The Reproductive Biology of the Recent Articulate Brachiopod Terebratulina retusa (Linnaeus)

James, Mark Andrew (1989) The Reproductive Biology of the Recent Articulate Brachiopod Terebratulina retusa (Linnaeus). PhD thesis, University of Glasgow.

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The reproductive biology of the Recent articulate brachiopod Terebratulina retusa (Linnaeus) has been examined in detail. Two populations of T. retusa were sampled. The first from a deep water brachiopod-Modiolus assemblage in the Firth of Lorn and the second from a shallower site at Kenmore Point, Loch Fyne. Limited assessements were made of local environmental conditions, including temperature and current profiles. Aspects of the gross reproductive anatomy and important ultrastructural features of the gonads are documented in addition to detailed ultrastructural descriptions of gametogenesis. Using the information gained from the morphological studies, the reproductive cycles of two geographically separate populations of T. retusa have been analysed using a quantitative stereological technique. T. retusa are dioecious. Maturity in both sexes is achieved when the shell length exceeds approximately 5.5 mm. There is no obvious external sexual dimorphism except for slight differences in the coloration of the gonads; testes are white/cream, ovaries are yellow/orange. The gonads occur as four palmate lobes, a pair in each valve. Gonads are formed within a mantle sinus, the vascula genitalia, which is an anterior extension of the coelom, that opens posteriorly into the visceral cavity and to the exterior via a pair of metanephridia. The latter serve as gonoducts during spawning. Gametes are borne on genital lamellae formed from a reticulate lattice of connective tissue. The lamellae are an extension of the ileoparietal band and are fused along one margin to the inner mantle epithelium. Developing oocytes are closely affixed to the genital lamellae and originate from a pool of proliferating germ cells at its base. Vitellogenic oocytes that are at an advanced stage are released from the genital lamellae, but are retained within the vascula genitalia. Liberated oocytes continue to accumulate yolk and eventually occlude the vascula genitalia, before being spawned. Histochemistry revealed the outer mantle epithelium to contain seasonally variable quantities of protein, glycogen and glycoprotein. Functionally, the outer epithelium is presumed to act as a storage tissue and the cycling of reserves it contains is thought to be intimately associated with reproduction. Two forms of phagocytic cell were identified within the gonads of spent animals. These appear to be actively involved in the resorption of atretic material. Oogenesis has been divided into six stages, defined according to ultrastructural changes. Each stage is thought to be a key factor in the development of the gamete, particularly the process of vitellogenesis. Special attention has been given to the possible mechanisms involved in the aquisition and assimilation of nutrients within the oocyte and the possible physiological consequences. Each vitellogenic oocyte is contained within a follicular envelope. As maturation proceeds, accessory cells proliferate within the follicular envelope. A variety of intra-accessory cell and oocyte/accessory cell communications were identified. The process of elaboration of the oolemma was observed. Lipid was identified as the major nutrient reserve of the oocyte. Late mature oocytes measure approximately 130 um in diameter when spawned. Spermatogenesis was also examined ultrastructurally. As with oogenesis, gametes are derived from a pool of proliferating germ cells which are distinct from other gonial tissues. Primary spermatocytes form clusters at the base of the genital lamellae, giving rise to a short-lived secondary spermatocyte stage which divides again to produce spermatids. The spermatids undergo structural modifications, eventually forming mature sperm. Successive stages of development are displaced from the genital lamella by division of cells beneath. Sperm are of the 'primitive type'. The head consists of an axially displaced acrosome, spherical nucleus and large doughnut-shaped mitochondrion. Stereological analysis revealed a single synchronised spawning event between late November and the end of January in the Firth of Lorn. The Loch Fyne population spawned repeatedly throughout the spring and summer, with the greatest spawning activity occurring in the late autumn. T. retusa from Loch Fyne were more fecund than those from the Firth of Lorn. The initiation of gametogenesis appears to be mediated by the mobilisation of reserves stored in the outer mantle epithelium during the winter. Primary productivity increases dramatically in the spring and both vitellogenesis and spermatogenesis show a corresponding increase. T. retusa from Loch Fyne successfully overwinter gametes and as a consequence are capable of spawning during the spring. The reproductive strategy and the mechanisms of physiological and environmental constraint which may have dictated the adoption of such a strategy are examined. Finally, the limitations and possible evolutionary consequences of brachiopod reproduction are addressed with reference to their palaeoecology.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Zoology
Date of Award: 1989
Depositing User: Enlighten Team
Unique ID: glathesis:1989-77828
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jan 2020 11:53
Last Modified: 14 Jan 2020 11:53

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