Recruitment, settlement and ontogeny of the serpulid 'Spirobranchus cariniferus' (Gray, 1843)
Serpulids are a globally represented group of polychaetes and can be found in many habitats from the intertidal fringe to the subtidal environment and even in deep-sea ecosystems. These tube-dwelling worms are often described as pioneer species in new or disturbed habitats. Serpulids secrete a calcareous tube and often occur in aggregations. These patches can range from several centimetres to several metres in diameter and may even form reef systems. Accumulations of tube-dwelling worms provide a new habitat for other species and, therefore, serpulids are considered bioengineers. Serpulid aggregations are known to enhance biodiversity and species abundance and may increase water quality through their filter activity. Despite their ecological importance, their ecology and ontogeny have received little attention. Spirobranchus cariniferus, a New Zealand endemic intertidal serpulid, is a substantial contributor to intertidal ecosystems. For this and other Serpulidae, the link between larval development and larval settlement is missing. However, this connection is essential to understand recruitment and ecology of tube-dwelling worms. Therefore, in this thesis, I describe the ontogeny of S. cariniferus from larval development to recruitment and reproduction. In the first data chapter, I present my findings on the recruitment of S. cariniferus in the field. This serpulid settles aggregatively in the field but not necessarily in response to the presence of adult conspecifics, as has been previously reported. Abiotic factors such as sunlight or wave disturbance have a more substantial effect on recruitment rather than the occurrence of adult individuals of the same or a competing species. Additionally, this chapter provides support for the hypothesis that larvae of S. cariniferus may accumulate near the substrate before settlement. Many sessile marine invertebrate taxa occur in either aggregations or as solitary individuals, with potential benefits and disadvantages associated with each configuration. For S. cariniferus, solitary and aggregative individuals can be found in the same habitat. Therefore, the second data chapter compares growth and mortality for individuals living alone or in aggregation. While solitary and aggregative individuals elongate their tubes at a similar rate, further correlations of body to tube sizes lead to the conclusion that solitary worms focus more of their energy on tube length growth rather than body size increment compared to aggregative conspecifics. Mortality is highly variable but does not differ between both configurations. However, individuals living in a patch have a better ability to recover from damage to their tubes. In the last two decades, the idea that gonochorism is the general reproductive pattern for Serpulidae has been challenged, and instead it has been suggested by some that protandry is the more common trait. Therefore, with my third data chapter, I explore maturation and sex ratio of S. cariniferus and whether it changes for individuals living alone vs. in aggregation or based on size. While maturation depends on size, sex does not, and neither maturation nor sex ratio are dependent on whether individuals live in aggregation or not. Further, the ratio of females to males did not favour either sex consistently. For the first time in this species I found evidence of possible hermaphroditism. Through spawning trials and histological sections, I identified nine individuals which simultaneously contained oocytes and sperm cells. I suggest therefore, that S. cariniferus has alternating sexes rather than protandry as a reproductive strategy. In the fourth and final data chapter, I describe the metamorphosis and settlement behaviour of S. cariniferus larvae. For this serpulid species, settlement and metamorphosis are separate and distinct steps that involve both behavioural and morphological changes to the larvae. Further, this entire process can be quite prolonged (i.e. over several days), and at some points can be reversed. It is therefore very important that observations last longer than 24–48 hours, when studying serpulid settlement. As far as I am aware, this is the first study on a serpulid species to examine aggregative settlement in the field in relation to the presence of adult conspecifics and abiotic factors, and also to explicitly test for consequences of solitary vs. group living on growth and mortality. It is also the first to show evidence of hermaphroditism in this species. I hope my research and this thesis stimulates a more inclusive and holistic investigation of serpulids in the future. Larval development, settlement patterns and ontogeny need to be studied in detail if we want to understand the evolution, ecology, impacts and benefits of these and other sessile marine invertebrates.