Eve C. Southward

Food sources, behaviour, and distribution of hydrothermal vent shrimps at the Mid-Atlantic Ridge

Five species of bresilioid shrimp were investigated at seven hydrothermal sites on the Mid-Atlantic Ridge: Menez Gwen, Lucky Strike, Rainbow, Broken Spur, TAG, Snake Pit and Logatchev. Samples were prepared for analysis of stable isotopes, elemental composition and lipids. Shrimp behaviour was observed from the submersible ‘Alvin’ and in the laboratory aboard RV ‘Atlantis’. The distribution and zonation of the shrimp species was recorded. Juvenile shrimp of all species arrive at the vents carrying reserves of photosynthetic origin, built-up in the pelagic larval stages. These reserves are used while the shrimp metamorphose to the adult form and, in Rimicaris exoculata and Chorocaris chacei , while they develop epibiotic bacteria supporting structures, the modified mouthparts and the inside of the carapace. The main food of adult R. exoculata is filamentous bacteria that grow on these structures. The intermediate sizes of C. chacei also feed on such bacteria, but the final stage gets some food by scavenging or predation. Mirocaris species scavenge diverse sources; they are not trophically dependent on either R. exoculata or mussels. Adults of Alvinocaris markensis are predators of other vent animals, including R. exoculata . The dense swarms of R. exoculata , with their exosymbionts, can be compared to endosymbiont-containing animals such as Bathymodiolus and the vestimentiferan tube-worms of the Pacific vents. Such associations, whether endo- or ectosymbiotic, may be necessary for the development of flourishing communities at hydrothermal vents.

On the distribution of the intertidal barnacles Chthamalus stellatus, Chthamalus montagui and Euraphia depressa

There are three species of chthamalids in Europe. Two of them, Chthamalus stellatus and Chthamalus montagui , overlap extensively in geographic range: they occur together along the whole Atlantic coasts of Britain, Ireland, France, Spain and N. Africa, and both also occur in the Mediterranean Sea. They have almost identical north-eastern limits to their distribution in the English Channel and N. Scotland, but C. stellatus may extend farther south along the W. African coast. These two species also show overlap in their vertical distribution, though C. montagui is usually commoner in the upper barnacle zone while C. stellatus may be dominant lower down. The species are separated by habitat, C. stellatus favouring wave-beaten open coast sites, C. montagui more embayed situations, but there is still extensive overlap. The third European species of chthamalid, Euraphia depressa, has a more restricted vertical distribution and is almost entirely confined to the Mediterranean Sea, where it favours high-intertidal levels in wave-beaten places, and also cryptic habitats.

Bacterial Symbionts in Pogonophora

Prokaryote organisms, with characteristics of Gram-negative bacteria, occur intracellularly in Pogonophora, as described here for seven small species. The tissue containing the bacteria lies between the two longitudinal blood vessels in the posterior part of the trunk and has a special blood supply. This tissue is probably homologous with the so-called trophosome tissue of the much larger vestimentiferan pogonophores, which also contains bacteria, and the term can be applied to all pogonophores. The presence of such bacteria-containing trophosome tissue may be a characteristic of the phylum. In both large and small species examined the bacteria appear to be chemoautotrophs and probably assist the nutrition and/or metabolism of their hosts. It is not yet certain if the bacterium-containing cells do originate from mesoderm or endoderm, but, if the latter, then the trophosome represents the remains of the missing gut. The trophosome tissue situated internally, and transfer of bacteria must take place early in the life history, in the egg or embryo.

