The embryonic, larval and early postlarval development of the terebratellid brachiopod Calloria inconspicua (Sowerby)

The present study of the terebratellid brachiopod Calloria inconspicua (Sowerby) was prompted by the discrepancy in locating the brood‐space in previous studies (Percival 1944; Doherty 1979); the unique position of the pigment spots in the larva; and the question of whether an initial shell preceded the definitive shell, as in Terebratalia transversa (Sowerby) (Strieker & Reed 1984, 1985b). Forty‐nine mature specimens out of a total of 378 C. inconspicua collected from Otago Harbour, South Island, on 16 February—30 March 1990, and 35 specimens out of about 300 collected on 14–15 April 1990 from Goat Island off Leigh, North Island, spawned in the laboratory. Mature males shed spermatozoa or fragments of their testes. Each female spawned several times within the observation period of 12 days, releasing either a single developmental stage or a mixture of several developmental stages, including occasional fragments of the ovaries. The ova, 54–192 ìm in diameter, varied widely in size even in an individual spawn. The spawn of different individuals also differed in colour from white or colourless through light orange or light mauve to dark orange or dark mauve. The natal colour was maintained throughout the embryonic and larval stages. Ova were fertilized either outside or inside the mantle cavity. Ova released into the mantle cavity, if not spawned immediately, were brooded for varying periods in the space between the brachial membrane and its arching cirri in the distal part of the median spiral arm of the plectolophe. The first cleavage of the zygotes was usually slightly unequal. The results of this study confirmed Percival's earlier report that the follicle cells are lost after fertilization, the fertilization membrane persists during the embryonic stage, occasional irregular meroblastic cleavages result in the death of the embryos, the coeloblastula has one layer of blastomeres, gastrulation is by invagination, the blastopore changes in shape. There are pigment spots in the anterior lobe, the wedge larval stage precedes the cylindroid larval stage, and the formation of the stomodaeum and of the first four pairs of lophophoral cirri in the anterior lobe proceed as described by Percival. Small ova developed either into viable larvae, or into inviable incomplete larvae, of miniature size. Measurements of the larval stages were made (Table 2). A constriction on the pedicle lobe of the pre‐settlement larva separated a distal pedicle‐forming region, at the apex of which a cup‐like depression formed. The constriction, pedicle depression and the behavioural change from photophilous to photophobic were presumed to indicate larval competence. During tentative settlement a small area of the pedicle apex adhered to the substratum by means of an adhesive substance. Next, rod‐like attachment processes extended from inside the pedicle to the substratum to anchor the adhered larva. Subsequently, this larva attached itself permanently by secreting a cuticle, after the pedicle depression was smoothed, then reversed its mantle. A day or two later, and before the stomodaeum opened into the gut rudiment, the postlarva secreted a pair of non‐hinged initial shell valves, 30–90 ìm wide by 15–50 ìm long, consisting of granular, mineralized substance with no growth‐disturbance lines. Initial shell valves became closely apposed only laterally. After the stomodaeum opened into the gut rudiment, new mineralized nepionic shell material with caeca, imbricated fibres and growth‐disturbance lines, formed on the inner surface of the initial shell to thicken it and to extend peripherally beyond it. Holoperipheral shell accretion was approximately equal between the two nepionic shell valves at first, but the ventral shell valve soon outgrew the dorsal shell valve posteriorly by producing a spatulate process to partially cover the pedicle ventrally. The development, larval settlement, and metamorphosis, and the formation of the initial and nepionic shells of articulate brachiopods, are discussed.

Keywords:

• Brachiopoda
• terebratellid
• Calloria inconspicua (Sowerby)
• spawning
• brooding
• embryology
• larval development
• larval behaviour
• settlement
• metamorphosis
• initial shell
• nepionic shell

Notes

Portobello Marine Laboratory, Portobello, New Zealand, and Leigh Marine Laboratory, Leigh, New Zealand. Present address: 144 Pasir Ris Road, Singapore 519131.