Adaptation to rock boring in botula and lithophaga (lamellibranchia, mytilidae) with a discussion on the evolution of this habit

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Abstract
All species of Botula and of Lithophaga live within borings they excavate in rock, In Botula boring is mechanical, into soft non-calcareous rocks. Firm attachment is made by byssal threads arranged in a large anterior and a small posterior group. The rock is abraded by the dorsal surfaces of the valves, which are deeply eroded; the forces responsible are contraction of the byssal and pedal retractors and the powerful opening thrust of the long ligament. There is no rotation within the boring. In Lithophaga boring is confined to calcareous rocks, byssal attachment being weak and the valves little eroded. The fused inner lobes of the mantle edges may protrude from between the valves anteriorly. They are applied to the head of the boring and it is postulated that they secrete an acid mucus. Some species of Lithophaga bore straight into rock, others rotate in the boring. Aggregations of foreign particles on the valves of Botula and Lithophaga are due to the effect of accumulations of mucous glands both anterior and posterior to the ligament. The origin and function of these glandular masses is discussed by reference to conditions in the allied Modiolus. They help in disposing of sediment. In Lithophaga the anterior glands, at least, probably secrete an acid mucus. Crystalline calcareous secretions which adventitiously lengthen the shells of certain species of Lithophaga are produced by the walls of the siphons, which may also lay down a calcareous lining to the boring. Evolution of the boring habit in the Mytilidae is discussed in relation to shell-form. The elongate shell of the dimyarian types Botula and Lithophaga is compared with those of Ensis or Tagelus and shown to be derived from that of a heteromyarian ancestor not unlike Modiolus. The effect of the tangential component in the growth of the mantle/shell counteracts that of reduction of the anterior end of the body and of the mantle/shell associated with heteromyarianism and is also concerned with lengthening of the ligament. Change in form in the Mytilidae made possible change in habit.