HORIZONTAL PIVOT AXIS AND STANDARDIZED SHELL OUTLINE: A new method to compare morphology of cardiids...

37

Phuket Marine Biological Center Special Publication 32: 37-51 (2014)

HORIZONTAL PIVOT AXIS AND STANDARDIZED SHELL OUTLINE:

A new method to compare morphology of cardiids (Bivalvia: Cardiidae) exemplified by the genera Cardium, Acanthocardia and Vasticardium;

The fossil Cardium hians Brocchi, 1814 replaced by Cardium paleobrocchi nom. nov.

Jorgen HyUeberg

DepartmentoJBiological Sciences, Section oJMarineEcology, Aarhus University, Building 1135 Ole Worms aile 1, DK-8000 Aarhus C, Denmark & Statens naturhistoriske Museum (ZMUC)

Universitetsparken 15, Copenhagen, Denmark Email: [email protected]

ABSTRACT. Identification of cardiids demands comparisons of a number of external and internal characters. Emphasis is commonly on external characters but I show that internal characters with focus on the hinge of right valves can yield supplementary characters which make it easier to compare interior aspects regarding teeth and nymph as well as obliqueness of shells. Photos of specimens are analyzed. The pivot axis along the nymph is turned into horizontal position of standardized shells. The angle of this axis with an axis passing the lower laterals is referred to as upwards or downwards inclination of the anterior shell. It is always turned upwards in Cardium and downwards in Vasticardium. Genus Acanthocardia, exemplified by A. acuieata, display both upwards and downwards inclinations. The right anterior laterals of Vasticardium cf. elongatum consists of an upper and a lower lateral but a small tooth (free or fused with the upper lateral) extends into a cavity. This condition is comparable to the right anterior laterals of Cardium costatum. Some authors have considered the living Cardium indicum Lamarck, 1819 a junior synonym of the fossil Cardium hians Brocchi, 1814, which is a homonym of Cardium hians Spengler, 1799. I therefore designate Cardium paleobrocchi nom. nov. for Cardium hians Brocchi, 1814. I argue that the fossil species is different from the living species and treat C. indicum as a name to be conserved. Based on the present standard square analysis I fail to separate Cardium maxicostatum Ter Poorten, 2007 from Cardium costatum Linnaeus, 1758 and treat C. maxicostatum as an infrasubspecific form of C. costatum. In cases where museums do not send type specimens on loan, the method of using photos may be the only option available. Limited budgets of museums is one reason because the cost of registered postage and insurance is very high, both for the museum and the receiver retorning the specimens after use.

Key words: external and internal characters, hinge, nymph, fossil species, living species

INTRODUCTION

Cardium costatum Linnaeus, 1758 (from Pliocene to Recent) is listed as the first species in Systeroa natura by Linnaeus (1758). In spite of being unique in tenns of shell morphology in comparison with other cardiids (Hylleberg, 2004; Savazzi and SiUgeback, 2004), the first species in the list by Linnaeus was nominated type of the genus Cardium by Children (1823). If it had not been for Children, the genus Acanthocardia could have been selected as a more appropriate type of the family. Lamarck thought along this line as early as 1799. He referred to Cardium aculeatum now classified as Acanthocardia aculeata (Linnaeus, 1758) as the characteristic species of genus Cardium

and listed the important characters (Lamarck, 1799: 86). Unfortonately, Lamarck did not use the word type species, so his reference to A. aculeata has been ignored. In summary, Cardium Linnaeus, 1758 is type genus of the family Cardiidae Lamarck, 1819 and Cardium costatum is type species of the genus with two living species: Cardium costatum Linnaeus, 1758 and Cardium indicum Lamarck, 1819, both from the west coast of Africa (Hylleberg, 2009). A third species, Cardium maxicostatum was described by Ter Poorten (2007) but I have failed to identify 1he characters separaling C costatum ftom C maxicostatum, except in 1arge specimens. Until further evidence of the presence of two species is provided, I treat C. maxicostatum as an infrasubspecific form of C. costatum.

38

Marine taxa in the Reference Collection ofPMBC

In accordance with earlier definitions, Sowerby (1852) diagnosed the geous Cardium as being equivalve, sub-equilateral, sometimes gaping posteriorly, ornamented on the outside by ribs radiating from the umbones; cardinal teeth, two in each valve, locked into each other crosswise, lateral teeth, two in each valve, remote; muscular impressions, two in each valve; palleal impression entire. Ligament external inflated.

The nymph is an important character in all cardiids hut few authors mention this structure of shell material on which the ligament rests arul binds the two valves together (Hylleberg, 2004: 427). The ligament consists of an outer layer of horny periostracum and an inner layer attached to the nymph. The inner layer is composed of two united layers of elastic matrix (lIylleberg, 1994, Fig. 2). The valves are closed by contraction of the adductor muscles and opened by the elastic ligament. The nymph constitotes a pivot axis along which valves are opened and closed. Obviously, these shell movements can only take place along a straight lioe. If the nymph is long and more or less convex as seen in some genera (e.g., Ciliatocardium and Clinocordium) the curved section does not take part in the opening-closing actions so the tangent plane is used for designation of the pivot axis.

