Two specimens of the crustose red algal genus Ramicrusta were collected off Lehua Island, Hawai‘i, at 11 m and 49 m depth in September 2018, and are here described as new species on the basis of both molecular and morphological comparisons. Ramicrusta hawaiiensis sp. nov. is distinguished from the 12 other known species in the genus by the combination of its gross habit and coloration, pronounced perithallial zonation, frequent and robust rhizoids, presence of hair cells, and by distinctive COI and rbcL marker sequences, and represents a mesophotic record of the genus Ramicrusta. Similarly, Ramicrusta lehuensis sp. nov. is distinctive in the combination of its gross habit and coloration, less-pronounced perithallial zonation, presence of frequent and robust rhizoids as well as hair cells, and is also distinct in COI and rbcL marker sequences. These records constitute the first for the genus Ramicrusta in Hawai‘i, and highlight the cryptic diversity of peyssonnelioid red algae in the Hawaiian Islands. This study also adds to the growing list of novel algal species being characterized from mesophotic depths in Hawai‘i—a habitat which is being shown to contain numerous undescribed taxa and records of species and genera previously unknown to the Islands. This initial record of Ramicrusta in Hawaiian waters is noteworthy given the recent reports of members of the genus overgrowing and killing corals in the Caribbean and South China Sea.

The marine green alga Caulerpa bikinensis W. R. Taylor was described as a new species in 1950 from dredged specimens (37–57 m depth) from Bikini Atoll and Rongelap Atoll in the Marshall Islands. Its relative rareness in collections from waters less than 30 m deep from these atolls indicated that it favored the mesophotic coral ecosystem (MCE) at depths of 30–150 m. Published and unpublished records of this species over the past 70 years show its habitat preference on the seaward slopes of atolls and in lagoons. Observations were based on collections from Pacific atolls of the Marshall Islands, Tuamotu Islands, Chuuk State in the Federated States of Micronesia, Cook Islands, and Johnston Atoll, and on the deep lagoonal reefs of Palau. Caulerpa bikinensis has also been recorded in shallow waters 1 m deep in dark microcavities on lagoonal pinnacles at Takapoto Atoll (Tuamotu Islands) and in Chuuk Lagoon, and 2 m deep at Raroia Atoll (Tuamotu Islands). Caulerpa bikinensis favors the deeper depths of the MCE for its low light intensity rather than its cooler temperature, and definitely shows a strong affinity to the high energy seaward slopes of calcareous Pacific atolls.

Mangrove forests are prevalent along tropical/subtropical coastlines and provide valuable ecosystem services including coastline stabilization, storm impact reduction, and enhanced coastal productivity. However, mangroves were absent from the Hawaiian Islands and their introduction to Moloka‘i in 1902 has provided an opportunity to examine their unique influence on coastal landscapes. Previous studies indicate an inability of native detritivores to utilize tannin-rich substrates, yielding poor cycling of mangrove-derived detritus in Hawaiian tidal zones. We hypothesize that in addition to altering detrital inputs, introduced mangroves facilitate the persistence of introduced species in the Hawaiian coastal zone by providing novel habitat for juvenile megafauna. To determine whether mangrove-dominated tidal zones harbor megafaunal assemblages distinct from open sandflats, we sampled in two mangrove (M1 and M2) and two adjacent sandflat (S1 and S2) sites along the southern coast of Moloka‘i, where the most mature mangrove forests occur in Hawai‘i. There were no statistical differences in total abundances between M1 and M2 or S1 and S2; therefore, results from individual deployments were pooled across the sites in order to conduct between-habitat (mangrove vs. sandflat) comparisons. Our mangrove study site had significantly higher abundances of megafauna, including several shrimp and crab species, compared to the sandflat site. The community composition within the mangrove site differed from the sandflat site, including higher abundances of non-native mangrove crabs (Scylla serrata), as well as native fish Bathygobius cocosensis and crustaceans (Thalamita crenata, Palaemon pacificus, P. debilis) than in the sandflat site, indicating that the mangrove site may provide niches for both invasive and native species. In addition, mean body length for several similar species was smaller in the mangrove site than in the sandflat site, suggesting that these mangroves may be providing a habitat for juvenile species. While our study was spatially limited to two mangrove and two adjacent sandflat sites, our results suggest that introduced mangroves in Moloka‘i may support small-bodied, native, and non-native megafauna, influencing coastal Hawaiian trophic dynamics. Our case study provides a baseline for megafaunal fish and invertebrate communities present prior to non-native mangrove removal as well as for monitoring potential community changes following expansion of mangrove habitats due to climate change.

