Aim The Point Conception (PC) biogeographical boundary is defined by a transition between cold northern and warm southern water masses, accompanied by shifts in numerous ecological and environmental parameters. While these factors can potentially contribute to the genetic differentiation of lineages whose distributions span this boundary, few organisms exhibit genetic signatures of restricted gene flow across this region. We examine the effects of the PC boundary and other potential dispersal barriers on phylogeographical patterns in the rocky supralittoral isopod Ligia occidentalis. This isopod has a limited dispersal potential, its distribution spans the PC region, and it exhibits high levels of allopatric genetic differentiation south of PC. Location Eastern Pacific coastline between southern Oregon and the Baja California Peninsula. Methods We conducted a thorough sampling of L. occidentalis from the PC area to its northern range limit in southern Oregon, and increased previous sampling coverage south of PC. We obtained sequences of the mitochondrial genes cytochrome c oxidase subunit I (COI) and 16S rDNA for the new localities and combined them with a previously published data set, resulting in a total of 58 sampling locations. We also obtained sequences of the nuclear gene sodium–potassium ATPase α‐subunit (NaK). We conducted phylogenetic and population‐genetic analyses. Results The geographical limit between the two most divergent clades of L. occidentalis (20–25% divergence for COI) distributed between southern Oregon and the Baja California Peninsula occurs at the PC biogeographical boundary. Levels of allopatric genetic divergences were greatly reduced north of PC (≤ 3.0% divergence for COI). Main conclusions The geographical limit between the two main L. occidentalis clades in California largely reflects the changes in sea surface temperature that define the PC biogeographical boundary. Reduced levels of genetic divergence among L. occidentalis lineages found north of PC suggest a recent expansion in the northern range of this isopod.

Studies of microbial associations of intertidal isopods in the primitive genus Ligia (Oniscidea, Isopoda) can help our understanding of the formation of symbioses during sea-land transitions, as terrestrial Oniscidean isopods have previously been found to house symbionts in their hepatopancreas. Ligia pallasii and Ligia occidentalis co-occur in the high intertidal zone along the Eastern Pacific with a large zone of range overlap and both species showing patchy distributions. In 16S rRNA clone libraries mycoplasma-like bacteria (Firmicutes), related to symbionts described from terrestrial isopods, were the most common bacteria present in both host species. There was greater overall microbial diversity in Ligia pallasii compared with L. occidentalis. Populations of both Ligia species along an extensive area of the eastern Pacific coastline were screened for the presence of mycoplasma-like symbionts with symbiont-specific primers. Symbionts were present in all host populations from both species but not in all individuals. Phylogenetically, symbionts of intertidal isopods cluster together. Host habitat, in addition to host phylogeny appears to influence the phylogenetic relation of symbionts.

The harpacticoid copepod Macrosetella gracilis is found in pelagic habitats in tropical and subtropical oceans associated with colonies of the N 2 fixing cyanobacterium Trichodesmium spp. ) and constituted an average of 10.8% of the total copepod population. However, we observed no statistically discernable correlation between M. gracilis and Trichodesmium spp. abundances, suggesting that availability of Trichodesmium spp. did not limit the abundance of M. gracilis during our study. In previous laboratory studies M. gracilis had been shown to have the ability to ingest Trichodesmium spp. trichomes and appeared immune to cyanobacterial toxins harmful to other species of copepods. Natural abundance of stable isotopes (δ 15 N, δ 13 C) in copepod tissue from field samples suggested that the diet of M. gracilis was not predominately composed of Trichodesmium spp. as proposed by previous research. Natural abundance of δ 15 N was similar for M. gracilis (3.06 ± 2.29), Miracia efferata (1.83 ± 0.88), and calanoid copepods (2.7 ± 1.95). No Trichodesmium spp. were observed in M. gracilis gut contents. Trichodesmium spp. was not a predominant food in the diet of this copepod, but colonies of the toxic cyanobacterium could provide shelter from predation and be used as a floating substrate for adult and juveniles of M. gracilis.

Sea star wasting disease (SSWD) describes a suite of disease signs believed to have led to catastrophic die-offs in many asteroid species, beginning in 2013. While most studies have focused on large, easily visible sea stars with widely-dispersing larvae, less information is available on the effect of this disease outbreak on smaller sea star species, such as the six-armed sea star Leptasterias spp. Unlike many larger sea stars, Leptasterias brood non-feeding young instead of broadcast-spawning planktonic larvae. Limited dispersal and thus limited gene flow may make these sea stars more vulnerable to local selective pressures, such as disease outbreaks. Here, we examined Leptasterias populations at sites along the California coast and documented abundance changes coincident with recent Pacific coast SSWD in 2014. Detection of Leptasterias in central California declined, and Leptasterias were not detected at multiple sites clustered around the San Francisco Bay outflow in the most recent surveys. Additionally, we categorized disease signs in Leptasterias in the field and laboratory, which mirrored those seen in larger sea stars in both settings. Finally, we found that magnesium chloride (MgCl2) slowed the progression of physical deterioration related to SSWD when applied to sea stars in the laboratory, suggesting that MgCl2 may prolong the survival of diseased individuals.