The genome of an apodid holothuroid (Chiridota heheva) provides insights into its adaptation to deep-sea reducing environment

Cold seeps and hydrothermal vents are deep-sea reducing environments that are characterized by a lack of oxygen, photosynthesis-derived nutrients and a high concentration of reducing chemicals. Apodida is an order of deep-sea echinoderms lacking tube feet and complex respiratory trees, which are commonly found in holothurians. Chiridota heheva Pawson & Vance, 2004 (Apodida: Chiridotidae) is one of the few echinoderms that resides in deep-sea reducing environments. Unlike most cold seep and hydrothermal vent-dwelling animals, C. heheva does not survive by maintaining an epi- or endosymbiotic relationship with chemosynthetic microorganisms. The species acquires nutrients by extracting organic components from sediment detritus and suspended material. Here, we report a high-quality genome of C. heheva as a genomic reference for echinoderm adaptation to reducing environments. Chiridota heheva likely colonized its current habitats in the early Miocene. The expansion of the aerolysin-like protein family in C. heheva compared with other echinoderms might be involved in the disintegration of microbes during digestion, which in turn facilitates the species' adaptation to cold seep environments. Moreover, several hypoxia-related genes were subject to positive selection in the genome of C. heheva, which contributes to their adaptation to hypoxic environments.

Studies on an endoparasitic ciliate Boveria labialis (Protozoa: Ciliophora) from the sea cucumber, Apostichopus japonicus

The morphology and infraciliature of an endoparasitic ciliate, Boveria labialis, isolated from respiratory trees of the sea cucumber, Apostichopus japonicus, were investigated using living observation and silver impregnation methods. Based on the present and previous studies, an improved diagnosis is supplied: marine Boveria, size in vivo about 30–100×15–30 μm, body slender and flask-shaped, with a protruding lobe measuring 5–15 μm in length; one ovoid macronucleus and one micronucleus; single contractile vacuole positioned in posterior 1¼ of body length; 17–26 somatic kineties; paroral membrane and membranelle 2 forming a double anticlockwise spiral of nearly two turns.

Origin of the holothurians (Echinodermata) derived by constructional morphology

According to a recent hypothesis, the holothurians originated by a process of extreme paedomorphism as "giant larvae", with "<i>de novo</i>" developed radial systems. However, the present approach, which follows the principles of constructional morphology, supports former views that the holothurian predecessor must have been echinoid-like. After constitution of a (reliable) early predecessor construction as a model with machine analogies, subsequent steps of structural transformation are explained by functional improvement and economy. Following results are discussed: (i) Holothurians have to be derived from a postlarval precursor; (ii) "Apodida" (as molecular-genetically derived first holothurians) must originally have been pedate; (iii) ophiocistioids would not be cladistic “holothurians” but a precursor construction of the taxon echinoids plus holothurians; (iv) the Lovenian structure of the calcareous ring of <i>Nudicorona</i> (Middle Devonian), possible radial series in <i>Palaeocucumaria</i> (Lower Devonian), and distribution of the podia in two new holothurian body fossils from the Lower and Middle Devonian (preliminary description as <i>Prokrustia tabulifera</i> n. gen., n. sp. and <i>Podolepithuria walliseri</i> n. gen., n. sp.) obviously corroborate homology of holothurian and other echinoderm radial systems; (v) different extent of podial and body wall skeletonization suggests the existence of respiratory trees by no later than the Middle Devonian. <br><br> Nach einer neueren Erklärung, die sich auf eine Theorie zur Homologisierung von larvalen und adulten Strukturen von Echinodermen stützt, sollen die Holothurien über extreme Paedomorphose, d.h. über „Riesen-Larven” mit neugebildeten postoralen Radial-Systemen entstanden sein. Dagegen läßt sich anhand eines konstruktions-morphologischen Verfahrens zeigen, daß Holothurien auf einen Echiniden-artigen Vorläufer zurückzuführen sind. So wird zunächst eine (wahrscheinliche) frühe Vorläufer-Konstruktion der Echinozoen nach Maschinen-Analogien konstituiert. Die daran anschließenden strukturellen Transformationen werden nach funktionellen und energetischen Kriterien begründet. Sie führen zwanglos zu Konstruktionen, die nicht nur rezenten Formen entsprechen, sondern offensichtlich auch durch bekannte und neue paläozoische Fossilien bestätigt werden. Im Einzelnen werden folgende Ergebnisse zur Diskussion gestellt: (i) Holothurien sind von einer post-larvalen Vorläufer-Konstruktion abzuleiten; (ii) die füßchenlosen „Apodida” (als molekulargenetisch früheste Holothurien) müssen zunächst vollständige Radien mit Podia besessen haben; (iii) bei den (nur paläozoischen) Ophiocistioiden handelt es sich nicht kladistisch um „Holothurien”, sondern sie repräsentieren eine Vorläufer-Konstruktion des Taxon Echiniden plus Holothurien; (iv) die Loven'sche Struktur des Schlundrings von <i>Nudicorona</i> (Mittel-Devon), die möglicherweise radialen Strukturen bei <i>Palaeocucumaria</i> (Unter-Devon) und die Verteilung der Podia in zwei neuen, vollständig erhaltenen Holothurien aus dem Unter- und Mittel-Devon (vorläufige Beschreibung als <i>Prokrustia tabulifera</i> n. gen., n. sp. und <i>Podolepithuria walliseri</i> n. gen., n. sp.) stützen die konstruktions-morphologische Begründung der Homologie der Radial-Systeme bei Holothurien und den übrigen Echinodermen; (v) das deutlich unterschiedliche Ausmaß der Skelettierung von Podia bei den neuen Holothurien-Funden scheint anzudeuten, daß die analen Respirations-Strukturen der Holothurien spätestens ab dem Mittel-Devon vorhanden sind. <br><br> doi:<a href="http://dx.doi.org/10.1002/mmng.20020050110" target="_blank">10.1002/mmng.20020050110</a>

