New sea urchin phylogeography reveals latitudinal shifts associated with speciation

Where do new species arise? When do they form and how do they diverge from a common ancestor? A new comprehensive study of Arbacia sea urchins provides surprising answers to these questions. By combining mtDNA phylogeographic markers with a nuclear locus (encoding the sperm acrosomal protein bindin) known to be susceptible to high rates of adaptive codon evolution, Lessios et al. (2012) show that new species and lineages arose relatively recently, most often in association with latitudinal shifts between the temperate zones and the tropics, and in one case, in association with a significant geological barrier to gene flow (the rise of the Isthmus of Panama). In addition to the ‘where’ and ‘when’ of Arbacia speciation, these new data resolve an important question about ‘who’Arbacia species are by revealing extensive allele sharing at both loci between a pair of broadly sympatric nominal species (that should perhaps be considered a single taxon). ‘How’Arbacia diverge from each other is less easily resolved: there is no evidence for reinforcement (via selection on bindin) as an important source of divergence between nominal species, and there are few other data to decide among the alternative hypotheses to explain Arbacia speciation.     

Early post-metamorphic,Carboniferous blastoid reveals the evolution and development of the digestive system in echinoderms

Inferring the development of the earliest echinoderms is critical to uncovering the evolutionary assembly of the phylum-level body plan but has long proven problematic because early ontogenetic stages are rarely preserved as fossils. Here, we use synchrotron tomography to describe a new early post-metamorphic blastoid echinoderm from the Carboniferous (approx. 323 Ma) of China. The resulting three-dimensional reconstruction reveals a U-shaped tubular structure in the fossil interior, which is interpreted as the digestive tract. Comparisons with the developing gut of modern crinoids demonstrate that crinoids are an imperfect analogue for many extinct groups. Furthermore, consideration of our findings in a phylogenetic context allows us to reconstruct the evolution and development of the digestive system in echinoderms more broadly; there was a transition from a straight to a simple curved gut early in the phylum''s evolution, but additional loops and coils of the digestive tract (as seen in crinoids) were not acquired until much later.     

Cambrian stalked echinoderms show unexpected plasticity of arm construction

Feeding arms carrying coelomic extensions of the theca are thought to be unique to crinoids among stemmed echinoderms. However, a new two-armed echinoderm from the earliest Middle Cambrian of Spain displays a highly unexpected morphology. X-ray microtomographic analysis of its arms shows they are polyplated in their proximal part with a dorsal series of uniserial elements enclosing a large coelomic lumen. Distally, the arm transforms into the more standard biserial structure of a blastozoan brachiole. Phylogenetic analysis demonstrates that this taxon lies basal to rhombiferans as sister-group to pleurocystitid and glyptocystitid blastozoans, drawing those clades deep into the Cambrian. We demonstrate that Cambrian echinoderms show surprising variability in the way their appendages are constructed, and that the appendages of at least some blastozoans arose as direct outgrowths of the body in much the same way as the arms of crinoids.     

Cambrian cinctan echinoderms shed light on feeding in the ancestral deuterostome

Reconstructing the feeding mode of the latest common ancestor of deuterostomes is key to elucidating the early evolution of feeding in chordates and allied phyla; however, it is debated whether the ancestral deuterostome was a tentaculate feeder or a pharyngeal filter feeder. To address this, we evaluated the hydrodynamics of feeding in a group of fossil stem-group echinoderms (cinctans) using computational fluid dynamics. We simulated water flow past three-dimensional digital models of a Cambrian fossil cinctan in a range of possible life positions, adopting both passive tentacular feeding and active pharyngeal filter feeding. The results demonstrate that an orientation with the mouth facing downstream of the current was optimal for drag and lift reduction. Moreover, they show that there was almost no flow to the mouth and associated marginal groove under simulations of passive feeding, whereas considerable flow towards the animal was observed for active feeding, which would have enhanced the transport of suspended particles to the mouth. This strongly suggests that cinctans were active pharyngeal filter feeders, like modern enteropneust hemichordates and urochordates, indicating that the ancestral deuterostome employed a similar feeding strategy.     

Vibrios of some deep-water invertebrates

Abstract The gut microbiota of 7 species of deep-water (300–400 m) invertebrates from the Gulf of Mexico was examined. High populations of Vibrio spp. were observed in crustaceans (ranging from 10⁵ to 7 × 10⁶ cells/g gut content) while relatively low populations of Vibrio spp. were found in annelids, the water column, and sediment. Although saprophytic Vibrio species were isolated, Vibrio fluvialis and Vibrio hollisae , potential human pathogens, were isolated from the crustaceans, Pleoticus robustus, Nematocarcinas sp., Plesionika sp., and Munida sp., and Vibrio vulnificus was isolated from Nereis sp. These observations confirm the finding of Ohwada et al. [18] that the gut of deep-water invertebrates has a bacterial flora abundant in Vibrio spp. These results also suggest that some marine invertebrates may serve as reservoirs for certain potential pathogenic Vibrio species.