Photoreceptors of serpulid polychaetes

This study describes ultrastructural investigations of photoreceptor organs in the Serpulidae, a widespread family of polychaete worms and an important component of most benthic marine communities.\ud \ud Previous studies of invertebrate photoreceptor structure and evolution have been based largely upon the examination of only a few species in each of the major taxa. The present research investigated the diversity of photoreceptor structures on a limited taxonomic level and thus determined the phylogenetic usefulness of ultrastructural variation. Serpulid photoreceptors have been almost entirely overlooked even though these worms possess a variety of such organs. Their structure provides useful information for suggesting directions of photoreceptor evolution in the polychaetes, and more particularly in the Order Sabellida. \ud \ud There are two developmentally and ultrastructurally distinct photoreceptor types in this family: simple ocelli situated within the brain which are present in all genera, and branchial photoreceptors which are variably developed and located on the elements of the branchial crown. The latter have sac-like lamellae formed as outgrowths of sensory cilia projecting into an extracellular invagination as their light receptive surface. In a number of Spirobranchus species, the sensory cells also feature a well-developed rhabdom situated above the ciliary lamellae. Branchial photoreceptors are usually surrounded by screening pigmented cells and are sometimes organized into elaborate compound organs. Cerebral ocelli are composed of two cells, a pigmented cell forming a cup, into the cavity of which an overlaying sensory cell projects an array of microvilli. The discovery of a novel mitochondria-rootlet association in the cerebral ocelli is discussed in terms of possible functional involvement in rhabdom formation and maintenance, stimulus transduction and receptor conformation.\ud \ud The life history of Spirobranchus gigansteus sp. A has been followed in detail in order to understand the changing photoreceptoral requirements of serpulid worms. Larvae were reared to settlement in the laboratory in 11-12 days at 28-30°C. Larvae swim and feed actively, and are positively phototactic during the planktonic stage. Trocophore development includes precocious development of a right ocellus and enlargement of a spacious blastocoele up to the two eyed metrochophore. Nectochaete formation commences from 6 days onwards, and by 10 days larvae have begun metamorphosis, a gradual process not completed until settlement. Larval eyes are retained through metamorphosis and into the juvenile. Prior to settlement, the larval worms pass through a distinct searching phase. Organization and behaviour of the larvae are detailed with particular reference to the musculature and photoreceptors. The enigma of how this conspicuous tropical worm settles onto living corals has now been resolved. Larvae would only settle and construct tubes if provided with fragments of living coral. In these cases settlement was rapid and always on the non-living edge. Juvenile worms are incorporated within the expanding coral skeleton. Field studies have confirmed these laboratory observations