Early development of congeneric sea urchins (Heliocidaris) with contrasting life history modes in a warming and high CO2 ocean.

The impacts of ocean change stressors - warming and acidification - on marine invertebrate development have emerged as a significant impact of global change. We investigated the response of early development to the larval stage in sympatric, congeneric sea urchins, Heliocidaris tuberculata and Heliocidaris erythrogramma with contrasting modes of development to ocean warming and acidification. Effects of these stressors were assessed by quantifying the percentage of normal development during the first 24 h post fertilization, in cross-factorial experiments that included three temperature treatments (control: 20 °C; +4: 24 °C; +6: 26 °C) and four pHNIST levels (control: 8.2; -0.4: 7.8; -0.6: 7.6; -0.8: 0.4). The experimental treatments were designed in context with present day and near-future (∼2100) conditions for the southeast Australia global warming hotspot. Temperature was the most important factor affecting development of both species causing faster progression through developmental stages as well as a decrease in the percentage of normal development. H. erythrogramma embryos were less tolerant of increased temperature than those of H. tuberculata. Acidification impaired development to the larval stage in H. tuberculata, but this was not the case for H. erythrogramma. Thus, outcomes for the planktonic life phase of the two Heliocidaris species in response to ocean warming and acidification will differ. As shown for these species, single-stressor temperature or acidification studies can be misleading with respect to determining species' vulnerability and responses to global change.

Genetic variation underlies temperature tolerance of embryos in the sea urchin Heliocidaris erythrogramma armigera.

Ocean warming can alter natural selection on marine systems, and in many cases, the long-term persistence of affected populations will depend on genetic adaptation. In this study, we assess the potential for adaptation in the sea urchin Heliocidaris erythrogramma armigera, an Australian endemic, that is experiencing unprecedented increases in ocean temperatures. We used a factorial breeding design to assess the level of heritable variation in larval hatching success at two temperatures. Fertilized eggs from each full-sibling family were tested at 22 °C (current spawning temperature) and 25 °C (upper limit of predicted warming this century). Hatching success was significantly lower at higher temperatures, confirming that ocean warming is likely to exert selection on this life-history stage. Our analyses revealed significant additive genetic variance and genotype-by-environment interactions underlying hatching success. Consistent with prior work, we detected significant nonadditive (sire-by-dam) variance in hatching success, but additionally found that these interactions were modified by temperature. Although these findings suggest the potential for genetic adaptation, any evolutionary responses are likely to be influenced (and possibly constrained) by complex genotype-by-environment and sire-by-dam interactions and will additionally depend on patterns of genetic covariation with other fitness traits.

Embryo fossilization is a biological process mediated by microbial biofilms.

Fossilized embryos with extraordinary cellular preservation appear in the Late Neoproterozoic and Cambrian, coincident with the appearance of animal body fossils. It has been hypothesized that microbial processes are responsible for preservation and mineralization of organic tissues. However, the actions of microbes in preservation of embryos have not been demonstrated experimentally. Here, we show that bacterial biofilms assemble rapidly in dead marine embryos and form remarkable pseudomorphs in which the bacterial biofilm replaces and exquisitely models details of cellular organization and structure. The experimental model was the decay of cleavage stage embryos similar in size and morphology to fossil embryos. The data show that embryo preservation takes place in 3 distinct steps: (i) blockage of autolysis by reducing or anaerobic conditions, (ii) rapid formation of microbial biofilms that consume the embryo but form a replica that retains cell organization and morphology, and (iii) bacterially catalyzed mineralization. Major bacterial taxa in embryo decay biofilms were identified by using 16S rDNA sequencing. Decay processes were similar in different taphonomic conditions, but the composition of bacterial populations depended on specific conditions. Experimental taphonomy generates preservation states similar to those in fossil embryos. The data show how fossilization of soft tissues in sediments can be mediated by bacterial replacement and mineralization, providing a foundation for experimentally creating biofilms from defined microbial species to model fossilization as a biological process.