ISSN 0704-3716

Translation of Fisheries and Aquatic Sciences adian r No. 5636

Studies on Scandinavian Ml illailli'4erpe-r.lt of Eleheries' g: 0,c'édilê

I. Mya it'‘4,1% 25 1995 Mjnistàré ofi&hot et dos

Original title: Studier over nordiske Mollusker

In: Scientific Reports of the Danish Society of Natural History, vol. 52, pp. 133-158

Original language: Danish

Available from: Canada Institute for Scientific and Technical Information National Research Council Ottawa, Ontario, Canada KlA 0S2

1995

28 typescript pages 1

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Translation Services Services de traduction MULTILINGUAL TRANSLATION DIRECTORATE - DIRECTION DE LA TRADUCTION MULTILINGUE Translation letterhead F E By 3 1995 Originator file No. Department Division/Branch City DFO Sciences, Biblioth. Translation Request No. Language Translator (initials) 3290785 Danish LT

Scientific Reports of the Danish Society of Natural History, Vol. 52, pp. 133-158

Scientific Reports 1900 [1901]

STUDIES ON SCANDINAVIAN MOLLUSKS. I. MIA. By Adolf Severin Jensen 1900

I. Mya

(Laid before the Meeting of 30 March 1900)

"... In the future, the greatest prize of a Molluscan collection should not be to obtain as many species as possible but - P. c. ; ... to exhibit as many series of forms as Inicrmation eeeenient possible".

P. u. F. Sarasin. 1899. 2

Introduction

The soft-shelled clam (Mya arenaria L.) has migrated to Scandinavian fjords and straits in times geologically quite recent.

Professor A.G. Nathorst was the first to draw attention to this". He found, at Alnarp in western Scania (between Malmö and Lomma), an old beach formation rising 8-10 feet above the sea level and in it, shells of mollusks still living around Oresund. However, Mira arenaria, which is today extremely common near the shore, was not found among the fossils.

This remarkable circumstance has been subsequently confirmed by studies

conducted on Danish soil. K. Rordam's treatise "Marine Alluvium in Northeast Zealand" reports that Dr C. G. Joh. Petersen carried out a comparison between the fauna of Ise Fjord and the extinct fauna of the postglacial strata belonging to this fjord system2) . This comparison lead to the interesting result that, on the one hand, several species (Tapes, Ostrea) no longer living in Ise Fjord are found in the old faunae, and that on the other hand, Nya arenaria, today one of the most characteristic and most frequently occurring

1) On upheaval and subsidence in Scania. Reports of the Geological Society at Stockholm [Geol. Feiren. i Stockh. Fbrhandlingar] Vol. I, 1872-74, p. 281. 2) Geological Survey of [Danmarks geol. Undersogelse] No. 2, 1892, pp. 106-111. 3

species, is absent. As the formation of these strata approximately or partially coincided with that of the kitchen middens,-and Mya arenaria is never found in the kitchen middens, the species must have arrived after the Stone Age. Dr Petersen points out quite correctly that this bivalve is thus highly significant.as an index fossil for age determination of a postglacial stratum: "The absence of this bivalve from one of the strata in question is alone practically a proof of that the stratum is old, while its presence indicates that the stratum is recent, that is, formed later than the kitchen middens." •

Furthermore, Mya arenaria is known to have migrated to West Prussia, Gotland and the Aaland Islands after the formation of the Tapes- (or Littorina-) containing strata".

Mya arenaria may have migrated also to the coast of during a later

period, as it is absent from postglacial deposits3) .

A treatise by D. Rayemaekers4) , a Belgian army surgeon, must not be left without mention, as it widens our knowledge of the prehistory of the soft- shelled clam. Dr Rayemaekers studied clayey sand strata lying below the marsh (the polders), north of the town of Antwerp, and found in them a saltwater fauna (Cardium edule, Scrobicularia piperata, Tellina baltica), but neither he nor the geologists who had examined these alluvial strata could detect the

1) See G. Berendt, Reports of the Royal Soc. of Phys.Econ., Konigsberg [Schriften d. kgl. physik. 8konom. Gesellsch. zu Ketnigsberg] Vol. VIII, 1867, p. 70, and M. Mendthal: Studies on The Mollusca and Annelida of the Vistula Lagoon (Inaug. dissert.), 1889, p.5. G. Lindstrbm: On Gotland's Present-Day Mollusks, 1868, pp. 44-45. V. Madsen, Reports of the Geol. Soc. at Stockholm [Geol. F6ren. i Stockh. F8rhandl.] Vol. 14, Part 7, 1892, p. 589. • 2) Report of its Occurrence in Shell Banks on Orlandet (M. Sars: Fossil Animal Remains from The Quaternary Period, 1865, p. 106) is based, as subsequently stated by M. Sars himself, on a mix-up with Lutraria elliptica which is very similar in shape and size (See Reports of the Scient. Soc, Oslo [Forhandl. Vidensk.-Selsk. Christiania] 1867 (1868), p. 58). e ) Époque à laquelle Mya arenaria a disparu de la région du Bas-Escaut belge. Ann. Soc. roy. malacol. de Belgique. T.XXX. 1895. Buul. des séances, p. V. 4 slightest trace of Mya arenaria, although this bivalve now lives along the Belgian coast and in the latitudes of the Scheldt River estuary. Dr Rayemaekers' observation, which was independent of Scandinavian observations, since he was not familiar with the contributions of Nathorst or Joh. Petersen", indicates therefore, that Mya arenaria also arrived in Belgium during the last part of the alluvial period.