Respiration and biometry in the sea cucumber Holothuria forskali

Relationships between wet body weight, dry body weight and ash-free dry body weight (AFDW) were established for the aspidochirote sea cucumber Holothuria forskali (Echinodermata: Holothuroidea); a wetdry weight ratio of 6–38:1 was found. Length-weight relations were also determined. Low oxygen tensions and mechanical trauma induced H. forskali to eviscerate (70% of individuals tested). Respiratory measurements of intact and eviscerated sea cucumbers were determined at 17°C. For intact animals, oxygen consumption (ul h1) was directly related to AFDW (the slope of the regression line, b=0–60), whereas weight-specific oxygen consumption (Vo2; ul g1AFDW h) was inversely related to AFDW (b=0–54). Oxygen consumption of eviscerated sea cucumbers was independent of AFDW (b=0-\5), but Vo 2 was inversely related to AFDW (t–0–85). There were no significant differences between the respiratory rates of intact and eviscerated individuals, indicating that H. forskali is not so dependent upon respiratory trees for oxygen uptake as previously assumed.

The relative significance of body surface and cloacal respiration in Psolus fabricii (Holothuroidea: Dendrochirotida)

Oxygen consumption at 10 °C was measured for Psolus fabricii, in whole animals and in specimens with the cloaca blocked to prevent use of the respiratory trees. In both sets of experiments, oxygen consumption was exponentially related to body size, but larger animals exhibited a significantly greater proportion of oxygen exchange by cloacal respiration. An animal of 80 g wet weight achieves 75% of its respiratory needs by actively pumping water through the cloaca, whereas body surface respiration alone is calculated to be adequate for a 1.9-g animal. It appears that because of their success in shallow areas of dynamic water motion, adult Psolus depend less than other holothurians on body surface respiration.

Seasonal atrophy of the visceral organs in a sea cucumber

The gut, gonad, respiratory trees, and circulatory system of the commercial sea cucumber Parastichopus californicus are annually lost as a result of atrophy of these organs and not, as originally supposed, through spontaneous, seasonal evisceration. Visceral loss is preceded by cessation of feeding–locomotory behaviour. Torpor ensues, and the visceral tissues are absorbed through a progressive process which includes phagocytosis by the sea cucumber's coelomocytes and, in some instances, the scavenging activities of endosymbionts. Regeneration of the viscera occurs within several weeks. Similar seasonal atrophy of the visceral organs has not been reported to occur in other coelomate organisms. We hypothesize that visceral atrophy in P. californicus is an expression of seasonal diapause induced by reduced food availability.