Variation in thermal tolerance plays a key role in determining the biogeographic distribution of organisms. Consequently, identifying the mechanistic basis for thermal tolerance is necessary for understanding not only current species range limits but also the capacity for range limits to shift in response to climate change. Although variation in mitochondrial function likely contributes to variation in thermal tolerance, the extent to which mitochondrial function underlies local thermal adaptation is not fully understood. In the current study, we examine variation in thermal tolerance and mitochondrial function among three populations of the intertidal copepod Tigriopus californicus found across a latitudinal thermal gradient along the coast of California, USA. We tested (1) acute thermal tolerance using survivorship and knockdown assays, (2) chronic thermal tolerance using survivorship of nauplii and developmental rate, and (3) mitochondrial performance at a range of temperatures using ATP synthesis fueled by complexes I, II, and I&II, as well as respiration of permeabilized fibers. We find evidence for latitudinal thermal adaptation: the southernmost San Diego population outperforms the northernmost Santa Cruz in measures of survivorship, knockdown temperature, and ATP synthesis rates during acute thermal exposures. However, under a chronic thermal regime, survivorship and developmental rate are more similar in the southernmost and northernmost population than in the mid-range population (Abalone Cove). Though this pattern is unexpected, it aligns well with population-specific rates of ATP synthesis at these chronic temperatures. Combined with the tight correlation of ATP synthesis decline and knockdown temperature, these data suggest a role for mitochondria in setting thermal range limits and indicate that divergence in mitochondrial function is likely a component of adaptation across latitudinal thermal gradients.

In order to protect the diverse ecosystems of coastal California, a series of marine protected areas (MPAs) have been established. The ability of these MPAs to preserve and potentially enhance marine resources can only be assessed if these habitats are monitored through time. This study establishes a baseline for monitoring the spawning activity of fish in the MPAs adjacent to Scripps Institution of Oceanography (La Jolla, CA, USA) by sampling fish eggs from the plankton. Using vertical plankton net tows, 266 collections were made from the Scripps Pier between 23 August 2012 and 28 August 2014; a total of 21,269 eggs were obtained. Eggs were identified using DNA barcoding: the COI or 16S rRNA gene was amplified from individual eggs and sequenced. All eggs that were successfully sequenced could be identified from a database of molecular barcodes of California fish species, resulting in species-level identification of 13,249 eggs. Additionally, a surface transport model of coastal circulation driven by current maps from high frequency radar was used to construct probability maps that estimate spawning locations that gave rise to the collected eggs. These maps indicated that currents usually come from the north but water parcels tend to be retained within the MPA; eggs sampled at the Scripps Pier have a high probability of having been spawned within the MPA. The surface transport model also suggests that although larvae have a high probability of being retained within the MPA, there is also significant spillover into nearby areas outside the MPA. This study provides an important baseline for addressing the extent to which spawning patterns of coastal California species may be affected by future changes in the ocean environment.

Long-term monitoring of marine ecosystems is critical to assessing how global processes such as natural environmental variation and climate change affect marine populations. Ichthyoplankton surveys provide one approach to such monitoring. We conducted weekly fish egg collections off the Scripps Institution of Oceanography Pier (La Jolla, CA, USA) for three years (2014-2017) and added a second sampling site near the La Jolla kelp forest for one year (2017). Fish eggs were identified using DNA barcoding and data were compared to previous work from Pier surveys from 2012-2014. We documented large interannual variability in fish egg abundance associated with climatic fluctuations, including an El Niño event captured during our sampling years. Overall egg abundance was reduced by > 50% during periods of anomalously warm water in 2014-2016. Fish egg abundance rebounded in 2017 and was accompanied by a phenological shift of peak spawning activity. We found interannual fish egg abundance may be linked with upwelling regimes and winter temperatures. Across the period of joint sampling, we found no distinct differences in community composition between the Pier (soft bottom) and kelp forest habitat we sampled (2 km distant). Long-term monitoring of fish spawning can contribute to our understanding of how natural environmental variation such as El Niño events affect fish reproductive activity. This understanding may extend to trends in marine resource availability associated with climate and aid in evaluating the efficacy of existing management efforts.