Structure and swimming behavior of the larva of Haliclona tubifera (Porifera: Demospongiae).

Abstract

Early developmental stages of Haliclona tubifera are retained by the parent and larvae are released on illumination of dark-adapted colonies. Larvae are parenchymellae without provision for feeding on particulate matter and without identifiable sensory and neuronal elements. Larvae, however, are strongly photonegative on release and remain so throughout the duration of larval existence. The anterior and lateral epidermal fields are composed predominantly of elongate flagellate cells which form a pseudostratified columnar epithelium. Interspersed among elongate cells are globular flagellate cells which, based on ultrastructural criteria, are thought to be secretory in function. The posterior surface is not flagellated and is composed of a cuboidal epithelium. A 5-7 cells wide ring of epidermal cells with flagella 50 μm in length forms a prominent flagellar band separating the posterior from lateral fields. Measurements of swimming speeds in the horizontal plane indicate that H. tubifera larvae swim at speeds intermediate between Halichondria melanadocia, which have a posterior tuft of 25 μm flagella and Haliclona sp, with a posterior ring 75 μm flagella. Morphometric analysis of larval length, larval diameter, and flagellar band diameter shows significant correlations in all pair-wise tests. Band diameter varies less than larval length or width suggesting that it is a conserved trait. A negative allometry exists between band diameter and larval length. As larval length increases elongate flagellar cells of the lateral and anterior surfaces become relatively more important for locomotion. A prominent cup-like sheath of subepidermal cells separates the epidermis from the central region of the larval interior. These cells are not associated with collagen as are cells similar in appearance and location in several other haplosclerid larvae. Their function remains unknown. Spicules occur in the posterior region. As in the adult, these spicules are hastate oxeas, but they are significantly different in length, width, and length/width from those of the adult. Their function is unknown, but several obvious potential functions in both larval and postlarval life exist and await further study. Archeocytes are the dominant cell type of the central region. Bacteria are found inter- and intracellularly. Based on morphological data, they appear to be of a single type, pleomorphic rods without thick walls, but further study utilizing isolates in culture is needed to establish their diversity and identity. Intracellular bacteria are associated with a cell type provisionally designated a bacteriocyte. Three stages of bacteriocytes are described based on anatomical criteria, presence/absence of bacteria, and what appear to be stages in degradation of bacteria. The significance of this association and of the bacteriocyte cell type remain to be explored. © 1993 Wiley-Liss, Inc.