Competition and coexistence in tropical Australian shipworms

Shipworms are a fascinating group of marine bivalves (Family Teredinidae) with a highly-specialized wood-feeding niche, and are an engaging topic of research from ecological and economic perspectives. However, the underlying biology of many shipworms remains poorly understood, hindering our understanding of their broaderscale demographics and community ecology. Taking advantage of the abundant shipworm fauna in tropical Australia, the aim of this study was to provide an in-depth investigation of shipworm demographics, diversity and life histories, focusing on recruitment, growth and reproduction. Timber recruitment panels were used to quantify 2, 4, 6 and 12 month abundances of tropical Australian shipworm species, recording 62,075 individuals from 6 genera and 19 species (Chapter 2). Species exhibited differing peak recruitment seasons and reproductive modes, with free-spawning, shortterm brooding and long-term brooding occurring. However, despite a higher diversity of spawning species overall (58%), abundance was strongly driven by the lower diversity of short-term brooding species (37%), comprising 95.7% of recruitment. Competition for habitat was high, with up to 1200 individuals per panel (~3 individuals per cm³), and widespread mortality (40 to 90%) in panels over 4 months old. These results demonstrate that reproductive mode is a key contributor to recruitment success, with the constraints of a patchy, ephemeral habitat favoring a 'middle of the road' strategy of short duration larval brooding, most effectively balancing fecundity, larval retention and dispersive ability. Life history strategies play a significant role in the competition and coexistence of species, but the life histories of many shipworm species are poorly understood. The life histories of two shipworm species differing in reproductive mode, the larval-brooding Teredo parksi and the free-spawning Bankia carinata (Chapter 3) were examined and recorded for the first time, comparing size, reproductive development and fecundity. B. carinata (26.15 ± 1.00mm average length) were significantly larger than T. parksi (16.09 ± 0.25mm). Both species reached sexual maturity within 2 months, at body lengths of 2-4 mm. Fecundities were similar for both species in individuals under 40 mm in length. However, after this time spawners were more fecund by over a factor of ten, reaching a clutch size of 3x10⁶ eggs by 100 mm in length providing the major contrast between the two reproductive modes. The rapid growth, precocious development and high fecundity of these two competing species highlight the evolutionary adaptations that allow them to be successful in a specialized ecological niche. The reproductive strategies of brooding and spawning shipworms both constitute successful adaptation to their shared niche. Incorporating data on maternal provisioning and reproductive effort permits a more complete model of shipworms' life history trade-offs. Therefore, maternal provisioning and reproductive effort were compared for Teredo parksi and Bankia carinata (Chapter 4). The number and size of larvae and eggs were quantified and coupled with ultimate analysis of body tissue, larvae and eggs, to generate a model of reproductive energy allocation. Results showed that T. parksi possessed significantly higher maternal provisioning and reproductive effort than B. carinata. Larvae of T. parksi contained more energy than the eggs of B. carinata, with 3.13x10⁻⁴ J of energy per larva, and comprise over 30% of both total body mass and energy. This demonstrates that brooding and spawning strategies in shipworms are not simply a number-size trade-off, and that higher overall reproductive investment by brooding species may be responsible for their dominant position in the composition of shipworm communities. In Chapter 5, a species of shipworm, Lyrodus turnerae, is described from specimens in tropical Australia and Papua New Guinea. The new species is characterized by the morphology of the pallets, with a dark periostracal cap sharply descending into a thin calcium cup. New and existing collection records of L. turnerae are presented, indicating a distribution range from coastal Papua New Guinea and the Bismarck Archipelago, extending along eastern Australia to New South Wales. An updated key to the genus Lyrodus is also presented, incorporating new taxonomic information. In conclusion, this study provides a novel, in-depth investigation of the demographics, diversity and life histories of tropical Australian shipworms. By quantifying the life histories of competing shipworm species and recording larger scale patterns of recruitment and abundance, this work has established how the biology of shipworms on the individual and species level shape broader processes of community composition and regional diversity.