The size, morphology, color, and shell shape of shelled molluscs are extremely variable factors which make quantitative comparison of specimens within and between species very difficult. My thesis for this study is that no matter the size and obliqueness of shells, the opening-closing actions should be a functional aspect showing less variability. I describe a method to allow quick comparisons of shell shape and proportions such as length to width ratios. I compare inner aspects of shells after orientation of the nymph in a horizontal position. All images of shells are standardized to fit within a 10xi0 em frame. The necessary adjustment of standardized shells are made on a computer where images can be rotated and reduction or eulargement easily carried out. I compare interior aspects of cardiid shells with emphasis on teeth numbers and shape of valves. I compare the right anterior laterals of C. costatum

with other species of cardiids.

MATERIALS AND METHODS

The comprehensive collection of Cardium costatum (Fig. I), all recorded by Hylleberg (2009: 139-142),

the smaller collection of Cardium indicum, and one specimen of C. paleobrocchi nom. nov. were studied at the Zoological Museum of Copenhagen Uuiversity (ZMUC), recently renamed Statens naturhistoriske Museum. Three valves of C. cos/alUm in the collection of JH are included. The left valve of Lamarck's original specimen of Cardium indicum was studied in Museum national d'Histoire naturelle (MNHN) in Paris (lIylleberg, 2004: 843). Photos of specimens of Acanthocardia aculeata in Linn. Soc. of London, Museum d'Histoire naturelle, Geneve MHNG, and in the ZMUC (lIylleberg, 2009: 843-S44) are analysed. Material of Vasticardium elongatum was studied at the ZMUC (Hylleberg, 2009: 303). Specimens of Vasticardium cf. elongatum in the collection of JH are included.

Standardized sheD measurements I have used images of shells obtained with a digital Olympus 500 or a fiat-bed scanner Studio Scan Hsi. Afiat-bed scanner yields the best images since problems with tilt of shells are avoided on the glass plate of the scanner. The steps performed on a specimen are shown in Fig. 2. The image is pasted within a square measuring 100 x 100 mm. Shell dimensions are measured in mm along the edges of this square arul displayed as percentages of the fixed width and variable length.

The relative width of the nymph is measured as the distance between two verticallioes: one through the umbo of the shell and one through the posterior edge of the nymph.

A line TT is drawn along the ventral edges of anterior and posterior hinges using the line tuol of Photoshop. Intersection of this line with the pivot line PP displays how many degrees the anterior part of the shell is raised or lowered relative to the horizontal nymph axis.

Sequence of actions (Fig. 2): Step I. Square imported from prepared template. Step 2. Image rotated to give a horizontal nymph

(photo shop ). Step 3. Filter: select style, find edges. Image is moved

to upper comer for easy cutting. Step 4. Image: mode: color changed to gray scale.

Smudging removed, black and white levels adjusted. Image cut to a width of 9.9 em.

Step 5. Image imported to the template and posi tioned. Step 6. Horizontal and vertical lines are pasted on the

image. If the valve is longer than wide the 10 x 10 cm square is extended to accommodate the lower shell part in CDEF.

39

Phuket Marine Biological Center Special Publication

Step 7. Measurements made on the image. The line IT touching the ventral parts of the anteriorposterior laterals is used to measure the angle of upwards or downwards inclination of the anterior shell, i.e., the angle formed by IT in terseeting PP. For right hand valves, the sections AK, KL, BL and BP, Px, and xD are measured in mm corresponding to relative values marlred on the sides of the square. The SQ ratio is Bx divided by 100 (AB). SQ is < I at lengths shorter thao wid1h; SQ ~ 1.00 at length ~ width; SQ > I at lengths higher than width. For left hand valves the corresponding lettering is laterally reversed. (BL,Bx and xD are part of the standard measurements, see fig. 2)

General abbreviations (cf. fig. 2) AB: antero-posterior distance ~ shell width. AK.: anterior part of shell. BD: dorso-ventral distance ~ length, height. BP: height of dorsal part f: flare ~ projection where anterior shell edge meets

with umbo. KL: nymph width. LB: posterior part of shell. PP: horizontal pivot axis. Px: distance from pivot axis to ventral edge of shells

< standard length. SQ: shell length divided by the constant shell width. IT ~ antero-posterior axis passing below the lateral

teeth.

Abbreviations used with Cardium hinge Right valve

Ral-I: anterior lamella forming the roof uotil fusion with Ral-3.

Ral-2: anterior lamella, distinct in central part of the hinge, extending into cavity uoder the roof.

Ral-3: anterior lamella on the lower part of the hinge line.