Molecular evolution and diversification of the vestimentiferan tube worms

The Vestimentifera, or deep-sea tube worms, comprise an ecologically and anatomically unusual group of marine invertebrates, with poorly understood biogeography, ecology, phylogenetic affinities and evolutionary radiation. To gain insight into evolutionary diversification within the group, we have used a molecular biological approach. We report the cloning of a region of 28S ribosomal DNA from representatives of five vestimentiferan genera plus, for comparison, a polychaete and a perviate pogonophore. Phylogenetic analyses using these DNA sequences confirm that Ridgeia and Tevnia are closely related genera. The analyses also lead us to propose the hypothesis that the earliest vestimentiferan lineage to diverge gave rise to the genus Lamellibrachia only. In addition, our comparative DNA sequence data now provide a means to use molecular methods for identification of deep-sea tube worms; we employed this approach to demonstrate that the first vestimentiferan specimen from the eastern Atlantic Ocean belongs to the genus Lamellibrachia . DNA-based identification should have wide applications in the study of vestimentiferan biogeography and ecology.

A Comparative Study of the Setae of Pogonophora and Polychaetous Annelida

Pogonophora are long thin tube-dwelling invertebrates similar in size to polychaetous annelids, but lacking both mouth and gut, and found mainly in the deep sea. Current discussion of their systematic position had revealed a need for more information about particular aspects of their morphology and embryology (Southward, 1971 a ). The earlier attribution of Pogonophora to the echinoderm-chordate group, Deuterostomia, based mainly on their apparently tricoelomate structure, has been questioned recently, following the discovery of the previously unknown posterior end (Webb, 1964 a ; Ivanov, 1965). This posterior end is a small and delicate structure composed of several coelomate segments separated by septa and provided with short bristles. It has such a strong resemblance to the posterior end of an annelid, though of course it does not contain a gut, that there has been much speculation about the possibility of a relationship between Pogonophora and Annelida and some reconsideration of the features thought to indicate affinity to the Deuterostomia (Livanov & Pornrieva, 1967; Webb, 1969 b ; Ivanov, 1970; Norrevang, 1970 a, b ; Southward, 1971 a ).

Fatty acids of hydrothermal vent Ridgeia piscesae and inshore bivalves containing symbiotic bacteria

Ridgeia piscesae from a hydrothermal vent and lucinid and thyasirid bivalves from inshore Canadian and UK waters, known to contain sulphur-oxidizing symbiotic bacteria, had lipids rich in 16:0,16:l(n-7) and 18:l(n-7) fatty acids in both bacteria-rich trophosome or gill tissue and in tissues without symbiotic bacteria. The results are consistent with the animals deriving these fatty acids from their sulphur-oxidizing symbionts. Ridgeia piscesae, Lucinoma annulata, Parvilucina tenuisculpta, Lucinoma borealis and Myrtea spinifera also contained substantial amounts of the non-methylene-interrupted dienoic fatty acids 20:2A5,13 and 22:2A7,15. It is proposed that these fatty acids are produced by chain elongation and A5 desaturation in animal tissues of 18:l(n-7) produced by the bacterial symbionts. Thyasira flexuosa did not contain 20:2A5,13 or 22:2A7,15 but instead contained 18:l(n-ll) and 20:l(n-13) which were not present in the other species analysed. It is proposed that 18:l(n-ll) and 20:l(n-13) arise from the A9 desaturation of 20:0 and 22:0, respectively, followed by chain shortening of the mono-unsaturated fatty acid products of A9 desaturation. It is considered that 20:2A5,13 and 22:2A7,15 are formed in the animals in response to a relative excess of 16:0, 16:l(n-7) and 18:l(n-7), accompanied by a relative deficiency of (n-3) and (n-6) polyunsaturated fatty acids. The results are discussed in relation to the lipid nutrition of marine invertebrates containing bacterial symbionts.

