Animal evolution at the ocean's water-air interface.

Innovation is a key to evolutionary success and entrance into novel ecosystems.1 Species that float freely at the ocean's surface, termed obligate neuston (also called pleuston, here referred to simply as neuston), include highly specialized taxa from distinct evolutionary lineages that evolved floating morphologies.2 In 1958, Soviet scientist, A.I. Savilov,3 stated that floating animal species are derived from benthic ancestors, rather than species from the adjacent pelagic zone, and that floating morphologies are homologous to benthic attachment structures. To test Savilov's hypothesis, we constructed molecular phylogenies and ancestral states for all major floating groups for which molecular data were available. Our results reveal that four of the five clades examined arose directly from a substrate-attached ancestor, although that substrate was not necessarily the benthos, as Savilov stated, and instead included epibiotic and rafting ancestors. Despite their diverse evolutionary origins, floating animals use gas-trapping mechanisms to remain at the surface,4,5,6 and many of these gas-trapping structures appear to be homologous to substrate attachment structures. We also reconstruct the trophic habits of floating mollusks and their sister species, revealing that prey preference remains conserved upon entering the ocean's surface ecosystem. Colonization of the ocean's surface seems to have occurred through successive evolutionary steps from the seafloor. Our results suggest that these steps often included transitions through epibiotic (where species attach to other living organisms) or rafting (where species attach to floating debris) habits. The water-air interface, despite its unique properties, may, in some ways, be just another substrate.

High concentrations of floating neustonic life in the plastic-rich North Pacific Garbage Patch.

Floating life (obligate neuston) is a core component of the ocean surface food web. However, only 1 region of high neustonic abundance is known so far, the Sargasso Sea in the Subtropical North Atlantic gyre, where floating life provides critical habitat structure and ecosystem services. Here, we hypothesize that floating life is also concentrated in other gyres with converging surface currents. To test this hypothesis, we collected samples through the eastern North Pacific Subtropical Gyre in the area of the North Pacific "Garbage Patch" (NPGP) known to accumulate floating anthropogenic debris. We found that densities of floating life were higher inside the central NPGP than on its periphery and that there was a positive relationship between neuston abundance and plastic abundance for 3 out of 5 neuston taxa, Velella, Porpita, and Janthina. This work has implications for the ecology of subtropical oceanic gyre ecosystems.

The mysterious ecosystem at the ocean's surface.

Life on the ocean's surface connects worlds. From shallow waters to the deep sea, the open ocean to rivers and lakes, numerous terrestrial and marine species depend on the surface ecosystem and the organisms found therein. Organisms that live freely at the surface, termed "neuston," include keystone organisms like the golden seaweed Sargassum that makes up the Sargasso Sea, floating barnacles, snails, nudibranchs, and cnidarians. Many ecologically and economically important fish species live as or rely upon neuston. Species at the surface are not distributed uniformly; the ocean's surface harbors unique neustonic communities and ecoregions found at only certain latitudes and only in specific ocean basins. But the surface is also on the front line of climate change and pollution. Despite the diversity and importance of the ocean's surface in connecting disparate habitats, and the risks it faces, we know very little about neustonic life. This Essay will introduce you to the neuston, their connections to diverse habitats, the threats they face, and new opportunities for research and discovery at the air-sea interface.

Environmental entrainment demonstrates natural circadian rhythmicity in the cnidarian Nematostella vectensis.

