The first two complete mitogenomes of the order Apodida from deep-sea chemoautotrophic environments: New insights into the gene rearrangement, origin and evolution of the deep-sea sea cucumbers
Two new species, Pseudostichopus langeae and Psolus griffithsi, and a new South African record, Molpadia musculus Risso, are described from some deep-sea material collected off the South African west and south coasts. This material also contains the well known Pseudocnella insolens (Théel), Ocnus capensis (Théel), Rhopalodinopsis capensis Heding and an indeterminate Thyone sp.
The origin and evolution of the faunas inhabiting deep-sea hydrothermal vents and methane seeps have been debated for decades. These faunas rely on a local source of sulfide and other reduced chemicals for nutrition, which spawned the hypothesis that their evolutionary history is independent from that of photosynthesis-based food chains and instead driven by extinction events caused by deep-sea anoxia. Here I use the fossil record of seep molluscs to show that trends in body size, relative abundance and epifaunal/infaunal ratios track current estimates of seawater sulfate concentrations through the last 150 Myr. Furthermore, the two main faunal turnovers during this time interval coincide with major changes in seawater sulfate concentrations. Because sulfide at seeps originates mostly from seawater sulfate, variations in sulfate concentrations should directly affect the base of the food chain of this ecosystem and are thus the likely driver of the observed macroecologic and evolutionary patterns. The results imply that the methane-seep fauna evolved largely independently from developments and mass extinctions affecting the photosynthesis-based biosphere and add to the growing body of evidence that the chemical evolution of the oceans had a major impact on the evolution of marine life.
Comparative ultrastructural features of the ovary and vitellogenesis have been described for six shallow water and four bathyal species of sea cucumbers representing four major holothuroid orders. Ovarian structure is similar in all ten species except for features of the peritoneal cells of the outer layer and the follicular inner epithelial cells surrounding the developing oocytes. The peritoneal cells vary from monociliated squamous or cuboidal cells to large columnar cells. Ultrastructural evidence suggests that these cells might be capable of incorporating materials from the perivisceral coelom. The follicular inner epithelial cells of two deep-sea species resemble podocytes, a feature previously unre-ported in holothuroid ovaries. It is suggested that these cells function to increase nutrient exchange between the genital haemal sinus and the oocyte during vitellogenesis. In all ten species, the oocytes appear to participate in yolk synthesis through the interaction of the Golgi complex and rough endoplasmic reticulum. The similarity in the ultrastructural features of vitellogenesis suggests that the process of yolk synthesis has been highly conserved in holothuroids. Endocytotic activity was detected in seven of ten species but it is uncertain if this is directly related to vitellogenesis. Cilia and intracellular structures resembling striated ciliary rootlets were observed in the oocytes of four of the ten species studied. The significance of this finding is unclear but could indicate that germ cells have a somatic cell origin.
Did shifting seawater sulfate concentrations drive the evolution of deep-sea methane-seep ecosystems?