This leads to the following question: Where did Mya arenaria live prior to coming here, and from what areas did it migrate ?

At first sight, the question does not seem to be difficult to answer. Based on present ideas about the geographic distribution of the species, the answer might be that it has come from the north.

All authors discussing the present distribution of this bivalve are unanimous about reporting that it is an essentially arctic and circumpolar species. Thus, many cata1ogues 2) on local faunae of the arctic regions indicate that Nya arenaria occurs at Labrador, Greenland and Spitsbergen, in the , at the northern shore of Siberia, and in the Bering Sea.

That the idea about migration from the north has not been alien to the above authors either, is indicated among other things by the following statement by Crosse and Debeaux: "De l'océan Glacial arctique, qui paraît être sa véritable patrie, elle est descendue dans les mers du nord de l'Europe, jusques et y compris la Manche et une partie de nos côtes de 1'Océan."3)

1)The main object of the Treatise is quite different and comprises those Mya arenaria (and other saltwater mollusks) that occur in their respective locations in the sand strata above the polders and consequently derive from the very latest part of the alluvial period. For this, reference is made to the Treatise itself and the supplementary notes, p. CXXV (1.c.). 2)The naines of the catalogues will be listed later. 3) Journ. de Conchyl. 3. sér. T.III, 1863, p. 254. • 5

However, if we extend our study so that it covers also the distribution of the species during the Ice Age, we immediately discover facts that make it difficult to accept a concept of arctic origin: Mya arenaria is not known from glacial strata in either Denmark"or the Scandinavian Peninsula2) .

-:• Thus, we face the rare circumstance that a species with a present-day range of disribution reportedly encircling the Polar regions, did not live in northern Europe during the Ice Age but migrated there later. This is definitely contrary to the nature of arctic animal forms: they have taken advantage of changing climatic conditions and spread to areas where they are either no longer found or, if they have staid, can be viewed as relicts from the Ice Age.

My task shall be to prove, on the following pages, that this discrepancy between the present and past distribution of the species is not attributable to some remarkable characteristic of the animal but to a faulty notion of the species designation "Mya arenaria" by authors concerned, in other words, that tIikigieeeikeetitaieWeiltâtedatteleielfyiteaedieleielielfeitethlkeetfiÉteteaeabealle admit that I have not been in a position to go after all the specimens mentioned in the literature, but I have examined a significant amount of

material from so many different localities, that I venture to claim that all reports on the occurrence of Mya arenaria in the arctic regions are based on erroneous identification.

Before proving this claim correct, I would like to present some observations

on the relative boundaries of the Scandinavian Ilya species.

1) It is not reported from the Vendsyssel diluvial systems by Jap. Steenstrup (in Johnstrup: On The Geological Conditions of The Northern Part of Vendsyssel, 1882) or by Jessen (Geological Survey of Danmark. I. Rk. No. 3. 1899). 2) M. Sars: Fossil Animal Relics from The Quaternary Period. 1865. - Thudén: On The Mollusks Occurring in The Postpliocene or Glacial Systems of Bohus County, 1866, p. 17. 6

Relative boundaries of the northern Mya species

A closer examination of differences between Mya arenaria and M. truncata might seem superfluous; they are assumed to be - and they really are - quite easy to tell apart at first sight. If, for example, a collection of Mya from Denmark has to be determined, they can be quickly classified by a look at the shell outline: The posterior end of M. arenarla is somewhat elongated with an arcuately tapered posterior edge, while that of M. truncate ends abruptly with a straight posterior edge. The authors who had to identify arctic Nya species obviously assumed that shell outlines stay essentially unchanged in arctic regions as.well, and they failed to examine the shells for other, more important characters. The latter are described in the following:

The most reliable distinguishing marks are found in the hinge plate of the left valve and a corresponding socket in the right valve as well as in the left valve umbo. In Mya arenaria, a ridge (boundary of the elastic part of the hinge plate) stretches over the plate, obliquely backward from the umbo, from where it then runs to the side of the plate in the form of a small prominence (p in Fig. 1 b); the plate itself extends quite considerably past this prominence, before its outer edge curves so that the prominence is left a good way behind at the back edge of the hinge plate. A similar ridge is found in Mya truncata, but the posterior edge of the hinge plate does not extend past the tip (p in Fig. 2 b) of the ridge but curves back immediately at the outer edge, so that the prominence constitutes the outer back corner of the hinge plate itself. The pit corresponds to the ligament and consequently, it is much higher (in the up-to-down direction) in M. arenaria (Fig. 1. a) than in M.

truncata (Fig. 2. a). Besides, the latter species has, in front of the pit, a prominent tooth (c in Fig. 2. a), which is absent or appears only weakly in M. arenaria (see Fig. 1. a). It must be iminted out, that in the left valve of M. arenaria, the tip of the umbo looks as if it were worn off, and the inside of the umbo is hollow (see Fig. 1. b); specimens with hinges still in place show that the hollow in the umbo provides a place of attachment for a fold of the outer ligament, which means that the disintegration of the umbo 7 can not be attributed to wear but to a characteristic structural detail. In M. truncate, the umbo of the left valve is intact (see Fig. 2. b)".