The native Hawaiian plant, Scaevola taccada, commonly grows in tropical coastal areas throughout the Pacific and Indian oceans. Oral tradition suggests that the fruit of this species may have been used as a natural sunblock. To examine this, an extract of S. taccada fruit was tested to determine whether it had UV-absorbing properties, and if so, to determine its stability and potential toxicity to coral. Scaevola taccada contained UV-absorbing compound(s) with a sun-protection factor of 20, on average, that absorbed in the UVB and UVA (19.6% and 24.4% absorbance, respectively). The UV stability of the fruit extract over time and to various treatments (fresh, heated, and frozen) did not change (ANOVA, p = .396, F = 1.018). In addition, we examined the toxicity of S. taccada juice by exposing coral fragments of Porites compressa (n = 7) to four treatments (fresh and heated S. taccada extract, cytotoxic Montipora capitata eggs, and filtered seawater control). Changes in color and photosynthetic yield in treated tissue areas were measured after 4 days. Only the concentrated S. taccada fruit extract and M. capitata eggs resulted in significant changes in pigmentation or photosynthetic yield of the coral (p ≤ .0001). Further investigation is needed to isolate and expand tests on the UV-absorbing component(s) to understand this toxicity more clearly.

A new species of mesophotic marine red algae, Ethelia hawaiiensis sp. nov., is illustrated and described. Ethelia hawaiiensis is distinguished from other members of the genus by its large diameter assurgent filament cells, and in having thallus cavities that are frequently inhabited by microalgae, as well as in DNA sequence. Analyses of mitochondrial COI, plastid rbcL, and nuclear SSU sequences demonstrated that E. hawaiiensis was distinct from other species of Ethelia and that it was not phylogenetically closely related to other known species. Both COI and rbcL analyses placed E. hawaiiensis within a clade of other Ethelia sequences, while the SSU analyses, which only included two previously described species of Ethelia, resolved E. hawaiiensis as sister to a clade comprising Ethelia plus the Peyssonneliales. Morphological differences between E. hawaiiensis and other members of the genus are discussed.

Ruppia cf. maritima is one of the few native Hawaiian brackish water flowering plants, but its identity has never been examined using genetic analysis. The ability of this seagrass to tolerate a wide range of salinities and temperatures is reflected in its morphological variability among locations worldwide. Three populations on the island of Hawai‘i were sampled, and molecular analyses of the nuclear gene ITS and two chloroplast genes trnH-psbA and rbcL were used to examine the identity of Hawaiian Ruppia. Concatenated analyses showed that the populations contained little intra- or interpopulation variability, and indicated greatest genetic similarity to specimens from Japan, India, Vietnam, and Africa. Slight variations in tree topologies were present among the individual nuclear and two plastid markers; however, all Hawaiian specimens nested within other sequences reported as R. maritima. Molecular phylogenetic analyses demonstrate that there are multiple clades of samples from around the world labeled as R. maritima, and that the Hawaiian samples are allied with one of these clades. The geographic isolation and geologic age of each Hawaiian island, as well as the disjunct distribution of Ruppia populations among islands and within each island suggest a multiplex biogeography and evolutionary history of Hawaiian Ruppia.

The new species Hypnea corona Huisman & Petrocelli is described to accommodate specimens from Australia, New Zealand, Italy, and Japan that were previously included in the Hypnea cornuta complex but were shown by recent molecular studies to represent a species-level clade. Hypnea corona produces distinctive stellate, starch-filled propagules that are easily dislodged and serve as vegetative reproductive structures. However, similar structures are found in closely related species and thus H. corona is distinguished primarily by unique rbcL barcode sequences.

Two introduced invasive red macroalgae, Eucheuma denticulatum (Burman) Collins & Hervey and Gracilaria salicornia C. Ag., progressively declined in abundance in Kāne‘ohe Bay, Hawai‘i, following ∼30 years of increase since their introduction in the mid-1970s. The declines were not the result of biological control or mechanical control efforts, but instead were the result of grazing by native herbivores, probably fishes, as demonstrated by field experiments conducted in 2014 with algal thalli exposed to, and protected from grazing. Two long-term time-series of the abundance of herbivorous fishes at Kāne‘ohe Bay patch and fringing reef sites indicated that the abundance of herbivorous fishes increased from 2007 through 2014 over the period when these macroalgae declined in cover. Analyses of data of algal cover from surveys of Kāne‘ohe Bay reefs carried out between 1996 and 2018 indicate that the declines in E. denticulatum and G. salicornia began in approximately 2008, several years before control programs commenced on some of the reefs in the bay. In addition to intense herbivory, high summer water temperatures in 2014 and 2015 may have contributed to the decline in E. denticulatum. Naturally occurring declines in the abundance of alien invasive marine algal species have rarely been reported in the literature. These reductions in macroalgal abundance are the second instance of decline in invasive macroalgae in Kāne‘ohe Bay since 2006, and have reduced competition between macroalgae and corals on reef flats and reef slopes across the bay.