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Repair scars on Mactra violacea from the eastern coast of India: a new classification and a model for describing shell breakage on bivalves

Subhronil Mondal, Subhendu Bardhan, Sumanta Mallick, and Arindam Roy

Plain Language Abstract

Shell-breaking predation can be non-lethal; in that case the prey survives the attack and repairs the breakage. However, breakage can also be non-predatory in origin produced during burrowing or impact from the saltating clasts. If the damage is sub-lethal, it is repaired. In this way, both non-predatory and predation-induced repaired breaks are preserved as scars on the prey shell. Morphologically these two types of scars are difficult to distinguish from each other. Here, we have shown that a bivalve species, Mactra violacea, from the eastern coast of India, have both types of damage preserved as repair scars. We argue to use a new classification scheme which considers intensity, rather than the cause, of the breakage, because severity of the damage determines whether the individual will survive that injury or not. Survival and recovery may be advantageous for reproduction and subsequent adaption.

Resumen en Español

Cicatrices de reparación en Mactra violacea de la costa oriental de la India: una nueva clasificación y un modelo para describir la rotura de la concha en los bivalvos

Las cicatrices no letales en la concha de los bivalvos, que se conservan como cicatrices de reparación, pueden tener su origen en una acción de depredación o de otro tipo. Cuando ciertos cangrejos (peeling crabs) son los principales grupos de depredadores, los daños no causados por depredación son debidos al impacto de clastos en movimiento o al desgaste durante la elaboración de la madriguera. En ambos casos, estas cicatrices de reparación son similares, y es difícil identificar cuál de los dos factores fue el causante. Debido a que la supervivencia de un individuo está relacionada con la gravedad del daño en la concha, con independencia de la causa, hemos desarrollado un esquema de clasificación para categorizar las cicatrices de reparación en relación con la intensidad del daño. Por otra parte, hemos proporcionado un modelo para analizar cómo la gravedad de las cicatrices puede ser utilizada de forma eficaz para estudiar la adaptación de las especies contra las causas que rompen las conchas, ello usando Mactra violacea como especie de estudio. Los individuos que sobreviven a la rotura de la concha pueden adaptarse en sus rasgos morfológicos de acuerdo al proceso denominado escalation para resistir el daño a largo plazo.

Palabras clave: rotura de la concha; costa de la India; tamaño-refugio; eficacia de la presa

Traducción: Enrique Peñalver

Résumé en Français

Cicatrices de réparation sur Mactra violacea provenant de la côte est de l 'Inde: une nouvelle classification et un modèle pour décrire la rupture de la coquille chez les bivalves

Les dommages de coquille non-létales, qui sont conservés en tant que cicatrices de réparation sur la coquille de bivalve, peut être d'origine prédateur ou non-prédateur. Lorsque les crabes de déroulage sont les principaux groupes prédateurs, les dommages non-prédateurs sont produits par l'impact des clastes de saltation ou par l'usure au cours de l'enterrement. Dans les deux cas, ces cicatrices de réparation se ressemblent presque, et il est difficile de déterminer quel facteur est causalement responsable. Parce que la survie d'un individu est liée à la gravité du dommage de la coquille indépendamment de la cause, ici, nous avons développé un système de classification pour classer les traces réparés à partir de l'intensité des dégâts. En outre, nous avons fourni un modèle pour analyser comment la gravité des cicatrices peut être efficacement utilisée pour étudier l'adaptation des espèces contre les causes de dommages de coquille, en utilisant Mactra violacea comme espèces étudiées. Les individus qui survivent les dommages de coquille peuvent s'adapter à une escalade de traits morphologiques pour résister aux dommages à long terme.

Mots-clés: rupture de la coquille; Côte indienne; taille-refuge; efficacité des proies

Translator: Kenny J. Travouillon

Deutsche Zusammenfassung

Korrekturnarben bei Mactra violacea von der indischen Ostküste: eine neue Klassifikation und ein Modell um Schalenbrüche bei Bivalven zu beschreiben

Schalenverletzungen die nicht zum Tod führen und die auf der Bivalvenschale als Ausbesserungssnarben erhalten sind, können räuberischen oder nicht räuberischen Ursprungs sein. Wenn Krabben die Hauptgruppe räuberischer Organismen sind, werden nicht-räuberische Verletzungen durch den Zusammenprall mit saltierenden Klasten oder durch den Verschleiß und die Abnutzung beim Graben produziert. In beiden Fällen sehen die Korrekturnarben beinahe gleich aus und es ist schwierig den kausal verantwortlichen Faktor zu bestimmen. Da das Überleben eines Individuums ungeachtet des Grundes mit dem Schweregrad der gebrochenen Schale in Zusammenhang steht, haben wir hier ein Klassifikationsschema entwickelt mit dem wir die Ausbesserungsspuren auf der Basis der Schadensstärke kategorisieren. Darüber hinaus haben wir ein Modell zur Verfügung gestellt mit dem der Schweregrad der Narben effektiv genutzt werden kann die Adaption der Arten gegen verschiedene Ursachen die zum Schalenbruch führen zu untersuchen, indem wir Mactra violacea benutzen. Individuen die Schalenbrüche überleben mögen sich steigenden morphologischen Eigenschaften anpassen, um Schäden auf lange Sicht zu widerstehen.

