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Adaptation to Rock Boring in Botula and Lithophaga (Lamellibranchia

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Adaptation to Rock Boring in Botula and Lithophaga (Lamellibranchia 383 Adaptation tto RocRock BorinBoring iin BotulaBotula and Lithophaga (Lamellibranchia,, MytilidaeMytilidae) with a Discussion on ththe EvolutioEvolution oof thithis Habit By C. MM.. YONGYONGE (From(From thethe UniversityUniversity ofof Glasgow)Glasgow) SUMMARY Alll speciespeciess ooff BotulaBotula anand ooff LithophagaLithophaga livlive withiwithin boringborings thethey excavatexcavate in rockrock,, IIn BotulaBotula borinboring iis mechanicalmechanical,, intinto sofsoftt non-calcareounon-calcareous rocksrocks.. FirFirm attachmenattachment is madee bby byssabyssall threadthreadss arrangearranged iin aa larglarge anterioanteriorr anand aa smalsmalll posterioposterior groupgroup.. The rocrock iiss abradeabraded bby ththe dorsadorsall surfacesurfaces ooff ththe valvesvalves,, whicwhich arare deepldeeply erodederoded;; ththe forceforces responsiblresponsiblee arare contractiocontraction ooff ththe byssabyssall anand pedapedall retractorretractors anand ththe powerfupowerfull openinopening thrusthrustt ooff ththe lonlong ligamentligament.. TherThere iis nno rotatiorotation withiwithin ththe boringboring.. IIn LithophagaLithophaga boring iis confineconfined tto calcareoucalcareouss rocksrocks,, byssabyssall attachmenattachmentt beinbeing weaweak anand the valvesvalves littllittlee erodederoded.. ThThe fusefused inneinnerr lobelobes oof ththe mantlmantle edgeedges mamay protrudprotrude frofrom betweebetween ththee valvevalvess anteriorlyanteriorly.. TheThey arare applieapplied tto ththe heahead oof ththe borinboring anand iit iis postulatepostulated thathatt thethey secretsecrete aan aciacid mucusmucus.. SomSome speciespecies oof LithophagaLithophaga borbore straighstraightt intinto rockrock,, otherss rotatrotate iin ththe boringboring.. Aggregationss ooff foreigforeign particleparticles oon ththe valvevalves oof BotulaBotula anand LithophagaLithophaga arare dudue to ththee effeceffectt ooff accumulationaccumulationss ooff mucoumucous glandglands botboth anterioanterior anand posterioposterior to ththe ligaliga- ment.. ThThee origiorigin anand functiofunction ooff thesthese glandulaglandularr massemasses iis discussediscussed bby referencreference to conditionconditionss iin ththe allieallied Modiolus.Modiolus. TheThey helhelp iin disposindisposing oof sedimentsediment.. IIn LithophagaLithophaga ththee anterioanteriorr glandsglands,, aatt leastleast,, probablprobably secretsecrete aan aciacid mucusmucus.. CrystallinCrystalline calcareoucalcareous secretionsecretionss whicwhich adventitiousladventitiously lengthelengthen ththe shellshells ooff certaicertain speciespecies oof LithophagaLithophaga arare produced bby ththe wallwallss ooff ththe siphonssiphons,, whicwhich mamay alsalso lalay dowdown aa calcareoucalcareous lininlining to ththee boringboring.. Evolution ooff ththe borinboring habihabitt iin ththe MytilidaMytilidae iis discussediscussed iin relatiorelation tto shell-formshell-form.. Thee elongatelongate shelshelll ooff ththe dimyariadimyarian typetypes BotulaBotula anand LithophagaLithophaga iis comparecompared witwith thosthosee ooff EnsisEnsis oorr TagelusTagelus anand showshown tto bbe derivederived frofrom thathatt oof aa heteromyariaheteromyarian ancestoancestorr nonott unlikunlike Modiolus.Modiolus. ThThe effeceffectt oof ththe tangentiatangentiall componencomponentt iin the growtgrowth off ththee mantle/shelmantle/shelll counteractcounteractss thathatt oof reductioreduction oof ththe anterioanterior enend oof ththe bodbody and off ththe mantle/shelmantle/shelll associateassociated witwith heteromyarianisheteromyarianism anand iis alsalso concerneconcerned witwith lengtheninlengthening ooff ththe ligamentligament.. ChangChange iin forform iin ththe MytilidaMytilidae madmade possiblpossible changchange iin habithabit.. CONTENTS PAGE SUMMARSUMMARYY .......................... 38383 INTRODUCTIOINTRODUCTIONN .......................... 38384 OBSERVATIONOBSERVATIONSS OONN BOTULBOTULAA SPPSPP.. .................... 38385 (a(a)) HabitaHabitatt 38386 (6(6)) ObservationObservationss oonn intacintactt animalanimalss .. .. .. .. .. .. .. .. .38.386 (c(c)) BehaviouBehaviourr withiwithinn borinboringg .................. 38387 (d)(d) ForFormm ooff ththee borinboringg ...................... 383888 (e)(e) MethoMethod ooff borinboring 38388 (/(/)) MateriaMateriall adherenadherentt tto ththe shelshelll 38389 OBSERVATIONOBSERVATIONSS OONN LITHOPHAGLITHOPHAGAA PLUMULPLUMULAA ................ 393900 (a)(a) HabitaHabitat t .......................... 393911 (b)(b) MantlMantlee anand siphonsiphonss .................... 39392 [Quarterl[Quarterly JournaJournall ooff MicroscopicaMicroscopicall ScienceScience,, VolVol.. 