Rc-I: upper central tooth Rc-2: lower central tooth Rpl-I: posterior part, indistinct lamella forming the

roofuotil fusion with RpI-3. Rpl-2: lamella absent. Rpl-3: posterior lamella, distinct, projecting on the

lower part of the hinge line. Left valve

Lal-I: upper lamella forming the roof fused with Lal-3

LaI-2: indistinct lamella extending into cavity uoder the roof.

Lal-3: lower lateral lamella, projects anteriorly. Lc-I: upper central tooth Lc-2: lower central tooth Lpl-I: posterior part, projects posteriorly on the upper

part of the hinge line Lpl-2: lamella absent. Lpl-3: posterior part lamella on the lower part of the

hinge line left valve. Abbreviations used with Vasticardium right valve. Ral-I: anterior lamella joined withRal-3 via the roof. Ral-2: anterior small lamella, free or fused with

Ral-I, extending into cavity. Ral-3: anterior lamella on the lower part of the hinge

line

RESULTS

Cardium costatum Linnaeus, 1758.

Valves in standard squares. Upwards inclination of anterior part: I-33°, most values between 10 and 30° (Fig. 3). Nymph length: 11-16 % of standard shell width. SQ ratio: 0.80--1.06, most values between 0.83 and 1.00.

Rightvalve, binge line. Anteriorpart: Ral-I expaoded into the roof arching over Ral-2 (Fig. 4B). The roof is narrow in adults. (Fig. 5, compare B and H). Ral-2 is a curved lamella. It continues at an angle with the shell edge and dips into a deep cavity uoder the roof (r) towards umbo (Fig. 5B, H). The cavity uoder the roofis wide and deep. Savazzi and Siilgeback (2004) refer to the cavity as a hollow socket. The roof is thin in juveniles (Fig. 4) and solid in adults (Fig. 5A). Ral-3 is longer than Ral-2 and forms a vertical, flat, erect or gently curved, triangular projection. Ral-3 continues as a curved list along the ventral edge of the nymph and terminates inside the umbonal cavity. Shell edge straight, or slightly curved, with anterior, rouoded shoulder (Fig. 5A). The shell edge expands dorsally into a rouoded triangular flare (~ spur of authors) attached to the umbo (Fig. 5C).

Central part. Two, prominent, erect, pointed orrouoded cardinal teeth. The taller, anterior tooth is labelled Rc-I and the posterior, shorter tooth Rc-2 (Fig. 5C). A ventral list connects the teeth. A relatively long nymph (Fig. 5A, C) is part of the central valve.

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Marine taxa in the Reference Collection of PMBC

Figure 1. Cardium costatum. ZMUC collection. Occurrence along the Atlantic coast of Africa north and south of equator.

A,-__________________ -,B

10x 10cm

CL-------------------~ D

I extension SO > 1.00 I EF steps 2-4

step 1

49% 36% A,-______ ~~~~----_,

t 50 TT forms an angle with PP I

step 6 right valve

step 7

step 5

35% 16% 49% Br-____ ~L~~K=_------~A

14% 14%

77%

SO=0.91 9% CL-----~---L--------~D

left valve

Figure 2. Cardium costatum. ZMUC collection JH # 2618. Sequence of actions performed on right and left valves from image to final treatment in standard square. Length/width ratio in standard square is termed SQ. ABCD: 10 x 10 cm standard square, DF: part of shell longer than the standard square, Dx: part of shell shorter than the standard square, PF: distance from pivot axis to ventral edge of shells > standard length, xx: punctured line marking ventral part of shell ..

41


35° upwards inclination of anterior part

• •

• sa index

o ... 1 ••••••••••••••••••••••••••• o 0 0 0 ...... ......

~ g: ig ~ 8 g:

Figure 3. Cardium costatum. Anterior TT-PP angles as a function of SQ values. Dots = specimens in the ZMUC collection. X-I = left valve, type specimen, Linn. Soc. London, X-2 = right valve, type specimen, Linn. Soc. London. X-3 = right valve, holotype C. maxicostatum Ter Poorten, 2007. X-4 paratype, right valve C.

maxicostatum Ter Poorten, 2007.

Figure 4. Cardium costatum. Comparison of juvenile specimens. A. Off Nigeria, 17 m. ZMUC, JH# 2513. B and C. Cameroon S. of Victoria, 11m, mud. Ral-1, RaI-2, Ral-3, r = roof, f= flare, n = nymph. ZMUC, JH# 2754. (Hylleberg, 2009: 140-141).

42


right valve c B

-------1

left valve

Lc

1----------_ /

right valve

Figure 5. Cardium costatum. ZMUC collection. A-G adult specimen. A: right valve interior view, positioned according to horizontal pivot axis. B. Right anterior laterals: Ral-l, RaI-2, Ral-3, r = roof. C. Right cardinal teeth Rc-l and Rc-2, f= flare, u = umbo, n = nymph. D. Right posterior laterals: Rpl-l, Rpl-3, n = nymph, r = roof. E: Left posterior laterals Lpl-l, Lpl-2, r = roof, n = nymph. F. Left cardinal teeth Lc-l and Lc-2, two sockets labelled I and 2, f= flare. G. Left anterior laterals: Lal-l, Lal-2 obscure list, La13, r = roof.

