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Easy to See, Difficult to Describe: Towards a Standardised Description of Bivalve Larval Shell Shape

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Easy to See, Difficult to Describe: Towards a Standardised Description of Bivalve Larval Shell Shape Easy to see, difficult to describe: towards a standardised description of bivalve larval shell shape N. Malchus Dept. de Geologia, Universitat Autònoma de Barcelona, Spain INTERNATIONAL CONGRESS ON BIVALVIA 22-27 JULY 2006 Introduction Basic eight-step procedure Examples Early ontogenetic shells of bivalves show two to three growth 1: Draw contour from valve (fill white); add scale bar. Rotate valve 1: P2 of unknown Jurassic taxon (pectinoid?), RV internal, round. phases: prodissoconch 1 (P1) (sometimes missing), first (if necessary) until hinge axis lies horizontally. Characteristic Touch point with circle posteroventral (arrow). This is a constant prodissoconch 2 (P2), and interdissoconch (IntD). Each of it details like wings or prominent elements may be added as a feature in this species that allows distinction of left and right valves carries eight basic character sets: shape (e.g., contour and sketch. from the exterior.All scale bars represent 100 micrometers. profile), size, sculpture, coiling, hinge teeth, ligament, internal shell margin characters, and micro- and ultrastructures, each with a minimum of two character states (absent/present). Thus, early ontogenetic shells carry between 33 and 48 basic character sets/characters/states; these could be successfully employed in taxonomy, evolution-developmental studies and a large number of derived applications. 100 Unfortunately, elementary taxonomic identification and Philobrya meleagrina Bernard, syntype, right valve, external comparisons between different publications are largely hampered by the heterogeneity of description modes. However, most modes 2: draw a circle larger than object (in this example 90 mm versus contain line drawings or photos of shape, specifically of the shell's 70 mm), fill black and place contour over circle (do not forget scale contour as a kind of “least common denominator”. The problem bar). here is that the similarity of contours complicates the assessment 2: Jurassic bakevellid sp. M2, RV, exterior, contour mirrored (cf. of differences with respect to taxonomic importance, even when example 3); strongly umbonate, oval-triangular, touch point these differences are still perceivable optically. It is even more posteroventral. difficult to describe the shells in a consistent, comparable way. The present contribution therefore proposes a simple graphic method which should aid in an easier perception of significant differences and a more homogenous description. The long-term objective would be to integrate such representations in determination keys, taxonomic bivalve web pages, and image 100 analysis techniques. For this, contours could be classified referring to simple geometric figures such as oval, round, triangular, etc. 3: Align contour and circle midpoints (there is a function for this task in most graphic programmes). Length HingeIV I Axis 3: Same species (M2) as Figure 2; RV interior. Compare matching contours of mirrored (2) and non-mirrored (3) specimens. P1 P2 Height III II 100m m 100m m 100m m 100 aligned A BC Figure 1: Larval shell contours of left valves (Malchus, Warén 4: reduce circle (here: 90 mm = 100%) until it touches the contour 2005, modified).A ,Glycymeris violacescens, prodissoconch 1 at a single point (it rarely touches more). The present reduction is (P1; length 146mm m), prodissoconch 2 (P2, length 209m ).B , 83.3% of the original drawing. Limopsis cristata, lecithotrophic, non-brooding prodissoconch (L = 175mm). C ,Philobrya wandelensis , lecithotrophic prodissoconch (L = 479mm), anterior and posterior wings with well 4: Recent pinnid (Atrina ?), RV interior. Umbonate, asymmetric visible growth increments, few “radial” elements, in this case triangular, more elongated anteroventrally, touch point narrow gutters; the characteristic pitted surface is not shown. anteroventral. Definitions 100 100 Def. 1:contour : outline of shell or valve with the commissure reduced plane parallel to drawing/paper plane Def. 2:hinge axis : axis parallel to the straight hinge of the larval 5: group contour and circle; here, also mirror horizontally because shell or drawn through the main hinge teeth below the straight the contour was drawn on a RV viewed from external). hinge (i.e., excluding lateral teeth) (Fig. 1B) Note that hinge axes from P1, P2, IntD of a specimen may diverge, but this should not generally influence the result in a significant way. 5: JurassicGrammatodon concinnus , RV exterior, mirrored. Def. 3:length of shell/valve : longest anterior-posterior distance Umbonate, oval in length, almost symmetrical, touch point parallel to the hinge axis (Fig. 1B). posteroventral. Def. 4:height : longest dorsal-ventral distance at 90º to the length axis (Fig. 1B). 100 100 mirrored Prerequisites and suggestions 6: draw a cross (diameter larger than circle diameter) and align with centre of circle/contour. Pre. 1: use high magnifying light or SEM microscope (with image capturing device) and drawing programme that allows to follow the eight-step procedure (explained below). Pre. 2: distinguish left and right valves 6: RecentTeredo ? navalis, LV interior, mirrored. Umbonate, Pre. 3: use preferably single valves almost symmetrically oval in height, touch point ventral. The present study has used LVs wherever possible, with the contour drawn from exterior view. Contours from RV interior should give comparable results if shells are equivalve. Contours drawn from LV interior and RV exterior must be 100 100 mirrored horizontally. centred Pre. 4: annotate details: which valve? exterior or interior?; add scale bar. 7: number resulting quadrants 8: resize figure + scale bar The scale bar is indispensable in the final figure. clockwise from I to IV (optional); describe contour Pre. 5: orientate specimen properly This is necessary for length/height measurements and for a non-distorted image (photo) from which the contour is drawn. l p rsa os IV I do te o IV I ro r d te o Pre. 6: rotate specimen (preferably) or contour until the length axis n r a s a Literature lies horizontal l Bernard F 1897, Journal de Conchyliogie 1:1-47. [poster features the first SEM figure of It is easier to rotate a specimen rather than the contour into the correct position. aP.meleagrina synype.] This should be done before composing the final figure. Note that the hinge axis Malchus N 2004, Acta Palaeontologica Polonica 49: 85-110. [referring to example a l is not visible from external view under SEM. However, it can be generally n a figures 1-4.] r te t n Malchus N, Warén A 2005, Marine Biology Research 1: 350-364. estimated sufficiently well. Problems occur in shells with prominent gyrate ro e v v o umbos, and estimations require much experience. In all cases, the line en r tr III II te between the most distant anterior and posterior points of P1 as seen from the III II al os Acknowledgments 100 p 100 exterior allows a first approximation to be made. The proposal is a contribution to a Ramón y Cajal research contract (2001-2006) at the Universitat Autònoma de Barcelona and to project number DGI-BTE2003-03606 of the Spanish Ministry of Science and Education (MEC)..
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