! 1992 Rhode Island Sea Grant ISBN 0-938412-33-7 This publicationis sponsoredin part by RhodeIsland Sea Grant under NOAA Grant No. NA 89 AA-D-SG-082. The views expressedherein are those of the author and do not necessarilyreflect the views of NOAA or any of its sub-agencies.This publicationis alsosponsored by RhodeIsland Cooperative Extension System under Grant No. Si 402, and the Northeast RegionalAquaculture Center, U.S. Depart- ment of Agriculture. This is RhodeIsland Sea Grant publicationNo, RIU-B-92-001 P1276! and publicationNo. 2674 of the RhodeIsland Agricultural Experiment Station,College of ResourceDevelopment, University of RhodeIsland. Additional copiesof this publicationare available from Rhode Island Sea Grant, University of RhodeIsland Bay Campus, Narragansett, RI 02882-1197. ~ c;a~+ . Printedonrecyeied paper. The NortherniNI ~ Quahog: The Biology of Mercnusria mercenaria Michael A. Rice AssistantProfessor of Fisheriesand Aquaculture Departmentof Fisheries,Animal, and Veterinary Science University of Rhode Island Kingston, Rhode Island 02881 Edited by CaroleJaworski and Malia Schwartz Rhode Island Sea Grant Information 0%ce Design by Donna Palumbo O'Neill Rhode Island Sea Grant Information Of6ce Drawn from living clam .04mm less than one- fiftieth of sn inch! in length. S, siphons, two tubes, one of which conducts water bearing food and oxygen to the body within the shell, the other conducting a stream containing waste matter to the exterior. F, foot, the organ of locomotion.B, byssus, a delicate thread for attachment, which is not presentin the adult. From Bulleti n of the ¹w York State hfuseum, le. 43, Vol, tt8, April 190I. INTRODUCTION GENERAL BIOLOGY 1 Taxonomic Position and Common Names 1 GeographicDistribution and RecognizedSubspecies Similar Bivalve Species Anatomy 25 27 Respiration and Circulation Filter Feeding and Digestion Reproductive Biology and Life Cycle 8 Growth 11 ECOLOGY AND ENVIRONMENT 13 Ecology 13 Factors Influencing Growth of Adult Populations 13 Factors Influencing Larval Settlement and Juvenile Survival 14 Quahogs as Prey 16 Structure of Unexploitedand ExploitedQuahog Populations 18 Quahog Assemblages as Modifiers of the Environment 20 Effects of Environmental Factors 20 Effects of Temperature 20 Effects of Salinity 21 Effects of Oxygen 22 Effects of Pollutants 23 FISHERIES 27 Fishing Methods and Gear 27 Fishery Management Regulations 32 %ster Quality and the Fisheries 32 Economics of the Fishery 34 AQUACULTURE $7 Hatchery Methods 37 Nursery Methods 39 Growout Methods 40 Aquaculture Diseases and Pathology 40 CONCLUSION REFERENCES Acknowledgements I wish to acknowledge a number of individuals and orlmnixations who were very helpful in the production of this book, Elizabeth Watkins and Vincent Kncena provided many of the line drawings. Recognition is due to William Brown Publishers, Macmillan Publishers, the Joiirnal of Shellfish Reaearch,Trunsactions of the American Fisheries Society, and Scientific American for granting permission to reproduce previously published mate- rial. I especially thank Dr. Joseph DeAlteris, Dr. Lindy Eyster, Dr. Jan A. Pechenik, Dr. Conrad Recksiek, Ms. Kathleen Castro, Mr. Gef Hlmlin, Ms. Carole Jaworski, and Mr. Robert Rheault for providing valuable comments on the manuscript. Intvoduction Thenorthern quahog, Mercenaria mercenaria Linnaeus!, isof interest to manypeople, especially to thoseliving along the eastern seaboard ofthe UnitedStates. This species supports valuable commercial and recreational shellfisheriesallalong the Atlantic coast. Seafood connoisseurs eqjoya host ! ofdelectable recipes alldependent onits delicate Qavor. Inaddition, some ! marinescientists find the quahog tobe the ideal organism tostudy a hostof ! fundamentalscientific problems. Theimportance ofthe quahog tomany peoplehas not been overlooked bythe legislature ofthe state of Rhode Island, whichdesignated the quahoghs the oKcial statemollusk. Thisbook is patterned after a previousRhode Island Sea Grant publica- tionon the lobster J. StanleyCobb's 976! TheAmericun Lobster: The BiolagyofHonmrus americanus! andis intended tobe a companionvolume. Theaim of this book is to providean overview of someof theinformation availableabout the quahog, and to provide a startingpoint for those who wishto delve further into the Mercenarta literature. Although the references sectionof this book contains a numberof recent citations, the serious re- searchershould be aware of an excellent annotated bibliography 74! con- tainir~ over 2,200 citations. Photographby DonnaPalgznbo O'A'eili GenemlBiology Taxonomic Position and Common Names Menenaria mercenaria Linnaeus, 1758!is a molluskof the class Bivalvia formerlyPelecypoda!, subclass Lamellibranchiata, order Heterodonta,and family Veneridae. Thus, its