I have examined these characters on several hundreds of specimens of both species, and hençe, I am convinced that they always proe correct. They constitute a reliable basis for determination, even of small specimens in which shell contour differences are known to be very subtle or downright imperceptible2) .

Now that the usefulness of these characters as a diagnostic tool has been established, they shall be used to revise the existing material on arctic "Mya arenaria".

”.• .1 : 17 s ,

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Fig. 1 Mya arenaria Fig. 2 Mya truncate a. Right valve b. Left valve p. Tip of diagonal ridge c. Tooth

"Acknowledgement of the circumstances here described is nothing new, since I find that they have been partly noted by authors whose monographs contain detailed descriptions of M. arenaria and M. truncate. I refer especially to Meyer and Möbius (Fauna of Kiel Firth, 2 Vol., 1872); but as far as I know, no author has pointed out - or perhaps, to be more correct, has not found it necessary to point out - the significance these circumstances have for species determination, as the diagnoses indicate only very generally, that the hinge plate is relatively bigger in M. arenaria than in M. truncate. 4) These characters provide besides the advantage that they could be used for identifying shell fragments, in case only the part closest to the umbo is preserved. 8

Fig. 3 Left valve of Mya truncata f. ovata from Greenland

Fig. 4 Mya truncata f. ovata, viewed from inside and above. a = right valve b = left valve p = tip of diagonal ridge c = locking tooth

Fig. 5 Left valve of Mya truncata f. ovata from Spitsbergen. 1/1. 9

Examination of some bivalves known as Mya arenaria from arctic regions

At first I will examine some specimens from West Greenland. One of them is shown in Fig. 3. Considering merely the contours of this bivalve, it is no surprise that it was referred to M. arenaria: due of its convex, arcuate posterior end, it deviates greatly from a typical mature M. truncate and is, in this respect, closer to some short forms of M. arenaria l) .However, a closer look at the shell reveals other features, which point to the direction of M. truncate, namely anteriorly strongly curving valves, the high palliai sinus, the thick and dark periostracum ("epidermis") lying roughly folded at the posterior edge of the shell, and the detail that when closed, the valves gape posteriorly only, not both anteriorly and posteriorly as they do in M. arenaria. To this must be added the critical detail that with regard to the umbo of the left valve and the hinge plate and hinge pit, these specimens look like typical M. truncate, which will be obvious if Fig. 4. a and Fig. 4. b are compared with the corresponding figures showing a typical M. truncate (Fig. 2. a and Fig. 2. b).

The same results are obtained if other similar forms from Iceland, Spitsbergen (Fig. 5) and Siberia (Fig. 6) are examined. The last-mentioned specimen is certainly the one that most resembles M. arenaria, not just by its contours, but also by the flatness of its shell (see Fig. 6. c); but the details of the hinge plate and the umbo (Fig. 6. c) show immediately, however, that it must be referred to the series consisting of forms of M. truncata.

Regardless of that such specimens do not have quite similar contours, as is obvious from the Figures, I believe that they constitute - in relation to

typical specimens - a distinctive form of Mya truncata, which could be suitably named forma ovate.

1) This species varies quite a lot with regard to the most posterior part of its shell, which sometimes is longer and quite tapered and at other times shorter and slightly tapered. 10

I should add that all available specimens of the form ovata are dry shells, and thus, no information can be given about the soft parts.

For completeness, I include the measures of a number of specimens:

Mya truncata L. forma ovata a. Long. .40; AIL 25m; Crass. 151••■ • Greenland b. -- 53 — 35 — . 20 e. — 58 —38 — 25 1.2 — 66 —45 —27'g e.°J — 70 —45 —29 f. / ' — 78 —51 » —30 Iceland _ a. — 32 --18,6 7.- 145 b. — so —32 — 18,5 c. — 60 — 370 (Tab'. dextra) Spitsbergen a. — 50 — 35 — 21 . Siberian Arctic Ocean b. — 74 — Si a. U.- — 49 — 34,5 } (valy.sinistra) b.1£ — 70 -- 4%5

a. b.

: ■•■-• 1' er rav) .f.t1- 1

C.

Fig. 6 Mya truncata f. ovata from Cape Gostinnyi, north coast of Siberia. a = viewed from outside b = from inside c = from above. 1/1. 11

The following is a critical survey of references found in the literature to the occurrence of Mya arenaria in arctic regions. I find it necessary to treat the matter in great detail, to achieve complete clarity. I feel very much obliged to draw up this account, because in almost all cases I have examined the very same shells on which the literature references are lased.

