Contrasting life histories in shipworms: growth, reproductivedevelopment and fecundity

Trade-offs are implicit in life history strategies, and contribute to the coexistence of competing species. Shipworms, a family of obligate wood-feeding marine bivalves (Teredinidae), form communities where larval-brooding species are ten-fold more abundant than free-spawning species. Shipworm metacommunities are shaped solely by interactions between and amongst shipworm species, making this group ideal for examining the follow-on effects of life history on recruitment success and community structure. Using timber recruitment panels, tropical Australian shipworms were collected over a 12 month period, and the growth, reproductive development and fecundities of brooding and spawning species were quantified. Life histories of both brooding and spawning species reflected the ephemeral nature of wood habitats, with rapid growth, precocious development and high reproductive output. Spawning species (23.13 ± 0.63 mm average length) were significantly larger than brooding species (11.94 ± 0.09 mm). 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, after which spawners were more fecund by over a factor of ten, reaching a clutch size of 3 × 10⁶ eggs by 100 mm in length. Results show that growth, reproductive development and fecundity are not sufficient to explain patterns of recruitment success for tropical shipworms, and rather that the brooding of larvae conveys the most substantive advantage in colonizing new habitat and defining population structure.