Considerable advances in chronobiology have been made through controlled laboratory studies, but distinct temporal rhythms can emerge under natural environmental conditions. Lab-reared Nematostella vectensis sea anemones exhibit circadian behavioral and physiological rhythms. Given that these anemones inhabit shallow estuarine environments subject to tidal inputs, it was unclear whether circadian rhythmicity would persist following entrainment in natural conditions, or whether circatidal periodicity would predominate. Nematostella were conditioned within a marsh environment, where they experienced strong daily temperature cycles as well as brief tidal flooding around the full and new moons. Upon retrieval, anemones exhibited strong circadian (∼24 h) activity rhythms under a light-dark cycle or continuous darkness, but reduced circadian rhythmicity under continuous light. However, some individuals in each light condition showed circadian rhythmicity, and a few individuals showed circatidal rhythmicity. Consistent with the behavioral studies, a large number of transcripts (1640) exhibited diurnal rhythmicity compared with very few (64) with semidiurnal rhythmicity. Diurnal transcripts included core circadian regulators, and 101 of 434 (23%) genes that were previously found to be upregulated by exposure to ultraviolet radiation. Together, these behavioral and transcriptional studies show that circadian rhythmicity predominates and suggest that solar radiation drives physiological cycles in this sediment-dwelling subtidal animal.

Phylogenetic analysis of cnidarian peroxiredoxins and stress-responsive expression in the estuarine sea anemone Nematostella vectensis.

Peroxiredoxins (PRXs) are a family of antioxidant enzymes present in all domains of life. To date, the diversity and function of peroxiredoxins within animals have only been studied in a few model species. Thus, we sought to characterize peroxiredoxin diversity in cnidarians and to gain insight into their function in one cnidarian-the sea anemone Nematostella vectensis. Phylogenetic analysis using all six known PRX subfamilies (PRX1-4, PRX5, PRX6, PRXQ/AHPE1, TPX, BCP-PRXQ) revealed that like bilaterians, cnidarians contain representatives from three subfamilies (PRX1-4, PRX5, PRX6). Within the PRX1-4 subfamily, cnidarian sequences fall into two clades: PRX4, and a cnidarian-specific clade, which we term CNID-PRX. This phylogenetic analysis demonstrates that the three PRX subfamilies present in Bilateria were also present in the last common ancestor of the Cnidaria and Bilateria, and further that diversification of the PRX1-4 subfamily has occurred within the cnidarian lineage. We next examined the impact of decreased salinity, increased temperature, and peroxide exposure on the expression of four prx genes in N. vectensis (cnid-prx, prx4, prx5, and prx6). These genes exhibited unique expression patterns in response to these environmental stressors. Expression of prx4 decreased with initial exposure to elevated temperature, cnid-prx increased with exposure to elevated temperatures as well as with hydrogen peroxide exposure, and expression of all prxs transiently decreased with reduced salinity. Predicted subcellular localization patterns also varied among PRX proteins. Together these results provide evidence that peroxiredoxins in N. vectensis serve distinct physiological roles and lay a groundwork for understanding how peroxiredoxins mediate cnidarian developmental processes and environmental responses.

Evolution and development of scyphozoan jellyfish.

Scyphozoan jellyfish, or scyphomedusae, are conspicuous members of many ocean ecosystems, and have large impacts on human health and industry. Most scyphomedusae are the final stage in a complex life cycle that also includes two intermediate stages: the larval planula and benthic polyp. In species with all three life-cycle stages, the metamorphosis of a polyp into a juvenile scyphomedusa (ephyra) is termed strobilation, and polyps can produce one ephyra (termed monodisc strobilation) or many ephyrae (termed polydisc strobilation). In contrast to species with planula, polyp and medusa stages, a handful of scyphozoan species possess modified life cycles with reduced or absent stages. The evolutionary patterns associated with strobilation and life-cycle type have not been thoroughly investigated, and many studies of ephyra development and strobilation induction are not yet synthesized. Herein, I place the development of scyphomedusae in an evolutionary context. I first review the current evolutionary hypotheses for Scyphozoa. Next, I review what is known about scyphomedusa development across a broad diversity of species, including the first signs of strobilation, the formation of strobila segments, and the morphogenesis of ephyrae. I then review cases where the canonical scyphozoan life cycle has been modified, and take advantage of phylogenetic hypotheses to place these observations in an evolutionary context. I show that the evolution of monodisc strobilation occurred at least twice, and that the loss of intermediate life-cycle stages occurred several times independently; by contrast, the reduction of the medusa stage appears to have occurred within a single clade. I then briefly review the major natural cues of strobilation induction. Finally, I summarize what is currently known about the molecular mechanisms of strobilation induction and ephyra development. I conclude with suggestions for future directions in the field.