Critical survey of arctic Mya arenaria mentioned in the literature

Greenland

Fabricius mentions Mya arenaria in his Fauna Groenlandica, p. 405, as early as in 1780, but in a posthumous manuscript he reports" that the individual on which the description was based may have been a M. truncata specimen with a more than normally arcuate anterior (?) end. He adds that he did subsequently have a genuine M. arenaria sent to him from Greenland. As indicated below, I have good reason to believe that Fabricius erred this time as well.

Moller reports both fossil and living M. arenaria from Greenland (Index Moll. Groenl., 1842, p. 21). However, the Greenlandic specimen in the local museum here, which was identified by Moller as M. arenaria and originated in his collection, is M. truncata f. ovata. Therefore, it is hardly incorrect to refer Moller's M. arenaria to this form of M. truncata.

Morch reports in his different catalogues (Rink's Greenland, 1857, p. 89; Rupert Jones, Arctic Manual, 1875, p. 130; Rink, Dan. Greenland, 1877, p. 440) M. arenaria from Greenland, by referring to Fabricius and Moller.

Posselt (Reports on Greenland [Medd. om Gronland] XXIII, 1898, p. 98) quotes Jeffreys' Brit. Conchology (III, 1865, p.65), according to which Walker

1) Posselt refers to this in Rep. on Greenl. XXIII, 1898, p. 98 Notes 12

had collected M. arenaria in South Greenland at depths of 10-120 fathoms, but P. overlooked a subsequent correction by the same author. In the correction

(Scient. Proc. Dublin Soc. N. S. II, 1880, P. 127), Jeffreys states that he had an opportunity to submit for revision the materiall) collected by Dr. Walker during the Fox Expedition. The revision revealed, among other things, that W's "Mya arenaria (L.), Godthaab, and Melville . Bay, 10-120 fathoms" consisted of young of Mya truncate L. - Posselt states in the same publication that he himself had seen fossil shells from Greenland (originating in a clay terrace far inside Nordre Strom Fjord); these are, however, M truncata f. ovata.

The local mineralogical museum here has a number of fossil Mya collected by Pfaff and K.J.V. Steenstrup in various localities in North Greenland (Lerbugten in the vicinity of Claushavn; Pattorfik at Umanak Fjord); Traustedt has referred them to M. arenaria, and they appear under this name in Rep. on Greenland IV, 1883, p. 235, but they also have proven to be M. truncate f. ovata.

Nordenskiiild collected fossil shells, partially in the saine areas as the

authors mentioned above. Lovén examined this material and included M. arenaria in the list (Synopsis of The Reports of The Royal Academy of Science, 1870, p. 1018). It has not been possible to procure the specimens in question (they are not in the Zoological Department of the National Museum), but there can be no doubt that the determination is incorrect.

Finally, E. Bay's Treatise on The Geology of East Greenland (Rep. on Greenland XIX, 1896, p. 172) reports M. arenaria from uplifted strata in the inner Scoresby Sound, but the shells concerned have proven to be M. truncata f. .ovata. •

1) Published by Walker in Journ. Roy. Dublin Soc. III, 1860, p. 70. :• ;.• '

13

have The present account will show that eateeileets - those that been obtainable - en1a nâ are baaed on

etete.egte4etteeteede0**iiimNeLtherkk•-• '*'••• :Ikeee.e,foss . L -cl the speciîa are

Iceland

According to Mohr (An Attempt to Iceland's Natural History, 1786, p. 128), Mya arenaria is frequently found on the shores of eastern Iceland. However, whether he had the right species in mind is doubtful. - In his Faunula

Molluscorum Islandiae (Scient. Rep. of The Nat. Hist. Soc., 1868, P. 218), Morch can only present a reference to Mohr.

Posselt indicates subsequently, in the mentioned Study on Greenland's Brachiopodes and Mollusks (p. 98) that Iceland is the place of origin of the species and backs up his claim with some specimens (from Hbfdhi and Ônundar Fjord in western Iceland), which turned up in excavations during subsequent years; however, they must be referred to M. truncata f. ovata.

raeeenatzieqsbow- e- -effl fi teaiZ 14 Spitsbergen

Kroyer brought two specimens of a Mya form from Bell Sound to the Zoological Museum, which were referred to M. arenaria by *arch"; they have proven to

belong to the form ovata of M. truncata. •

Meyer and Möbius mention, in The Fauna of Kiel Firth [Fauna der Kieler Bucht], Vol. 2, 1872, p. 119, Spitsbergen as the place of origin for M. arenaria and refer to Malmgren as an authority on the subject. As Malmgren had produced no detailed account of Spitsbergen's mollusk fauna2) in a scientific treatise, the statement made by Meyer and Möbius is probably based on some kind of misunderstanding.'

