Category: Hydrocarbon Seep

Two new bivalves from chemosynthetic ecosystems

In two new lead-authored publications in Venus (Journal of the Malacological Society of Japan), we described two new bivalves from chemosynthetic ecosystems of the Western Pacific. One of them is a large vesicomyid clam ‘Calyptogena‘ marissinica Chen, Okutani, Liang & Qiu, 2018 from a methane seep in the South China Sea, and the other is Thermomya sulcata Chen, Okutani, Watanabe & Kojima from a vent in southern Mariana Trough, which is the first cuspidariid found in any chemosynthetic ecosystems.

The two new species:

‘Calyptogena’ marissinica Chen, Okutani, Liang & Qiu, 2018
VESICOMYIDAE
-1372m, Taken by ROV Haima, From ‘Haima’ methane seep, Off southern Hainan Island, Northern sector of the South China Sea, 146.9mm, Paratype #1 (NSMT-Mo 79001)
Known only from the ‘Haima’ hydrocarbon seep in the South China Sea, ‘Calyptogena’ marissinica is a large vesicomyid clam with an average shell length of about 150mm and the largest specimens exceeding 210 mm. It probably relies on endosymbiotic bacteria living inside cells of its much enlarged gills like other vesicomyids, although this warrants further research to confirm. It lives in clusters or colonies and is usually positioned half-buried in mud. Like all large vesicomyid clams, the periostracum is lighter in colouration and more glossy in young individuals. Although a little-varied species, the proportional position of umbo varies slightly among individuals even from the same colony, and the radial ridge running from the umbo to the postero-ventral corner is stronger in younger individuals. It seems to be closely related to ‘Calyptogena’ similaris from the Nankai Trough, Japan, which is far more elongate and with a more concave ventral margin. Calyptogena nanshaensis Xu & Shen 1991 is the only other large vesicomyid reported from the South China Sea, but it clearly differs from ‘C.’ marissinica in hinge morphology as well as having a straight dorsal margin, a bluntly acute posterior end, and also being much smaller in size (only up to 70 mm shell length).

Thermomya sulcata Chen, Okutani, Watanabe & Kojima, 2018
CUSPIDARIIDAE
-2849m, Snail site (12°57.189’N 143°37.166’E), Southern Mariana Trough, Leg. Shigeaki Kojima during DSV Shinkai 6500 Dive #1228 on-board R/V Yokosuka cruise YK10-11, 2010/ix/14, 8.1mm, Holotype (NSMT-Mo 78997)
Thermomya sulcata is the first ever Recent cuspidariid bivalve to be recovered from deep-sea hydrothermal vents or any chemosynthetic ecosystems, and is a handsome species characterised by a compressed, blunt rostrum and sharply raised commarginal ribs on the shell discs. Such upturned and sharp commarginal ribs are unusual among cuspidariids, and the monotypic genus Thermomya was erected to house this species. Furthermore, it is also the first member of the superorder Anomalodesmata to be found at deep-sea vents. So far only known from two specimens taken by the manned submersible DSV Shinkai 6500 from the 2849 m deep Snail hydrothermal site in the Southern Mariana Trough, but it is likely more widely distributed in the periphery of western Pacific vents where small burrowing animals are easily overlooked. It is most likely a carnivorous bivalve like other cuspidariids, using internally generated suction forces to hunt small crustaceans. The two known specimens are 8.1 mm (holotype) and 5.5 mm (paratype) in shell length. Prof. Okutani gave it an elegant Japanese name: “Yume-no-syakushi-gai”, literally meaning “Ladle Shell From Dreams”. This is in reference to the Japanese name, “Yume-Hamaguri” (= “Dream Clam”), of the famously beautiful and rare deep-water venerid clam Circomphalus hiraseanus (Kuroda, 1930) which is also characterised by sharply upturned commarginal ribs and a relatively small size within its family.

New paper on adaptation of vent/seep scale worms published!

New co-authored paper on the adaptation and evolution of vent scale worms (polynoid polychaetes) published in Scientific Reports! The article is open access and free to read here: http://www.nature.com/articles/srep46205

Scale worms inhabit a great variety of environments ranging from very shallow water down to kilometres deep and are often an important member of chemosynthetic ecosystems such as vents, often living in close proximity to hot black smokers. In this study, we sequenced the transcriptomes of two deep-sea scale worms inhabiting hydrothermal vents and hydrocarbon seeps and one shallow water counterpart that was rather closely related. By comparing the three transcriptomes, we were able to elucidate selective amino acid usage, positively selected genes, highly expressed genes, and potentially duplicated genes, thereby shedding light on how the scale worms evolved to become successful members of deep-sea chemosynthetic communities. These are the first deep-sea scale worm transcriptomes ever reported.

Highly expressed genes in B. pettiboneae (vent/seep), Lepidonotopodium sp. (vent) and H. imbricata (shallow water relative).(a) Percentage of genes participated in different cellular processes. (b) Expression level for gene groups participated in different cellular processes.

Most significant among our findings was the significance of genes related to haemoglobin. The two deep-sea polynoids chosen for this study, in the genera Branchipolynoe and Lepidonotopodium, are shown to have adopted different yet equally effective ways to cope with the oxygen-poor chemosynthetic ecosystems. Branchipolynoe rapidly evolved a novel tetra-domain haemoglobin which is highly effective in oxygen transport, whereas Lepidonotopodium increased the expression levels of standard single-domain haemoglobin to four times as high as Branchipolynoe. These results indicate that dealing with hypoxic environment is a key element in becoming successful in deep-sea vents and seeps.

Zhang Y [Yanjie], Sun J, Chen C, Watanabe HK, Feng D, Zhang Y [Yu], Chiu JMY, Qian P-Y, Qiu J-W (2017). Adaptation and evolution of polynoid scale-worms (Annelida: Polynoidae): insights from transcriptome comparison among two deep-sea and a shallow-water species. Scientific Reports, 7: 46205. http://doi.org/10.1038/srep46205