H: juvenile specimen (Hylleberg, 2009: ZMUC, JH# 2754).

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Posterior part (Fig. 50). The upper Rpl-I, short, fused with the shell edge. Anteriorly, forming a roof together with the prominent, erect, triangular Rpl-3. The fused bases of the two teeth arch over a short, narrow cavity, extending under the nymph (Fig. 50). Rpl-2 absent.

Left valve, hinge line. Anterior part (Fig. 5G). The upper lateral (Lal-l) forms the narrow roof. Lal-2 is an obscure list extending into the cavity. Lal-3 is the only distinct lamella (Fig. 5E). The shell edge is straight and projects into a triangular flare attached to the umbo (SF).

Central part. Two teeth (Fig. SF). Lc-I is low, with a vertical list on the right hand side facing umbo. Separated from the longer Lc-2 by a narrow cavity connecting two deep sockets to the left and right of the teeth. Both teeth with a the flat side facing the umbo. The nymph with concave or straight upper edge. The shell edge projects dorsally as a rounded triangular flare attached to the anterior side of umbo.

Posterior part (Fig. 5E). Upper lateral Lpl-Ilocated close to the shell edge, carries a flat triangular projection. Lpll joins with Lp-3 to form a narrow roof arching over a short, shallow cavity. Lpl-3 has a low angular projection. Lpl-2 absent.

Cardium maxicostatum Ter Poorten, 2007.

Valves in standard squares. Upwards inclination of anterior part: 21-23°. Nymph lengths: 11%. SQ ratios range from 0.97-1.06 (Fig 6, 3-4).

I have examined extemal and intemal characters of the total ZMUC material (Hylleberg, 2009) but I did not find the consistent combination of characters listed by Ter Poorten (2007). Hence, I could not separate Cardium costatum from Cardium maxico.statum. Similarly, standard square comparisons revealed a uniform pattern without gaps (Fig. 3). The characters used to separate C. costalum from C. maxicostatum overlap and the inner views of the type specimens deposited in the Linn. Soc. of London (Fig. 6) do not display obvious differences when compared with the type material of C. maxicostatum.

Cardium indicum Lamarck, 1819

Valves in standard squares. Upwards inclination of anterior part: 12-16°. Relative nymph width: 10-12 %. SQ ratios: 0.92-{).97. (Fig. 7B-0).

C. indicum (Hylleberg, 2004: 142) is a Mediterranean-West African species and it is the only living representative of genus Cardium spart from C. costatum and its form maxicostatom.

Cardium paleobrocchi nom. nov. for Cardium hians Brocchi, 1814.

Valve in standard squares. Upwards inclination of anterior part: 19°. Relative nymph width: 8 %. SQ ratio: 0.87 (Fig. 8).

Van Aartsen and Gianmusszi-Savelli (1991) advocated that Cardium hians Brocchi, 1814 should be called by its next available synonym Cardium indicum Lamarck, 1819. However, we are dealing with two different species so the tertiary fossil species C. hians Brocchi, 1814 must be given a new name because it is a homonym of Cardium hians Spengler, 1799. In consequence, I rename it Cardium paleobrocchi nom. nov. for Cardium hians Brocchi, 1814. Only one right valve is available at the ZMUC, JH # 2612 (a fossil from the Tertiary).

The analysis confirms that living C. indicum and fossil C. paleobrocchi nom. nov. are very close in terms of shell morphology (Fig. 9A-B). However, large spines on the exterior shell are conspicuous along the posterior part of the hinge of C. paleobrocchi nom. nov. In comparison similar spines on the posterior part of Cardium indicum are small or absent (Fig. 7A-0).

Acanthocardia aculeata (Linnaeus, 1758)

Valves in standard squares. Upwards inclination of anterior part: 0-7°. Oownwards inclination of anterior part: 6-7°. Nymph length 14-23 % of standard shell width. SQ ratio: 0.88-1.00 (Fig. 10). Variation in the angle ofTT with PP was examined in IS specimens at ZMUC (Hylleberg, 2009: 104-105). Eight specimens displayed downwards inclination, 3 specimens had zero inclination, and 4 specimens upwards inclination of the anterior part. Samples of the three types are shown in Fig. II.

Two laterals on anterior part: Ral-I curved upper lateral with low projection close to shell edge, Ral-2 lower lateral, curved, with larger triangular projection. A long slit separates the two laterals.

Two cardinal teeth, Rc-I upper tooth, small, Rc-2 taller, sockets on both sides of Rc-2. Long straight nymph. Flare small or absent, not attached to umbo.