Finally, Friele reports (Yearbook of The German Molluscan Society VI, 1879, p.270), that the Norwegian North Atlantic Expedition collected M. arenaria in

Magdalene and Advent Bay, at 10-30 fathoms. Upon an inquiry, Mr. Friele kindly informed me that some time ago, when he was revising the material, he noticed that this species had made its way into the list by a slip of the pen. tmkWew earme:

1) Even though they are not mentioned in Catal. des Moll. du Spitzberg (Ann. Soc. Malacol. de Belgique IV, 1869), March has added them to his own special issue of this treatise. The issue is owned by Mr. Herm. Lynge, who drew my attention to the addition in question. 2) Malmgren referred to spitsbergen's mollusk fauna in a short paragraph only (Swedish Expeditions to Spitsbergen and Jan Mayen, 1867, p. 252), mentioning that 130 species were collected during the Swedish Expedition in 1864. The Literature on The Polar Regions of The Earth [Die Literatur über die Polar-Regionen der Erde], 1878, by von Chavanne, Karpf and le Monnier, mentions Malmgren (p. 137) as the author of Contributions to Spitsbergen's Mollusk Fauna [Bidrag till Spetsbergens Molluskfauna], Stockholm, 1863, but this is, according to what professor Hj. Theel has told me, a mistake, because no such work was written by M. I am inclined to think that the confusion is based on a treatise by Torell with a similar name. 15

The Kara Sea

In The Scientific Observations of The Vega Expedition (Vega-Exped. Vetensk. lakttagelser, Vol. III, 1883, P. 437), Leche mentions, in his report on the processing of the Lamellibranchia collected by the Expedition, 4 very small specimens from the Kara Sea, taken at 5-10 fathoms. I have examined the specimens and come to a different conclusion: the very strong arcuation of the valves, the large tooth in the right valve and the features of the hinge ligament show that they are young of M. truncata.

K. arenaria is rePorted also from the Kara Sea (Kusov Sound, 20 f) by Collin (Zoological-Botanical Yield of the Dijmphna Expedition (Dijmphna-Togtets zoologisk-botaniska Udbytte], 1886, p. 144). This specimen as well has proven to be a young M. truncata.

« àïïiedialeààïCïhèiïïâ

The Siberian Sea

The Swedish expeditions to Novaya Zemlya and Yenisei in 1875 and 1876 collected a very significant marine molluscan material, for which Leche has given an account (Reports of the Royal Swedish Academy of Sciences, Vol. 16, No 2, 1878). P. 10 mentions M. arenaria, collected in fossil form at Cape Gostinnyi (Yenisei), between Gostinnyi and Korepovskoy, and at Korepovskoy. I have examined this material and come to the conclusion, that t-4.

Ogigeal erelee teinee4 °Vitt?* . The specimens from the second location mentioned conform very accurately with the form from West Greenland. A left valve from Gostinnyi is shown in Fig. 6, and I have already remarked (p. 140) on the great similarity of its contours with those of M. arenaria. Only a fragment was collected at Korepovskoy, namely the posterior half of a left valve, which is tapered to the degree that I would have been doubtful about the species, had not the hinge ligament and the umbo been preserved.

16

Below is a list of synonyms, which will provide a comfortable overview of the reports. "?" denotes only that I have not had access to the determination.

Mya truncata Linné. Additamenta ad synonymiam

1780. Mya arenaria Fabricius, Fauna groenlandica, p. 405 1786. ? Mohr, Attempt to A Natural History of Iceland, p. 128 1842. Moller, Index Molluscorum Groenlandiae, p. 21 1857. 'torch in Rink, Greenland, p. 89. List of The Mollusks • of Greenland 1860. - , Walker, Journ. Roy. Dublin Soc. III, p. 70 1868. ? Morch, Scientific Reports of the Natual History Society • p. 128. Faunula Molluscorum Islandiae. 1870. ? Lovén, Synopsis of The Reports of the Royal Academy of Sciences, No 10, p. 1018. Subfossil animal species collected in Greenland during the 1870 expedition. 1871. Morch, Geolog. Magazine VIII, No. 9, p. 398. On the Mollusca of the Crag-System of Iceland. 1) 1875. Morch in Rupert Jones, Arctic Manual, p. 130. Prodromus Faunae Molluscorum Groenlandiae 1877. Morch in Rink, Danish Greenland, p. 440. 1878. Leche, Reports of the Royal Swedish Academy of Sciences. Vol. 16, No 2, p. 10. Synopsis of The Marine Mollusca collected by The Swedish Expedition to Novaya Zemlya and Yenisei in 1875 and 1876 1879. Friele, Yearbook of The German Molluscan Society. VI, p. 270. Catalogue of The Mollusca Found at Spitsbergen during The Norwegian North Atlantic Expedition 1883. Leche, Scientific Observations of The Vega Expedition. Vol. III, p. 437 1883. Traustedt, Reports on Greenland. IV, p. 235 1886. Collin, The Zoological-Botanical Yield of The Dijmphna Expedition, p. 441 1896. Posselt, Reports on Greenland. XIX, p. 173 1898. Posselt, Reports on Greenland. XXIII, p. 98. Consp. Faun. Groenlandiae. Brachiopoda et Mollusca.

urr.

wihMya

These are the limits of my study, because . I obviously can . not comment on Mya arenaria from the rest of the Polar zone. However, I am hardly incorrect in

1) This synonymy will be explained in the subsequent text (p. 153). ••

17

claiming that reports from the latter regions are also based on confusion with M. truncata, and for this probability I can present at least one proof.

