Multiple predatory drill holes in Cardiolucina (Bivalvia: Lucinidae): Effect of conchiolin sheets in predation

The origin of multiple drill holes in the lucinid bivalve Cardiolucina okinawaensis ( Noda, 1988) from the upper Pliocene Shinzato Formation is discussed. These holes are thought to be produced by predatory naticid gastropods. The valve of C. okinawaensis often bears more than one drill hole, none or only one of which is completely penetrated. The holes are countersunk stepwise and often have a flat bottom when they are incompletely penetrated. Analyses of shell microstructure and growth pattern in the Pliocene C. okinawaensis and modern Cardiolucina quadrata reveal unique shell episodic growth that results in the secretion of discretely distributed, blocky prismatic myostraca and thin conchiolin sheets within the shell. The conchiolin sheets seem to have been produced in each growth unit and spread broadly along the interior shell surface to a varying extent, so that the shells have many conchiolin sheets in fully-grown individuals. Like corbulid bivalves, the conchiolin sheets prevent drilling predation effectively and therefore result in multiple incomplete drill holes. Observations of nine fossil and eight modern Cardiolucina species reveal that all the species examined have almost the same episodic growth pattern with C. okinawaensis and have varying numbers of conchiolin sheets. The conchiolin sheets have an effective function to retard the naticid drilling predation among the fossil and modern Cardiolucina species. However, the absence of any spatial and temporal trend in the number of conchiolin sheets, and the higher number of conchiolin sheets in larger species seem to suggest that the presence of conchiolin sheets in the Cardiolucina species is an example of exaptation rather than of adaptation to the naticid drilling predation.