Indoles induce metamorphosis in a broad diversity of jellyfish, but not in a crown jelly (Coronatae).

Many animals go through one or more metamorphoses during their lives, however, the molecular underpinnings of metamorphosis across diverse species are not well understood. Medusozoa (Cnidaria) is a clade of animals with complex life cycles, these life cycles can include a polyp stage that metamorphoses into a medusa (jellyfish). Medusae are produced through a variety of different developmental mechanisms-in some species polyps bud medusae (Hydrozoa), in others medusae are formed through polyp fission (Scyphozoa), while in others medusae are formed through direct transformation of the polyp (Cubozoa). To better understand the molecular mechanisms that may coordinate these different forms of metamorphosis, we tested two compounds first identified to induce metamorphosis in the moon jellyfish Aurelia aurita (indomethacin and 5-methoxy-2-methylindole) on a broad diversity of medusozoan polyps. We discovered that indole-containing compounds trigger metamorphosis across a broad diversity of species. All tested discomedusan polyps metamorphosed in the presence of both compounds, including species representatives of several major lineages within the clade (Pelagiidae, Cyaneidae, both clades of Rhizostomeae). In a cubozoan, low levels of 5-methoxy-2-methylindole reliably induced complete and healthy metamorphosis. In contrast, neither compound induced medusa metamorphosis in a coronate scyphozoan, or medusa production in either hydrozoan tested. Our results support the hypothesis that metamorphosis is mediated by a conserved induction pathway within discomedusan scyphozoans, and possibly cubozoans. However, failure of these compounds to induce metamorphosis in a coronate suggests this induction mechanism may have been lost in this clade, or is convergent between Scyphozoa and Cubozoa.

Identification of differentially expressed genes from multipotent epithelia at the onset of an asexual development.

Organisms that have evolved alternative modes of reproduction, complementary to the sexual mode, are found across metazoans. The chordate Botryllus schlosseri is an emerging model for asexual development studies. Botryllus can rebuild its entire body from a portion of adult epithelia in a continuous and stereotyped process called blastogenesis. Anatomy and ontogenies of blastogenesis are well described, however molecular signatures triggering this developmental process are entirely unknown. We isolated tissues at the site of blastogenesis onset and from the same epithelia where this process is never triggered. We linearly amplified an ultra-low amount of mRNA (<10ng) and generated three transcriptome datasets. To provide a conservative landscape of transcripts differentially expressed between blastogenic vs. non-blastogenic epithelia we compared three different mapping and analysis strategies with a de novo assembled transcriptome and partially assembled genome as references, additionally a self-mapping strategy on the dataset. A subset of differentially expressed genes were analyzed and validated by in situ hybridization. The comparison of different analyses allowed us to isolate stringent sets of target genes, including transcripts with potential involvement in the onset of a non-embryonic developmental pathway. The results provide a good entry point to approach regenerative event in a basal chordate.

Comparative muscle development of scyphozoan jellyfish with simple and complex life cycles.