Arth. Krause, to whom the report on M. arenaria's occurrence in the Bering Sea was attributed", notes that the specimens were only 8-14 mm long2) and that he referred them to M. arenaria,,because similar-sized young of M. truncata are already distinctly identifiable by their posteriorly truncate shape. Based on what I pointed out earlier about the reliability of this character, particularly in young individuals, there is good reason to believe that the species determination was incorrect. Furthermore, it can hardly be a coincidence that prof. W.H. Dall, a prominent mollusk expert in Washington, does not mention M. arenaria in any of his lists3) of Bering Sea mollusks.

Packard reports M. arenaria from Labrador, stating that both presently living and fossil ones are found in glacial strata4) .The Copenhagen Zoological Museum happens to have an American collection of mollusks, received from Packard in 1866. Among these are some shells deriving from Pleistocene strata in Maine, identified by Packard as Mya arenaria; they are, however, Nye truncata f. ovate. It seems probable that the "M. arenaria" in the work quoted hides just this form of M. truncate, which has been incorrectly identified by so many European scientists.

I) A Contribution to Knowledge of The Molluscan Fauna of The Bering Sea. Arch. f. Naturgesch. Jahrg. 51, 1885, P. 39. 2)Krause presents an assumption that the M. arenaria found in the Arctic North occurs only in stunted (dwarf) forms. He refers not only to the specimens available to him but also, among other things, to those brought back from the Kara Sea by the Vega Expedition, which were very small as well. Based on what has been pointed out earlier, there is no need to support this hypothesis. 3)Proc. Californ. Acad. V, Part III, 1874, p. 246. Proc. U.S.N. Mus. 1884, p. 340. Ibid. 1886, p. 218. The Fur Seals and Fur-Seal Islands of The North Pacgic Ocean III, 1899, p. 539. • 4)Observations on the glacial phenomena of Labrador and Maine. Mem. Boston Soc. Nat. History I, Part III. 1867. 18

On the true range of Mya arenaria

The result of the examination above can be briefly stated to mean that Mye

An attempt to roughly determine the true range of M. arenaria is hampered by the difficulty that we can not know with certainty whether the authors, whose reports we use as supportive material, had the correct notion of the species.

In the southern parts of the areas where Mya arenaria occurs and where M. truncata f. ovata does not seem to occur, the boundary can be set with fair certainty. In the Atlantic Ocean, the southern boundary lies in Europe at southwestern Francel) and in America at South Carolina2) , in the Pacific Ocean3) at Japan4) and northern China (between lat. 30 and 40° N) 5) .

Due to an absence of reasonably reliable material, I have to leave to others to point out the northern boundary of M. arenaria's range in the western Atlantic Ocean. I will limit myself to Europe.

1) Locard: Catal. gén. des Mollusques vivants de France - Moll. marine. 1886, p. 383. - Some authors report, however, that M. arenaria occurs in the Mediterranean, but it is not mentioned by the best expert of the Mediterranean molluscan fauna, Marchese di Monterosato (New Journal of The Mediterranean Mollusca [Nuova revista delle conchiglie Mediterranee], Palermo 1875). .PDall. Bull. U.S.N. Museum. No. 37, 1889, p. 70. M. arenaria seems not to be native to western North America; in any case, it does not, or more correctly did not (cf p. 151, Note 3), occur north of San Francisco Bay, as none of the authors could subsequently confirm Middendorff's report on its occurrence at Sitka (cf R.E.C. Stearns, Amer. Naturalist xV, 1881, p. 362). 4) Lischke: Japanese Marine Mollusca [Japanische Meeres-Conchylien], 1869, p. 138. - I think that I am in a position to confirm that what is known as Mya japonica Jay is really identical with the Atlantic M. arenaria. The local zoological museum has a large (105 mm), beautiful specimen brought here by captain Andréa from Hakodadi; for all its essential characters, it is a genuine M. arenaria 5) Crosse et Debeaux, Journ. de Conchyl. 3. sér., t. III, 1863, p. 253. 19

M. arenaria has spread from southwestern France to the British Isles, Belgium, the Netherlands, Germany, Denmark, and southern ; in the it is reported to occur up to 62°36' in the Bay of Bothnia". Further, G.O. Sars2) reports that it occurs along the entire Norwegian coast, .and there is no reason to doubt the correctness of this. Also, I would believe that the M. arenaria that is reported by Knipovich3) to live in the warm (but not the cold) area of the White Sea, is correct. Besides, as the last-mentioned scientist has been entrusted with carrying out a biological study of Russia's northern coasts, it is up to him to investigate in greater detail the range of M. arenaria in these areas.

Even though, due to the nature of the matter, this part of the present study ... is necessarily quite incomplete, it still shows that Mye arenaria kgs b* :00e#0403»:»1«8»keee.. It is true that in northern Europe it is naturally included in the arctic zone, but as is known, this region deserves to be called "arctic" only because of its location, eot because of its climatic conditions.

