Species’ vulnerability to climate change depends on the most temperature-sensitive life stages, but for major animal groups such as fish, life cycle bottlenecks are often not clearly defined. We used observational, experimental, and phylogenetic data to assess stage-specific thermal tolerance metrics for 694 marine and freshwater fish species from all climate zones. Our analysis shows that spawning adults and embryos consistently have narrower tolerance ranges than larvae and nonreproductive adults and are most vulnerable to climate warming. The sequence of stage-specific thermal tolerance corresponds with the oxygen-limitation hypothesis, suggesting a mechanistic link between ontogenetic changes in cardiorespiratory (aerobic) capacity and tolerance to temperature extremes. A logarithmic inverse correlation between the temperature dependence of physiological rates (development and oxygen consumption) and thermal tolerance range is proposed to reflect a fundamental, energetic trade-off in thermal adaptation. Scenario-based climate projections considering the most critical life stages (spawners and embryos) clearly identify the temperature requirements for reproduction as a critical bottleneck in the life cycle of fish. By 2100, depending on the Shared Socioeconomic Pathway (SSP) scenario followed, the percentages of species potentially affected by water temperatures exceeding their tolerance limit for reproduction range from ~10% (SSP 1–1.9) to ~60% (SSP 5–8.5). Efforts to meet ambitious climate targets (SSP 1–1.9) could therefore benefit many fish species and people who depend on healthy fish stocks.

Many dinoflagellate species are notorious for the toxins they produce and ecological and human health consequences associated with harmful algal blooms (HABs). Dinoflagellates are particularly refractory to genomic analysis due to the enormous genome size, lack of knowledge about their DNA composition and structure, and peculiarities of gene regulation, such as spliced leader (SL) trans-splicing and mRNA transposition mechanisms. Alexandrium ostenfeldii is known to produce macrocyclic imine toxins, described as spirolides. We characterized the genome of A. ostenfeldii using a combination of transcriptomic data and random genomic clones for comparison with other dinoflagellates, particularly Alexandrium species. Examination of SL sequences revealed similar features as in other dinoflagellates, including Alexandrium species. SL sequences in decay indicate frequent retro-transposition of mRNA species. This probably contributes to overall genome complexity by generating additional gene copies. Sequencing of several thousand fosmid and bacterial artificial chromosome (BAC) ends yielded a wealth of simple repeats and tandemly repeated longer sequence stretches which we estimated to comprise more than half of the whole genome. Surprisingly, the repeats comprise a very limited set of 79–97 bp sequences; in part the genome is thus a relatively uniform sequence space interrupted by coding sequences. Our genomic sequence survey (GSS) represents the largest genomic data set of a dinoflagellate to date. Alexandrium ostenfeldii is a typical dinoflagellate with respect to its transcriptome and mRNA transposition but demonstrates Alexandrium-like stop codon usage. The large portion of repetitive sequences and the organization within the genome is in agreement with several other studies on dinoflagellates using different approaches. It remains to be determined whether this unusual composition is directly correlated to the exceptionally genome organization of dinoflagellates with a low amount of histones and histone-like proteins.

Dinoflagellates are microbial eukaryotes that have exceptionally large nuclear genomes; however, their organelle genomes are small and fragmented and contain fewer genes than those of other eukaryotes. The genus Amoebophrya (Syndiniales) comprises endoparasites with high genetic diversity that can infect other dinoflagellates, such as those forming harmful algal blooms (e.g., Alexandrium). We sequenced the genome (~100 Mb) of Amoebophrya ceratii to investigate the early evolution of genomic characters in dinoflagellates. The A. ceratii genome encodes almost all essential biosynthetic pathways for self-sustaining cellular metabolism, suggesting a limited dependency on its host. Although dinoflagellates are thought to have descended from a photosynthetic ancestor, A. ceratii appears to have completely lost its plastid and nearly all genes of plastid origin. Functional mitochondria persist in all life stages of A. ceratii, but we found no evidence for the presence of a mitochondrial genome. Instead, all mitochondrial proteins appear to be lost or encoded in the A. ceratii nucleus.

Dinoflagellates are a major cause of harmful algal blooms (HABs), with consequences for coastal marine ecosystem functioning and services. Alexandrium fundyense (previously Alexandrium tamarense) is one of the most abundant and widespread toxigenic species in the temperate Northern and Southern Hemisphere and produces paralytic shellfish poisoning toxins as well as lytic allelochemical substances. These bioactive compounds may support the success of A. fundyense and its ability to form blooms. Here we investigate the impact of grazing on monoclonal and mixed set-ups of highly (Alex2) and moderately (Alex4) allelochemically active A. fundyense strains and a non-allelochemically active conspecific (Alex5) by the heterotrophic dinoflagellate Polykrikos kofoidii. While Alex4 and particularly Alex5 were strongly grazed by P. kofoidii when offered alone, both strains grew well in the mixed assemblages (Alex4 þ Alex5 and Alex2 þ Alex5). Hence, the allelochemical active strains facilitated growth of the non-active strain by protecting the population as a whole against grazing. Based on our results, we argue that facilitation among clonal lineages within a species may partly explain the high genotypic and phenotypic diversity of Alexandrium populations. Populations of Alexandrium may comprise multiple cooperative traits that act in concert with intraspecific facilitation, and hence promote the success of this notorious HAB species.