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1 36% 11 %

53% B L K A

~

W

P p <f

j -,J!. 0

x ------ ------------- x left valve SO = 0.84 C '--____ L-----'. _______ --' D

A 3 68% 21% B

I\) w -,J!. 0

p~=-------~~~ P

T 29°1

.I -...J -...J -,J!. 0

c ~.,__;_____;_--[7.~'¥,;l~~~-___1 D E Lr--,i9,--h_t _va_lv_e ____ ----'~ ____ ____' F ~

A

T

P

x

A

x C

2 13% 53% K L 34% B

-...J -,J!. 0

P

16° 1

T

-...J -...J -,J!. 0

x OJ

'--"-------------'-----'---------' 0 ~

11 % 4 62% r-~-~=_---_,K,_-L,_-2-7°-~_,B

'1------i p

-----xw D<f

Figure 6. Cardium costatum in standard squares positioned according to horizontal pivot axis, PP. 1. Right valve. Type specimen, Lin. Soc. of London, isolated by Hanley (1855). 2. Left valve. Type specimen, Lin. Soc. of London, isolated by Hanley (1855). 3. Cardium maxicostatum, right valve, holotype, Ter Poorten fig. 13 (2007).4. Cardium maxicostatum, right valve, paratype, Ter Poorten fig. 21 (2007). The anterior part, nymph width, and the posterior part (AK, KL, LB) respectively, are shown in right valves as percentages of total width A-B. AP, P-x shown as percentages of total length AD. The labelling is reversed on left valve. TT forms and angle with the horizontal pivot axis PP. SQ calculated as AxlAD in right valves. BxlBD in left valves. (AD, Ax and Bx are part of the standard measurements, see fig. 2)

Two laterals on posterior part: Rpl-1, low posterior lateral, Rpl-2 longer with a triangular pro-jection. Roof absent in both anterior and posterior parts.

Remarks: La Perna and D' Abramo (2009) found that the common constructional pattern of spines on shell surfaces of Acanthocardia constitute a useful taxonomic tool for defining the genus. Based on analysis of spines they concluded that most probably, aclinal variation exists through the geographic range of the widely distributed species Acanthocardia echinata.

Similarly, the present analysis of hinges of A. aculeata display much variation of characters, indicating the possibility of clinal variation also in this species.

Vasticardium cf. elongatum (Bruguiere, 1789)

Right valve in standard square. SQ index: 1.14. Downwards inclination of anterior part: 16°. Nymph length 20 % of standard shell width. Specimen from Thailand analysed (Fig. 12 A).

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A

x xoo ':?

C D°

55% 10%

35% 12% A K L B 30% L K 58% A

T ;:;:; <f- 12° t -; ~ T

p p P

T

C D 00 <f-...

-; OJ f'..

SQ = 0.97 x ______ _

C - - - D ~ X '---___ L-----'-______ ------' C

Figure 7. Cardium indicum. A. Right valve. Specimen at ZMUC, JH # 2597, Algeria (Hylleberg, 2009: 142). B. Same specimen ZMUC, JH # 2597 in standard square. C. Juvenile specimen ZMUC JH # 2630, French Equatorial Africa. D. Left valve. Lamarck's original specimen at MNHN, Paris. (Hylleberg, 2004: 843). Position, sections and SQ calculations as in Fig. 6.

Three laterals present on anterior part (Fig. 12 C). Ra1-1 straight, short, upper lateral close to the curved shell edge, Ral-2 centra11atera1 tooth, short, free or attached to the side of Ra1-I. Solid roof over Ra1-2, shallow cavity. Ra1-310wer lateral, straight, with larger triangular projection.

Two cardinal teeth. Rc-l small upper tooth. Rc-2 taller, sockets on both sides of Rc-2. Long straight nymph. Flare absent.

Rp1-1, low posterior lateral fused with curved shell edge, Rpl-2 longer with a triangular projection. Posterior roof absent.

Remarks. Species placed in genus Vasticardium are the only cardiids I have observed with structures which I interpret as three teeth in the anterior part of

the right valve. The basic arrangement is similar in Cardium costatum and Vasticardium cf. elongatum, but according to a cladistic analyses the two genera are far from each other although placed in the same subfamily (Schneider, 1995).

DISCUSSION

Detection of species. The first task is to define an operational system for identification of species. For practical reasons I use the concept of morpho species because it is operational in practice. The morphospecies approach is based upon the demonstration of gaps in the distribution of external and internal characters.

46


A T 60%

Pr--r--~------~

8% K L

x -------------SO = 0.87

32% B

-..J o ;R. o

------ x-"

w ;R. o

C '-------------------'----"-----------' D

o.

Caroiumhians Original drawing.

Figure 8. Cardium paleobrocchi nom. nov. A. Right valve. Specimen at ZMUC, (Hylleberg, 2009: 318, ill # 2612). B. Original drawing by Brocchi, 1814. (Hylleberg, 2004: 94). Position, sections and SQ calculations as in Fig. 6.

