K. K.. J. K. COCHRAN. Y. NOZAKI. I. THOMPSON. AND D. Middle Atlantic Bight. Fish. Bull., U.S. 80:21-34. S. JONES. MURAWSKI. S. A.. AND F. M. SERCHUK. 19~~: Determinationofshell dell"sitiOlll'ates. ,L\ rdi...ii.lml 1979. Shell length-meat weight relationships of ocean dica from the New York Bight using natural "'RA and quahogs. Arclira islandira. from the Middle Atlantic Z21Th and bomb-produced "C. Limnol. Oceanogr. 27: Shelf. Proc. Natl. Shellfish. Assoc. 69:40-46. 737-741. PALMER. R. E. TlIRNER. H. J .. JR. 1980. Observations on shell deformities, ultrastructure. and 1949. The mahogany quahog resource of Massachusetts. In increment formation in the bay scallop Argopec/en Report on investigations of improving the shellfish re irradiano. Mar. BioI. (Berl.) 58:15-23. sources of Massachusetts. Commonw. Mass. Dep. Con RHOADS. D. C.. AND R. A. LUTZ (editors). sel'\'.. Div. Mar. Fish.• p. 12-16. 1980. Skeletal growth of aquatic organisms. Plenum Press, WEYMOllTH. F. W.. H. C. MCMILLIN. AND H. B. HOLMES. N.V.. 750 p. 1925. Growth and age at maturity of the Pacific ra~or clam. ROPEs .•J. W.• AND A. S. MERRILL. Siliqua patula (Dixonl. U.S. Dep. Commer.. Bur. Fish. 1970. Marking surf clams. Proc. Natl. Shellfish. Assoc. Doc. 984. p. 201-236. 60:99·106. ZOLOTAREV. V. N. SHUSTER. C. N.. JR. 1974. Detennination of age and growth rate of the far east 1957. On the shell ofbivalve mollusks. Proc. Natl. Shellfish. mussel. Crellomytilus grayanus (Dunker). from its shell Assoc. 47:34-42. structure. Dok!. Akad. Nauk SSSR 21615):1195-1197. SKULADOTTIR. U. ITransl. by Plenum Publ. Corp.• N.Y.. 1974. p. 308 1967. Krabbadyr og skeldyr (Crustaceans snd mollusks). 309.1 Rsdstefna lsI. Verkfraedinga. 52:13-23. Proceedings of ZOLOTAREV. V. N., AND A. V. IGNAT'EV. the Conference of Islandic Professional Engineers. 1977. Seasonal changes in the thickness of the main layers ITransl.: Transl. Bur.• Fish. Res. BoardCan.. Bioi. Stn.. St. and temperature growth of the marine molluscan Andrews. N.B.. No. 1206.1 shells. BioI. Morya 5:40-47. {TransI. by Pelnum Publ. SORBY.H.C. Corp.. N.Y.. 1978. p. 352-358.1 1879. Address on the structure and origins oflimestones. Q. ZOLOTAREV. V. N.• AND N. I. SELIN. J. Geol. Soc. Lond. 35:56-95. 1979. The use of tags of shells to determine growth of the TAYLOR, J. D .. W. J. KENNEDY. AND A. HALL. mussel Crenomy/ilus grayallus. BioI. Morya 1:77-79. 1969. The shell structure and mineralogy of the Bivalvia. ITransl. by Plenum Publ. Corp.. N.Y.. 1979. p. 58-59.1 19