Giant clams (genus Tridacna) are iconic coral reef animals of the Indian and Pacific Oceans, easily recognizable by their massive shells and vibrantly colored mantle tissue. Most Tridacna species are listed by CITES and the IUCN Redlist, as their populations have been extensively harvested and depleted in many regions. Here, we survey Tridacna crocea and Tridacna maxima from the eastern Indian and western Pacific Oceans for mitochondrial (COI and 16S) and nuclear (ITS) sequence variation and consolidate these data with previous published results using phylogenetic analyses. We find deep intraspecific differentiation within both T. crocea and T. maxima. In T. crocea we describe a previously undocumented phylogeographic division to the east of Cenderawasih Bay (northwest New Guinea), whereas for T. maxima the previously described, distinctive lineage of Cenderawasih Bay can be seen to also typify western Pacific populations. Furthermore, we find an undescribed, monophyletic group that is evolutionarily distinct from named Tridacna species at both mitochondrial and nuclear loci. This cryptic taxon is geographically widespread with a range extent that minimally includes much of the central Indo-Pacific region. Our results reinforce the emerging paradigm that cryptic species are common among marine invertebrates, even for conspicuous and culturally significant taxa. Additionally, our results add to identified locations of genetic differentiation across the central Indo-Pacific and highlight how phylogeographic patterns may differ even between closely related and co-distributed species.

Aim The aim of this study was to test historical and contemporary influences on population structure in the giant clams, Tridacna maxima (Röding, 1798) and T. crocea (Lamarck, 1819). To refine the location of clade boundaries within a newly resurrected species, Tridacna noae (Röding, 1798). Location Indo‐Australian archipelago, including Indonesia, the Philippines, Australia, Papua New Guinea, the Solomon Islands, Republic of Kiribati, the Line Islands and Taiwan. Methods We used isolation‐migration (IMa) coalescent models and distance‐based redundancy analyses (dbRDA) to test the relative influence of barriers and continuous distances on historical divergence, gene flow and population structure of T. maxima and T. crocea. Continuous metrics of distance included present‐day and Last Glacial Maximum overwater distances along with probability of larval dispersal (LD) among sampling sites. We combined new mitochondrial cytochrome oxidase subunit I (mtDNA COI) sequences with existing data to compile the largest data set of these species yet analysed. Results The Pleistocene land barriers of the Sunda Shelf and Torres Strait were associated with old (>0.5 Myr) divergence times. The western and eastern boundaries of the Halmahera Eddy were also locations of significant, but more recent, divergence. No gene flow was detected across any of the four barriers tested. Larval dispersal distances between sampling sites were significant predictors of T. crocea population structure, accounting for differentiation above and beyond the contribution of barriers. We further delineated the species range of T. noae and showed that its two known clades are sympatric in central Indonesia. Main conclusions The strong signature of historical barriers on genetic differentiation argues against the assumption that Indo‐Pacific Tridacna are open meta‐populations. Despite similar life histories, T. maxima and T. crocea differ in their mtDNA population structure. The widespread species (T. maxima) exhibits population structure linked solely with historical factors, whereas T. crocea's population structure reflects both historical factors and LD distances.

The annual commercial catch from the Southern Zone of the South Australian rock lobster (Jasus edwardsii) fishery is ~1900 tonnes, representing ~50% of total landings from south-east Australia. A single minimum legal size (MLS) of 98.5 mm carapace length (CL) exists across the entire zone. Fecundity (F), size at onset of maturity (SOM) and relative reproductive potential (RRP) of female rock lobsters were investigated in two major fishing regions, i.e. the North Southern Zone (NSZ) and South Southern Zone (SSZ) with a view to providing a basis for future fine-scale spatial management of the resource. F ranged from 45,292 to 466,800 eggs per female and increased proportionally with CL according to the relationship: F = 0.0584 × CL3.1642. F was significantly higher in the NSZ compared to the SSZ but was attributed to differences in lobster size between regions. There was no significant difference in the number of eggs · g−1 of egg mass between areas. SOM, estimated as the size at which 50% of females reached sexual maturity (L50) was higher in the NSZ (104.1 mm CL) compared to SSZ (92.3 mm CL). Approximately 20% of lobsters above the MLS in the commercial catch in the NSZ were under the L50 estimate. RRP, as a measure of egg production, was calculated for each size-class from the product of F, SOM and population length–frequency. The modal RRP size-classes in the NSZ were 117.5–122.5 mm CL, while in the SSZ it was 97.5–102.5 mm CL. Only 6% of RRP was contributed by female rock lobsters below the MLS in the NSZ, compared to 34% in the SSZ. Regional differences in SOM and RRP in the Southern Zone of South Australia suggest that different MLSs may be beneficial, particularly if the fishery is to be effectively managed at finer spatial scales.

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