Figure 9. A. Recent Cardium indicum (Hylleberg, 2009: 142, ill # 2597). B. fossil Cardium paleobrocchi nom. nov. (ZMUC, Hylleberg, 2009: 318, JH # 2612). Right valves. 1 = Ral-l, 2+3 = fused Ral-2 and Ral-3. r = roof, n = nymph, f= flare.


B 1 32% L 15%K

-oR o N

p f------~ ...

T

-oR o <Xl <Xl

53% A 50%

h.--------1 P PI-----=-yo t 50 T T r-7;':~~~

x DL---~~~----~------------~ C C

42% K 19% L 39% B A4 45%

0 -oR 0

P P

T T

(Xl (J1

?t-

X (J1 SO = 1.0 Doe C

50% K 20% L 30% A 6 43%

~ ?t- P

6°!

T

(Xl I\)

?t-

SO = 0.88

K

14% K L

23% L

36% B

r------1 P

T 50!

x (J1

D ~

32% B ~ -oR 0

P

T 0°

(Xl O> -oR 0

0

39% B

?t-P

T

-..,J -..,J

?t-

x ~ I\)

-oR c L-__________ -L ____ ~ ________ ~Do

47

Figure 10. Acanthocardia aculeata. 1. Left valve. Type specimen, Linnean Society of London. 2. Right valve. Type specimen, Linn. Soc. of London. 3. Specimen identified by Lamarck, MHNG. 4. ZMUC, without loco JH # 3615.5. ZMUC, Montpellier, France JH # 3636. 6. ZMUC, Naples, Italy JH # 3617 (Hylleberg, 2004, 2009). Position, sections and SQ calculations as in Fig. 6.

48


Figure 11. Acanthocardia aculeata. Right valves. A. Upwards inclination of TT (2) with PP (1). Specimen from Mediterranean, Montpellier, France. ZMUC, JH 2009 # 3636. B. Zero inclination ofTT with PP. Specimen from Atlantic Ocean, Portugal. ZMUC, JH 2009 # 3618. C. Downwards inclination ofTT with PP, Mediterranean, Italy, Naples. ZMUC, JH 2009 # 3606. Scale bars = 10 mm.

49


33% K 20% L 47% A,----___ ---=;:::-_---,--_____ -----,B <Xl

pf-------i;~~~~=_----~ p # T

T

B C~-~~~--1---_+~-~D

E F

Figure 12. Vasticardium cf. elongatum. A. Right valve, interior view. B. The same valve positioned in standard square. C. Right anterior laterals: 1-3 = Ra-l to 3, r = roof over cavity. Specimen from Gulf of Thailand. JH collection. Position, sections and SQ calculations as in Fig. 6.

Most descriptions and identification of species put emphasis on external characters such as rib numbers and ornamentation. Schneider (2002) also applied interior characters in his cladistic analyses of recent and fossil species, but it is rare to find undamaged and well preserved fossils in contrast to living material. Obviously, the concept of morphospecies must be applied with fossils since soft bodies are absent. Ter Poorten (2007) divided C. costatum into two species based on the morphological variation of external characters, but description of morphospecies based on gaps has its limitations. Large specimes apparently display gaps while smaller individuals and juveniles do not. Only full separations do constitute gaps, thus constituting taxonomic characters of morphospecies. Examination of the right valve of C. costatum showed that the anterior laterals consisted of a roof-forming lamella and two distinct lamellae. The number of laterals and the shape of each lateral constitute characters distinguishing this taxon.

Internal shell morphology. I have found upwards inclination of the anterior part in Cardium costa tum, C. indicum, C. paleobrocchi nom. nov., Ringicardium ringens, Acanthocardia aculeata, and specimens of Cerastoderma cf. glaucum from the Adriatic, but in cardiids the general rule is that the anterior part is turned downwards relative to the pivot axis. In a few cases the pivot axis PP and the inclination axis TT

are parallel. Standardized size and orientation of shells

permit a quick comparison of closely related species, e.g., Cerastoderma edule with a long nymph and Cerastoderma glaucum having a short nymph (Hylleberg, 2004, p. 531; Petersen, 1958), but as expected all measurements of shells placed within the standardized squares display variation. The method is only a supplement to other methods.

Most molluscs including cardiid species are notoriously variable in morphological characters (Gray, 1835). The combination of the two ingredients, homogeneity of the genus and variability of the species, is a sure recipe for producing taxonomic chaos. Dr. B. Tursch, Universite Libre Brussels found that this has indeed happened many times (pers. comm.). Of course many of the specific names are genuine synonyms, but misidentifications are also very common. There is no hope of cleaning up this taxonomic mess in a quick way. We can only hope that experienced taxonomists will still be available to provide the best possible names. Studies of morphology can lay down part of the species identity and in the majority of cases this is what we have to work with. In the future, species identification may be supplemented by molecular techniques such as electrophoresis and DNA analyses, but there is a long way to go before such information is generally available.