Thus , it is seen that eieggfflg4SNeeikallYaiiegelikeelgeggingeggsik .614eIk

tkk; me

Following this more realistic recognition of Mya arenaria's distribution, the answer to the question of whence this mussel has come to us, will seem quite contrary to previous beliefs: ehniented ,teohti s.nuth. After the Ice

1) Cf G. Kozhevnikov: La faune de la mer baltique orientale etc.; Congrés internat. de Zool. 2e ne Sess. à Moscou 1892, I, p. 151. 4) Mollusca regionis arcticae Norvegiae, 1878, p. 92 and p. 355. 3) L'Ann. du Musée zool. de l'Acad. Imp. des Sc. de St. Pétersbourg. 1896, p. 29 and p. 32. 20

Agel) it spread toward north2) , but the migration must have been very slow, or it must have begun very late3) , because the species reached Belgium, Denmark, and Scandinavia only in the most recent phase of the alluvial period.

Next comes the question whether M. arenaria existed before . the Ice Age, and if it did, in which areas. I have been unable to dedicate very much time to this side of the history of the soft-shelled clam and must limit myself to a few remarks.

That a species possessing the characteristics of M. arenaria lived at the end of the Tertiary period at the British Isles can be considered a fact. In his large, beautiful ' work "A Monograph of The Crag Mollusca" 4) , Searles V. Wood reports that M. arenaria occurs both in "Red Crag" and in "Mammaliferous Crag" (1.c. II, p. 279), that is, in systems deposited in the period extending to the Ice Age. That Wood's determination was not incorrect is shown by his Figures 2.b and 2.f in Table XXVIII. The Figures, although not very good in all respects, show with certainty that he had a just about perfect notion of

1) M. arenaria is certainly mentioned by some English authors (for ex. Clement Reid) among the mollusks found in glacial systems on the British Isles, but I refuse to believe that the reports are correct. 2) Based on the view (justified at the time) that M. arenaria is an arctic form, Dr Joh. Petersen (1.c. p. 111) has used its late migration into Danish fjords as a support for the theory that formation of Tapes strata took place at a time when the Danish waters were, in hydrographic terms, more like the open sea. Without wanting in the least to shake this theory, I have to point out however, that the absence of M. arenaria from the strata mentioned can no longer be included among proofs for correctness of the theory. *3) My friend Herman Lynge has drawn my attention to an interesting treatise by Rob. Stearns (Mya arenaria in San Francisco Bay; Amer. Naturalist XV, 1881, p. 362), which indicates, that M. arenaria can spread very fast to localities where it has not occurred earlier. Prior to the year 1874, this bivalve was unknown in San Francisco Bay, and it could similarly not be found in the old shell mounds or the kitchen middens of the local indigenous peoples. A few specimens of Mya were found in 1874 near Oakland; they were immediately assumed to belong to an unknown species (M. hemphillii Newc.), but when compared with M. arenaria from the Atlantic shores of the United States, they proved to belong to the latter species. In the course of a few years, the soft-shelled clam spread for miles along the coast and reproduced so fast, that next to the oyster, it was the mollusk most frequently taken to the markets of Oakland and San Francisco. Stearhs thinks that the first specimens were introduced with Ostrea virginica, which was at that time being set out in San Francisco Bay. The soft-shelled clam has also started to show up at Santa Cruz (Monterey Bay), which lies 72 miles south of the estuary of San Francisco Bay. 4) The Palaeontological Society, 1848 -1882. 23. the species characteristics (comp. his figures of H. truncate, Fig. 1.a and 1.f, in the same Table). Besides, I have myself examined M. arenaria from Red Crag, which I will touch upon later.

Similar crag or Lower Palaeozoic systems are known in northern Europe only from Iceland. The bivalves in the Icelandic Crag are listed in a detailed

a.

b.

Fig. 7 Mya truncate f. ovata from Icelandic Crag, Hallbjarnastadir. a. Left valve viewed from outside b. viewed from above. 1/1 UNEDITED TRANSLA.7101\1 oniy For - 22 index by liorchl) , who mentions (p. 398) that M. arenaria is fairly common in strata examined at Hallbjarnastadir. However, a study of the specimens in question, which are kept at the local mineralogical museum here, showed that they are exceptionally elongated M. truncate f. ovate (see Fig. 7). - This does not allow a conclusion that M. arenaria did not occur in northern Europe during the Tertiary period, but it has to be emphasized that the species has not yet been found in the strata where it could be expected2) .

On changes of shell — shape in Mya truncata

The form ovate, with which we made acquaintance in the text above, adjoins very naturally the other variations of M. truncate and represents the extreme of a developmental direction of the species.

In my opinion, ovate is the most original form of the species, partly because it has preserved the appearance of a majority of very young M. truncata. If a large number of M. truncata young are examined, the posterior end of their shell is seen generally to be more elongated than that of the adults, sometimes significantly so, and besides, the posterior edge is arcuate or to a minor degree distinctly truncate. Also, this form, which is now relatively rare (of about 50 specimens from Greenland, only 2 could be referred to this form), seems to have been predominant in the Tertiary period. In any case, I find that of 11 specimens deriving from the Icelandic Crag system, no fewer than 10 belong to this form, and they are indeed characterized by a very much elongated shell shape (cf Fig. 7).