BACKGROUND: Simple life cycles arise from complex life cycles when one or more developmental stages are lost. This raises a fundamental question - how can an intermediate stage, such as a larva, be removed, and development still produce a normal adult? To address this question, we examined the development in several species of pelagiid jellyfish. Most members of Pelagiidae have a complex life cycle with a sessile polyp that gives rise to ephyrae (juvenile medusae); but one species within Pelagiidae, Pelagia noctiluca, spends its whole life in the water column, developing from a larva directly into an ephyra. In many complex life cycles, adult features develop from cell populations that remain quiescent in larvae, and this is known as life cycle compartmentalization and may facilitate the evolution of direct life cycles. A second type of metamorphic processes, known as remodeling, occurs when adult features are formed through modification of already differentiated larval structures. We examined muscle morphology to determine which of these alternatives may be present in Pelagiidae. RESULTS: We first examined the structure and development of polyp and ephyra musculature in Chrysaora quinquecirrha, a close relative of P. noctiluca with a complex life cycle. Using phallotoxin staining and confocal microscopy, we verified that polyps have four to six cord muscles that persist in strobilae and discovered that cord muscles is physically separated from ephyra muscle. When cord muscle is removed from ephyra segments, normal ephyra muscle still develops. This suggests that polyp cord muscle is not necessary for ephyra muscle formation. We also found no evidence of polyp-like muscle in P. noctiluca. In both species, we discovered that ephyra muscle arises de novo in a similar manner, regardless of the life cycle. CONCLUSIONS: The separate origins of polyp and ephyra muscle in C. quinquecirrha and the absence of polyp-like muscle in P. noctiluca suggest that polyp muscle is not remodeled to form ephyra muscle in Pelagiidae. Life cycle stages in Scyphozoa may instead be compartmentalized. Because polyp muscle is not directly remodeled, this may have facilitated the loss of the polyp stage in the evolution of P. noctiluca.

Sizing ocean giants: patterns of intraspecific size variation in marine megafauna.

What are the greatest sizes that the largest marine megafauna obtain? This is a simple question with a difficult and complex answer. Many of the largest-sized species occur in the world's oceans. For many of these, rarity, remoteness, and quite simply the logistics of measuring these giants has made obtaining accurate size measurements difficult. Inaccurate reports of maximum sizes run rampant through the scientific literature and popular media. Moreover, how intraspecific variation in the body sizes of these animals relates to sex, population structure, the environment, and interactions with humans remains underappreciated. Here, we review and analyze body size for 25 ocean giants ranging across the animal kingdom. For each taxon we document body size for the largest known marine species of several clades. We also analyze intraspecific variation and identify the largest known individuals for each species. Where data allows, we analyze spatial and temporal intraspecific size variation. We also provide allometric scaling equations between different size measurements as resources to other researchers. In some cases, the lack of data prevents us from fully examining these topics and instead we specifically highlight these deficiencies and the barriers that exist for data collection. Overall, we found considerable variability in intraspecific size distributions from strongly left- to strongly right-skewed. We provide several allometric equations that allow for estimation of total lengths and weights from more easily obtained measurements. In several cases, we also quantify considerable geographic variation and decreases in size likely attributed to humans.

Differential gene expression in the siphonophore Nanomia bijuga (Cnidaria) assessed with multiple next-generation sequencing workflows.

We investigated differential gene expression between functionally specialized feeding polyps and swimming medusae in the siphonophore Nanomia bijuga (Cnidaria) with a hybrid long-read/short-read sequencing strategy. We assembled a set of partial gene reference sequences from long-read data (Roche 454), and generated short-read sequences from replicated tissue samples that were mapped to the references to quantify expression. We collected and compared expression data with three short-read expression workflows that differ in sample preparation, sequencing technology, and mapping tools. These workflows were Illumina mRNA-Seq, which generates sequence reads from random locations along each transcript, and two tag-based approaches, SOLiD SAGE and Helicos DGE, which generate reads from particular tag sites. Differences in expression results across workflows were mostly due to the differential impact of missing data in the partial reference sequences. When all 454-derived gene reference sequences were considered, Illumina mRNA-Seq detected more than twice as many differentially expressed (DE) reference sequences as the tag-based workflows. This discrepancy was largely due to missing tag sites in the partial reference that led to false negatives in the tag-based workflows. When only the subset of reference sequences that unambiguously have tag sites was considered, we found broad congruence across workflows, and they all identified a similar set of DE sequences. Our results are promising in several regards for gene expression studies in non-model organisms. First, we demonstrate that a hybrid long-read/short-read sequencing strategy is an effective way to collect gene expression data when an annotated genome sequence is not available. Second, our replicated sampling indicates that expression profiles are highly consistent across field-collected animals in this case. Third, the impacts of partial reference sequences on the ability to detect DE can be mitigated through workflow choice and deeper reference sequencing.