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Marine taxa in the Reference CoUection ofPMBC

Nomenclature. Taxonomic literature encompass a continual shuttling of species from one generic allocation to another, and the status of names ranging from species, subspecies, forma, to plain synonymy (Hylleberg, 2004: 5, 840). Within the genus, a relatively few species are easy to recognise. Many more species are not easy at all and pose frustrating problems familiar to tropical maiacologists (Tursch, 1998). Difficulties in species delimitation exist in all groups of molluscs giving rise to the classical problem that a specimen seems to be right in the middle between two species. This fact is caused by intraspecific variability which may be due to genetic as well as environmental factors. For example, Roopuarlne et al. (2008) analyzed multiple samples of the venerid Anomalocardia squamosa from the Kungkrabaen Bay area in Thailand The study revealed an interesting pattern of variation related to microhabitat differences, most notably sediment type. Samples collected from fine mud substrata within the bay formed a fairly homogenous set, regardless of whether the specimens were live-collected or collected as dead and possibly transported shells, or present as subfossil individuals deposited in recent geological past. Three samples from coarse-grained sandy substrate were not assigoable to this large set. Roopnarine et al., (op. cit.) concluded thatA. squamosa is as a single, widespread species capable of very high levels of morphological variation and perhaps adaptation.

Can fossil and recent species be identical? Lamarck's name C. indicum referring to India is not the best name given to a species not found in India but it was Lamarck's choice. If Cardium paleobrocchi nom. nov. and C. indicum Lamarck, 1819 were identical the oldest available name must be applied (International Code of Zoological Nomenclature,

1999) but it can never be proven that the fossil and recent species are in fact identical. I maintain the name C. indicum as a nomen conservandum. It is not likely that a fossil species such as Cardium paleobrocchi nom. nov. has remained unchanged over many million years where dramatic changes in climatic and environmental factors have occurred. In addition, molecular techniques can ouly be applied to extant specimens. Living and extinct species should not carry the same name even if they look alike. For example, Deshayes (1844-1848) could illustrate the soft body of recent C. indicum (Hylleberg, 2004: 155-157), a possibility excluded with fossils. As pointed out by Gray (1835) soft bodies can yield valuable information when it comes to separation of species which may be difficult to distinguish based on external characters alone. Organization of siphons and presence/absence of pigmented eyes exhibit important characters for classification. Based on extemal shell characters alone, the genera Vepricardium and Fulvia are placed in different subfamilies Hylleberg (2009: 12-14) but they have pigmented eyes in common Vidal (2001), indicating a closer relationship.

Another example is Vasticardium fidele (Vidal, 1992) considered synonym of CanIium (Trachycardium) kenyanum Cox, 1930, a pleistocene fossil from Mombassa, Kenya (Ter Poorten, 2005). Extemally, the two species certaiuly look alike, but this does not make them identical. I continue to use the name Vasticardium fidele for the living species for reasons discussed above. Similarly, I make a distinction between the tertiary fossil A.frocardium ebaranum (Yokoyama, 1927) and the living speciesA.frocardium richardi (Audouin, 1826) considered to be synonyms by authors.

REFERENCES

Children, J.G. 1823. Lamarck's genera of shells translated from French by J.G. Children, F.R.S. &c, &c. &c. With Plates from original drawings, by Miss Anna Children. 1823. Quarterly Journal of Science Literature and the Arts, Vols. 14, IS, 16

Deshayes, G.P. 1844-1848. Exploration scientifique de I' Algerie pendantles Ann""" 1840, 1841, 1842. Publiee par Ordre du Gouvemernent et avec Ie Concours d'une Commission d' Academique. Sciences Physiques. Zoologie. Paris Jmprimerie Natiouale. Histoire naturelle des Mollusques. Tome Premier. Mollusques Acephales. Texte. Atlas. Paris, 606 pp. 142 pis. REMARKS. Plates seen at MNHN but the legends to these plates are not present in the text volume at the museum. Apparently they were never printed.

Gray, J.E. 1835. Remarks on the difficulty of distinguishing certain genera oftestaceous Mollusca by their shells alone, and on the anomalies in regard to habitation observed in certain species. Philosophical Transactions. Part II for 1835. London.

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Phu/cet Marine Biological Center Special Publication

Hylleberg, J. 1994. Indo-Pacific cockles (Bivalvia:Cardiidae). Part 1. Generic diagnoses and an overview of species with mention of taxonomic problems encountered in Thailand. Phuket Marine Biological Center Special Publication 13: 113-136.

Hylleberg, J. 2004. Lexical Approach to Cardiacea. Vol. 1. Bibliography, annotated and illustrated. Tropical Marine Mollusc Program (TMMP). Phuket Marine Biological Center Special Publication 29: 1-352.