The other forms could then have derived from this form through successive shortening of the most posterior part of the shell. This path takes us first

1)On the Mollusca of the Crag-System of Iceland. Geolog. Magazine VIII, No. 9, 1871. 2)If the Icelandic Crag is older than the English Red Crag - which I do not know - the last statement is incorrect. M. arenaria namely seems to have emerged simultaneously with Red Crag; in any case, it is not found in the Coralline Crag; cf Wood 1.c., p. 280. 23 to the form shown in Fig. 8 a, which is considered the typical M. truncata. A further shortening leads to the shell pictured in Fig. 8 b. Finally, at the end of the whole sequence, there is the peculiar form shown in Fig. 8 c, originally described by Hancock as a separate species: Mya uddevallensis.

Nature provides us with two ways to understand what biological circumstances may have brought about the different shell shapes:

1. Present-day distribution of the forms. The typical M. truncata is a southern form. The northern range of distribution is characterized largely by shorter forms, and the form uddevallensis is exclusively associated with shores of the Arctic Ocean.

2. Distribution of forms in the Quaternary strata. The typical M. truncata is the only form occurring in postglacial shell banks of southern.and western Norway. It is gradually replaced by shorter forms in the younger shell banks of the glacial systems, while in the older strata from the Ice Age, uddevallensis is the most common form".

This parallelism indicates clearly enough, that climatic conditions have affected the shaping of the shells and still continue to have the same effect, so that the more severe the climate is under which the animal lives, the shorter the shell becomes.

A question arises now about the more fundamental reasons behind the occurrence of short-shelled forms of Mya truncata under severe climatic conditions.

I believe that under such conditions, Mya burrow deeper than at other times, to seek protection from cold and ice. It is known today that other bivalves that burrow deep, such as Teredo and Xylophaga, develop smaller shells; large

1) Cf M. Sars: Fossil Animal Relics from The Quaternary Period, 1865, pp. 13, 57, 85, 86, 123.

I. 24

•

.at

1.••••• I•aft....•••mlo•ft• ....v. •

.1. •11• •1•0* *.m • b; .••• • ..;f1

• .1. - • 1: I

..; • :d .1 •• ■ ••

Fig. 8 Three left valves of Nya truncata, illustrating shortening of shells. a. From the Farce Islands (forma typica autt.) b. From Greenland c. From glacial strata (forma uddevallensis) 25 shells offer naturally more frictional resistance and obstruct efficient burrowing.

Thus, I assume that the arctic climatic conditions force Mya to burrow more deeply, which results in the shortening of their shell.

The fact that M. truncata occurs in arctic areas also in the elongated ovata form would seem to be contrary to the opinion above. However, in view of the great present-day rarity of this bivalve (cf p. 154), nothing keeps us from regarding it as a form that has preserved, exceptionally and for unknown reasons, its original character.

Anyhow, reliable information on these circumstances could be obtained only by observations in nature. That I take the liberty of searching for an explanation without instituting such observations is based on my wish to encourage those who have the opportunity, to examine for example the length of the respiratory channel of living specimens of the various forms.

In this connection, I also wish to draw attention to a remarkable parallel between M. truncata and M. arenaria. The local zoological museum here has some specimens of M. arenaria from the English Red Crag. These shells display the remarkable feature that they have a truncated posterior end (see Fig. 9), so that in this respect they resemble M. truncata"; however, the hinge ligament and umbo of the left valve (see Fig. 9. b) indicate with absolute certainty that they are just M. arenaria with a truncated posterior end2) . The strata in which this form occurs were deposited during a part of the Tertiary period that constituted the transition to the Ice Age; the temperature started to drop and the ice reached the British Isles. In my opinion, this example shows that the effect of climatic conditions on the shell shape of M. arenaria could be similar to their effect on M. truncata3) .

"These shells constitute an additional proof of that contours may deserve minor attention, since we now know of the existence of not only M. truncata specimens with the outlines of M. arenaria but also of the reverse. 26

2) This form was described by Sowerby as a species of its own, Mrs Zata (Mineral Conchology of Great Britain I, 1812, p. 185, pl. 81). Forbes and Hanley (Brit. Mollusca I, 1853, p. 172) have quite correctly recognized that such specimens must be referred to N. arenaria, but they regarded them as defgormations (cf Note 3)). e) Forbes and Hanley also wanted (1.c. p. 172) to associate the deviating shell shape with the special physical conditions under which the species lived. However, they thought the shells were deformed because of the abundance of fresh water produced by melting ice.

..; 27

a.

ftIfl

. J!°.

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b.

Fig. 9 Mya arenaria with "truncated" posterior end. From English Red Crag. a) exterior of left valve b) left valve from above. 1/1.

I would like to thank professor Hj. Théel, the Director of the Invertebrate Department of the National Museum at Stockholm, for the assistance he gave me by letting me examine the material collected by Swedish arctic expeditions. Without his help, the present report could not have been produced. I thank also professor N.V. Ussing, who allowed me to study the fossil Mya of the Mineralogical Museum.