9696,, parpartt 33,, pppp.. 383-410383-410,, 1955.1955.] 384 Yonge—Adaptation to Rock Boring in (c) Ciliary currents ........... 393 (d) Behaviour within the boring ......... 393 (e) Nature of the boring . -395 (/) Method of boring ........... 395 (g) Glandular areas ........... 397 OBSERVATIONS ON OTHER SPECIES OF LITHOPHAGA ....... 400 MATERIAL ADHERENT TO THE SHELL . .401 EVOLUTION OF THE BORING HABIT IN THE MYTILIDAE ...... 403 REFERENCES ............. 410 INTRODUCTION VARIETY of unrelated Lamellibranchia have become adapted for boring AL into rock. As suggested elsewhere (Yonge, 1951a), evolution of this habit would seem to have proceeded along one of two lines. It may have done so by further specialization of species already adapted for deep-burrowing in soft substrata, enabling them to penetrate stiff clay and finally rock. Examples are provided by the Pholads generally and also by Platyodon among the Myidae (Yonge, 195 ib). The alternative route may have been by way of the 'nestling' habit possessed by various bivalves that live attached by byssus threads within depressions and crevices among rocks. Initially they may enlarge the depres- sions and eventually become active borers into the rock. Such animals are exemplified by species of Hiatella {Saxicava) described by Hunter (1949), and by Tridacna crocea among the Tridacnidae (Yonge, 1936). Also to be included within the latter category are species of the Mytilidae, the best known being Lithophaga {Lithodomus) lithophaga, the date mussel of the Mediterranean. Like all species of this genus (or subgenus according to Thiele (1935)), L. lithophaga bores only into calcareous rock, doing so, it has been claimed, by the aid of an acid secretion from the mantle (Carazzi, 1892, 1903; List, 1902; Otter, 1937; Kiihnelt, 1930). Details of the procedure employed in boring have hitherto been somewhat obscure. Thiele (1935) subdivides the genus Lithophaga into subgenera of which Lithophaga s.s., further split by him into three sections, is one and Botula is another. Three species of the latter, according to Keen (1937), occur along the Pacific coast of North America. Two of these, B. falcata and B. cali- forniensis, were collected between tide-marks near San Francisco. They proved of particular interest because although they bore they do so mechani- cally. Acid secretion would in any case be useless because they occur in non- calcareous rocks. Examination of these animals proved of unusual interest in view both of the absence of previous work and of the light thrown on the evolution of the boring habit in the genus Lithophaga. The observations on Botula spp. were conducted in the Department of Zoology, University of California, Berkeley, while the author was Visiting Professor in the spring of 1949. The further work on Lithophaga plumula was carried out later that year at the Hopkins Marine Station, Pacific Grove. Some observations on other species of Lithophaga were subsequently made at Botula and Lithophaga 385 Bermuda in 1951. Thanks are here rendered to the late Professor H. Kirby and members of the staff at Berkeley, in particular Dr. R. I. Smith, and also to Dr. R. Boline, Acting Director at Pacific Grove. At Glasgow important help has been given by Dr. Vera Warrender, who made the drawings repro- duced in figs. 1 and 8, and by Dr. H. F. Steedman and Mr. G. Owen, who prepared sections. OBSERVATIONS ON BOTULA Botula (Adula) falcata (Gould) and B. (A.) californiensis (Philippi), respec- tively the hooked pea-pod shell and the Californian pea-pod shell (Keep and Baily, 1935), are true rock-borers. As shown in fig. 1, they differ markedly . 1 cm,. FIG. 1. A, Botula californiensis; B, B. falcata, both viewed from left side, showing erosion of shell dorsally and accumulation of material on posterior region of valves. in form. Both are long, but especially B. falcata, which is also straight, B. cali- forniensis being markedly concave ventrally. The valves of this species are more angular and the umbones more pronounced. Apart from the form, the thick periostracum also differs in the two species: that of B. falcata is thrown into numerous fine transverse ridges, which are absent in the other species, where the periostracum is rather darker in colour. In adult specimens of either species the periostracum is always eroded over a wide area dorsally, an important matter which will be discussed later. In both species the posterior region of the valves is covered with an amorphous encrustation. In the regions where specimens were collected B. falcata was much the commoner, and laboratory observations were confined to this species. But, at any rate between tide-marks, the two species have the same habits and bore into the same type of rock. Smith and Gordon (1948) report that both occur in Monterey Bay, boring into shale, down to depths of 15 fathoms. Here also B. falcata is much the more
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