Hylleberg, J. 2004. Lexical Approach to Cardiacea. Vol. 2. Records. Annotated and illustrated, A-M. Phuket Marine Biological Center Special Publication 30: 351--{)44.

Hylleberg, J. 2004. Lexical Approach to Cardiacea. Vol. 3. Records. Annotated and illustrated, N-Z. Phuket Marine Biological Center Special Publication 30: 645-939.

Hylleberg, J. 2009. Cardiidae (Mollusca: Bivalvia) in the collection of Statens naturhistoriske Museum, previously the Zoological Museum, University of Copenhagen (ZMUC). Annotated and revised Part I: 1-101. Part 2: 102-324.

International Code of Zoological Nomenclature (ICZN). 1999. The International Trust for Zoological Nomenclature. 4th Edition, London, 306 pp. ISBN 085301 0064. Abbot, 1991.

Lamarck, J.B. 1799. Sur les genres de laeche, du Calmar et du Pouple, vulgairement nommes Polypes de mer. Memoires de la Societe d'lfistoire Naturelle de Paris, p. 62-91. Baudouio. Prairial an VII.

Lamarck, de. 1819. Histoire naturelle des animaux saos vertehres, presentant les caracteres generaux et particuliers de ces aoimaux, leur distribution, leurs classes, leur families, leur genres, et la citation des principales especes qni s 'y rapportent, precedee d'une Introductioo offraot la determ:inatioo des caracteres essentiels de I' animal, sa distinction du vegetal et des autres corps naturels, enfin, l'Exposition des Principes fondamentaux de la Zoologie. De l'Imprimerie de A. Belin, Paris, Vol. 6, Part I: 1-252.

La Perna R. and M. D' Abramo. 2009. Morphometric and systematic study on three Acanthocardia species from the Mediterranean Pleistocene (Mollusca, Bivalvia, Cardiidae). Geodiversitas 31(3) : 669--{)82.

Petersen, G.H. 1958. Distinctioo between Cardium edule L. and C. lamarcki Reeve. Nature 181: 356-357. Roopnarine, P.D., J. Signorelli and C. Laumer. 2008. Systematic, biogeographic and microhabitat-based

morphometric variation of the bivalve Anomalocardia squamosa (Bivalvia: Veneridae: Chioninae) in Thailand. Raffies. B. Zoology. Supplement Series 18: 95-102.

Savazzi, E. and J. Siilgeback. 2004. A comparison of morphological adaptations in the cardiid bivalves Cardium and Budmania. Paleontological Research 8(4): 221-239.

Schneider, J.A. 1995. Phylogeny of the Cardiidae (Mollusca,Bivalvia): Protocardiinae, Laevicardiinae, LahjJJiinae, ThIongocardiinae subfam. n. and Pleuriocardiinae subfam. n. Zoo!. Scripta 24(4): 321-346.

Schneider, J.A. 1998. Phylogeny of the Cardiidae (Bivalvia): Phylogenetic relationships and morphological evolutioo within subfamilies Clinocardiinae, Lymnocardiinae, Fraginae, and Tridacninae. Malacologia 40(1-2): 321-373.

Schneider, J.A. 2002. Phylogeny of cardiid bivalves (cockles and giant clams): revision of the Cardiinae and the importance of fossils in explaining disjunct biogeographical distributions. Zool. J. Linn. Soc. 136: 321-369. With 29 figures.

Sowerby, G.B. 1852. A Conchological Manual. Fourth edition, London, i: 1-337, pis. 1-29. (col.), 2 Tables. Ter Poorten, J.J. 2005. The identity of Cardium (Trachycardium) kenyanum Cox, 1930, from Pleistocene

deposits in Mombasa, Kenya (Bivalvia, Cardiidae). Basteria 69: 9-11. Ter Poorten, J.J. 2007. Notes on the identity of Cardium costalUm Linnaeus, 1758, with the description of

Cardium maxicostalUm spec. nov. from tropical West Africa (Bivalvia, Cardiidae). Basteria 71: 57--{)9. Tursch, B. 1998. Taxonomic problems in the genus Oliva. Phuket Marine Biological Center Special Publicatioo

18(2): 263-284. VanAartsen, J. J., and R Giannuzzi-Savelli. 1991. New names for well-known European marine Mollusca.

Boll. Malacol. 27(1-4): I--S. Milano. Vidal, J. 1992. A remarkable new species of the subfamily Trachycardiinae (Mollusca, Cardiidae) from

the Indo-Pacific. Apex (Brussels) 7(1): 23-26. Vidal, J. 2001. Siphons and associated tentacles including eyes of Cardiidae (Mollusca: Bivalvia). Phuket

Marine Biological Center Special Publication 25(2): 405-410.

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HORIZONTAL PIVOT AXIS AND STANDARDIZED SHELL OUTLINE: A new method to compare morphology of cardiids...

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