shellhas two valves,it has sheet-likegills, it is clam-hkewith large hinge teeth, and is in thefamily of hard-shelledclams. In theliterature prior to theearly 1960s, the quahog is knownas Venus mercenaria L., 1758.The accepted o%cial malacological commonname the nameused by molluskan scientistsand shell enthusiasts! for M. mercenariais the northernquahog, but it is locallyknown as hard clam,hard-shell clam, round clam, or quahog quahaug!. In additionto these regional names, there are difFerent names for arumals of different sizes. In RhodeIsland, the smallestlegally sold quahogs approximately 48 milli- metersvalve length! are known as "littlenecks."Intermediate-sized hm length!and large >75mmlength! are known as "cherrystones and "chowder quahogs,"respectively. In someother localities, the term cherrystone"refers to the smallestanimals that can be legally harvested.Other market names indude topnecks and "topcherries forintermediate-sizedaninmls. In NewEngland, quahogs have been harvested since pre-colonial times. Accordingto the OxfordEnglish Dictionary, RogerWilliams, the flrst colonial governorof the ProvidencePlantation Colony, wrote in 1643,"Poquauhock, this the English caOHens, a little thick shel-fis sic!, which the Indians wade deepe sic! and dive for." Our modernword "quahog"is derived&om "poquauhock"in the languageof the NarragansettIndians living in whatis nowRhode Island!. They ate the meatand used the shellsto makewampum beads,which were a trading currency.The wampumbeads made &om the Phocognrph by Michaei A. Rice purple shell margins were especially valuable. Indeed, the speciesname "M. mercenaria" refers to their former value as a tradn~ currency. Geographic Distribution and Recognized Subspecies The quahog inhabits shallow coastal waters from The Gulf of St. Lawrence in Canada to Florida 81!. It has been introduced into Europe ; 115; 116; 117! and California 5; 56!. One recognized subspeciesis Mercenaria mercenaria notata, which is characterized by chestnut-colored, often chevron-shaped markings on the shell exterior 2; 96!. Field and laboratory studies suggest that the M. m. notata subspeciesoccurs with a frequency of 1 to 2 percent or less in the southern Atlantic states 6; 120!. The subspeciesMercenaria mercenaria texana is native to the northern coast of the Gulf of Mexico ; 55!. Similar Bivalve Species The southern quahog, Mercenaria campechiensis,is a closely related speciesthat has more prominent shell sculpturing and attains larger sizes. M. campechiensisis found in the more southern regions of the range of M. mercenaria and the two species are reported to form hybrids 8; 176!. The false quahog, Pitar morrA,uanus formerly Cailocardia!, is found in the geographic range of M mercenaria 96!. Individuals of the false quahog, P. morrhaanus, reach a maximum size of about 60mm valve length, have thinner shells than Mercenaria, and are dull gray with smoother shells. There is evidence that Pitar prefers a muddier substrate than does Mercenaria 00!. Anatomy The quahog, like all other clams, has its soR tissues surrounded by a shell consisting of two halves or valves. The quahog shell consists of calcium carbonate in a crystalline form aragonite! held in a network of complex organicmolecules 05!. The valvesare held togetherby a tough,but pliable, hinge ligament alongthe top or dorsalsection of the animal, called the hinge plate. The valve hinge, which consists of intermeshing teeth, forms the joint between the valves. The hinge allows for opening and closing of the shell. In close proximity to the hinge is the umbo or what is commonly known as the "beak."If you notethe roughly triangular shapeof the quahog,and orient the quahogso that the hinge and the umbo are at the top or dorsal,the widest edgeof the shell or the part that is oppositethe umbo is the ventral shell margin. The umbo is the oldest section of the shell, with subsequent shell growth radiating out from it. The concentricrings on the external surfaceof crenulated shell margin anterior posterior adductor adductor muscle musde scar erlor ot hinge actor anterior foot posterior retractor adductor musde labial palps excurrenl siphon anterior adductor ncurrent muscle siphon Figure1. Grossanatomy ofthe quahog, Mcrcenonamercenaria, Thesoft tissues aredis- sectedfree from the left valve toreveal the pallial lineand adductor musde scars. From Shuster, C,N.1966. "A three-ply representation ofthe majororgan systems ofa quahog."National Shell6shSanitation Program, U.S. Department foot inner outer ofHealth, Education, and Welfare. Pechenik, demibranch J,A.1991, Biokgy ofthe invertebrates nded.!. demibranch WilliamC. BrownPublishers. Used with permission. the shell indicate the generalgrowth pattern of the animal. The mantle is responsiblefor