Schlüsselwörter: Schalenbruch; Indische Küste; Größen-Zuflucht; Beuteeffektivität

Translator: Eva Gebauer

Arabic

439 arab

Translator: Ashraf M.T. Elewa

 


 

FIGURE 1. Map of the studied area with beach-bar-tidal flat-back swamp. The actual photograph is taken from the Google Earth (2013).

figure1

FIGURE 2. Several types of damages on the bivalve shells have been used as an indicator of breaks. 1-3,valves are gaped even when they are closed, 1 and 3, scars are mirror imaged on two valves; 4,internal view of the bivalve shows sign of repair due to severe break (visible at the external side); 5,a severe break (Grade I) has been repaired by carbonate secretion; 6-7, retention of broken fragment near a large break (arrows).

figure2

FIGURE 3. Different types of repair scars on Mactra violacea are presented here. See that the valves (7 and 10) have distinct undulation because of a severe break (Grade I).

figure3

FIGURE 4. Schematic diagram of the studied species, Mactra violacea, shows scar distribution (the pie chart at right) on the valves (i.e., the grids at left). Note that the venter (location 2 and 3 together) has maximum number of scars.

figure4 

FIGURE 5. The graph shows absolute frequency distribution of ‘length at death’ and ‘length at the incidence of scarring’ on bivalve shells. Note marked differences in absolute frequency distributions of these two lengths. Presence of non-random distribution of ‘length at death’ indicates that the samples are not winnowed from the actual habitat of the living bivalves.

 figure5

FIGURE 6. Ontogenetic changes in M. violacea. The figures indicate changes in depth of living, shell thickness at venter and ventral angle with ontogeny, supporting lower incidence of shell damages.

figure6

FIGURE 7. The model shows (left column) how species’ adaptation allows it to reduce frequencies of severe damages with time (i.e., from Phase I to Phase III). In addition to it, the model also shows how death due to scarring is minimized by moving away from the frequency distribution of length at scarring (see text). The right column presents many examples of these distributions, redrawn from Alexander and Dietl (2001).

figure7

 

TABLE 1. Types of shell damages, separated by different Grades, introduced in the paper.

Grade Characters of scars Synonyms
Soft tissue damage

Stretch
(with respect to length)

Depth
(with respect to width)
Predatory Non-Predatory
I visible >50% >30% Embayed, Cleft Creases, Arcuate-angular
II --- 5-50 5-30% Scalloped, Devoted Horseshoe
III --- <5% <5% Scalloped Arcuate-angular

 

TABLE 2. Distribution of damages and their severities are presented here. Note that most the damages are of Grade III and are concentrated on the ventral part of the shell. Stretch = the dimension of the scar (see text).

  Location on shell
Grade 1 2 3 4
    Type A (stretch and depth) Type B (soft tissue damage) Type A (stretch and depth) Type B (soft tissue damage) Type A (stretch and depth) Type B (soft tissue damage)
I 2 12 3 20 5 14 3
II 7 18 32 18
III 25 47 106 45
 

author1Subhronil Mondal
School of Geosciences
University of South Florida
4202 E. Fowler Ave., NES107
Tampa, Florida 33620-5250
USA
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Subhronil (Nil) Mondal is currently pursuing his Ph.D. in the University of South Florida, Tampa, Florida, USA. His works involve long- and short-term macroecological changes and their macroevolutionary impacts in the marine molluscan history of life.

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author2Subhendu Bardhan
Department of Geological Sciences
Jadavpur University
Kolkata-700032
India
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Subhendu Bardhan, received his Ph.D. from Jadavpur University and has been on the faculty there since 1981 where he is now a Professor in the Department of Geological Sciences. He works on various groups of mega-invertebrate fossils and has strong interest in unraveling palaeobiological intrigues, evolution and extinction. His current research interest involves coastal ecology of molluscs and the prey-predator interaction (drilling and peeling) on molluscs in space and time.

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author3Sumanta Mallick
Department of Geological Sciences
Jadavpur University
Kolkata-700032
India
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Sumanta Mallick is pursuing his Ph.D. on predator-prey interactions in Indian marine fossil record (across the K-T boundary) in Rajahmundry, Andhra Pradesh from Jadavpur University. His research interests include study on prey-predator interaction between gastropods and other faunas, and bibliographic surveys and database development on gastropod predation in space and time.

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author4Arindam Roy
Department of Geological Sciences
Jadavpur University
Kolkata-700032
India
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Arindam Roy is a Ph.D. student at Jadavpur University and doing his dissertation on the Oxfordian (Upper Jurassic) ammonite faunas of Kutch, Gujarat, India: a study of systematics, palaeobiogeography and evolution. His research interests include ammonites systematic and palaeobiogeography, their evolutionary mechanisms, and prey-predator interaction between gastropods and other faunas.

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