The tropical green algal genus Halimeda is one of the best studied examples of pseudo-cryptic diversity within the algae. Previous molecular and morphometric studies revealed that within Halimeda section Rhipsalis, Halimeda incrassata included three pseudo-cryptic entities and that the morphological boundaries between H. incrassata and Halimeda melanesica were ill-defined. In this paper, the taxonomy of H. incrassata is revised: two pseudo-cryptic entities are described as new species, Halimeda kanaloana and Halimed heteromorpha, while H. incrassata is redefined to encompass a single, monophyletic entity. Similarities and differences between the three species and H. melanesica are discussed. Monophyly of H. heteromorpha, which was questioned in a former study, is reinvestigated using sets of 32 ITS1–ITS2 and 21 plastid rps3 sequences and various alignment and inference methods. The phylogenetic relationships within Halimeda section Rhipsalis are inferred from nuclear 18S–ITS1–5.8S–ITS2 and concatenated plastid sequences (tufA & rpl5–rps8–infA) and interpreted in a biogeographic context.

A revised checklist of 425 taxa of benthic marine algae is provided for the French Polynesian region, including the Society Islands, Tuamotu Archipelago, Marquesas Islands and Austral Islands. The list comprises 117 Cyanophyceae, 42 Phaeophyceae, 96 Chlorophyceae and 170 Rhodophyceae, representing 79 new additions to the last checklist a decade earlier. Included are all previously published records and some yet unpublished additions, with the systematic arrangement and nomenclature updated. The flora of Tahiti and its adjacent islands is quite well known, whereas the one from Marquesas, Austral and Gambier Islands have yet to be adequately examined. Also, such algal groups as the crustose Corallinaceae are still poorly studied.

Monitoring potentially devastating coral-eating crown-of-thorns starfish (COTS) populations at scales relevant to management is a challenging task. Here, we investigated a citizen science approach to detect COTS outbreaks and prioritize management responses. Between 2014 and 2018, 38 000 COTS were recorded through 641 online observation reports submitted across New Caledonia, Vanuatu and Fiji by private stakeholders (51%), NGOs (22%), business operator (11%), research/government agencies (16%). COTS were observed in multiple areas, including in remote/inhabited reefs where they had never been reported. A three-level classification was developed to discriminate risk areas and propose operational guidelines to streamline management actions. About two-thirds of reports had low abundances (<10 starfish sighted) and could be addressed with low priority. Verification surveys at 65 reef sites confirmed outbreaks in half of the cases, along with high peak densities (7 000 ind.ha⁻¹). Combining professional and non-professional observers increased the detection range (+27%) and the number of COTS detections (+129%). Citizen reports were eventually followed by removal campaigns organized within diverse institutional frameworks. While citizen monitoring has intrinsic limitations, we advocate that it constitutes a complementary and promising approach to support the ongoing management efforts in all countries affected by COTS.

Distant-source swells are known to regularly inundate low-lying Pacific Island communities. Here we examine extreme total water level (TWL) and inundation driven by a distant-source swell on Fiji’s Coral Coast using observations and a phase-resolving wave model (XBeach). The objective of this study is to increase understanding of swell-driven hazards in fringing reef environments to identify the contribution of wave setup and infragravity waves to extreme TWL and to investigate coastal flooding during present and future sea levels. The maximum TWL near the shore was caused by compounding mechanisms, where tides, wave setup, infragravity waves, and waves in the sea swell frequencies contributed to the TWL. Waves and wave setup on the reef were modulated by offshore wave heights and tides with increased setup during low tide and increased wave heights during high tide. Numerical simulations were able to reproduce the mechanisms contributing to the extreme TWL and allowed an estimation of the inundation extent. Simulations of the same swell under the RCP8.5 sea-level rise scenario suggest the area of inundation would increase by 97% by 2100. A comparison between the numerical model, a multiple linear regression model, and two commonly used parametric models reveals that both XBeach and the linear regression model are better suited to reproduce the nearshore wave setup and TWL than the empirical equations. The results highlight the need for customized, site-specific coastal hazard assessments and inundation forecast systems in the South Pacific.