Different energy sources for three symbiont-dependent bivalve molluscs at the Logatchev hydrothermal site (Mid-Atlantic Ridge)

The vent mussel Bathymodiolus puteoserpentis , a large vesicomyid clam and a smaller thyasirid were collected from an area of sediment subject to diffuse hydrothermal flow. The mussels live on the surface, the vesicomyids are partly buried and the thyasirids burrow in the sediment. The fine structure of the gills differs in the three bivalves. Bathymodiolus puteoserpentis hosts two types of bacterial symbiont, one methanotrophic, and another probably thiotrophic. The other two bivalves have single types of symbiont of different shapes. Stable isotope ratios of carbon and nitrogen indicate thiotrophy in the vesicomyid and thyasirid, but a predominance of methanotrophy in the mussel. This is the first time that such an assemblage has been found at a hydrothermal site on the Mid-Atlantic Ridge (MAR), with the different faunistic elements exploiting different energy resources.

Further experiments on the value of dissolved organic matter as food for Siboglinum fiordicum (Pogonophora)

Adult and larval stages of Siboglinum fiordicum, collected from 32 to 35 m depth, accumulate measurable quantities of amino acids and glucose from low concentrations. The amino acids are absorbed against a considerable gradient. The glucose and the amino acids are metabolized in the tissues and substantial amounts are respired to give carbon dioxide or volatile organic acids. Under the experimental conditions almost all the metabolism follows aerobic pathways.

Ocean-ridge segmentation and vent tubeworms (Vestimentifera) in the NE Pacific

Abstract Vestimentiferan tube worms are important components of the hydrothermal vent and cold seep communities of the Pacific Ocean. The distribution and geographic intraspecies variation of Ridgeia piscesae and Lamellibrachia barhami were examined in the region of the Explorer (1 site), Juan de Fuca (33 sites) and Gorda (2 sites) Ridges and the nearby Cascadia Subduction zone. Isozyme electrophoresis, DNA restriction fragment length polymorphism (RFLP) and DNA sequencing techniques have been used. Ridgeia piscesae is widespread at hot vent sites along the ridges. The transform offset between Explorer and Juan de Fuca does not appear to impede gene flow (isozyme and RFLP data). Hydrographic conditions close to the Juan de Fuca Ridge favour bidirectional along-axis transport of planktonic larvae and some cross-axis transport. Rapid colonization by R. piscesae has been observed at a new vent site on CoAxial Segment of the Juan de Fuca Ridge, supporting the idea of a pool of larvae in the water overlying the ridge. The 360 km offset between southern Juan de Fuca and Gorda Ridges is associated with a significant level of genetic differentiation (shown by RFLP) which indicates some interruption to larval dispersal at this scale. The occurrence of the cold-seep species Lamellibrachia barhami is confirmed at one hydrothermal site, the sedimented Middle Valley on Juan de Fuca Ridge. Vestimentiferans appear to have a tremendous dispersal, location and adaptation capability. It is only at the thousand kilometre scale, that complete barriers to dispersal occur.

Vestimentiferans (Pogonophora) in the Pacific and Indian Oceans: a new genus from Lihir Island (Papua New Guinea) and the Java Trench, with the first report of Arcovestia ivanovi from the North Fiji Basin

Explorations by the German Research Vessel Sonne in the fore-arc and back-arc basins of the western Pacific Ocean have collected vestimentiferan tube worms from both warm vent and cold seep sites. Edison Seamount is a small volcanic cone on the southern flank of Lihir Island, in the Tabar-Feni island chain. Beds of vesicomyid clams on the summit (1450 m depth) are associated with hydrothermal effluent, whereas an uplifted scarp (1600 m depth) nearby is covered by low temperature gas-rich sediments. A methane anomaly has been detected in the water column above the scarp. The benthic fauna includes vestimentiferan tubeworms and bathymodiolid mussels. Bottom photographs show that the vestimentiferans occur singly or in small groups. Four specimens were collected by TV-guided grab. They are described as a new species of a new genus, related to the cold-seep genus Escarpia, of the family Escarpiidae. A single specimen obtained from the landward slope of the Java Trench (1500m) is identified as the same species, extending its area of distribution by some 6000 km westward. Arcovestia ivanovi, already known from hydrothermal vents in the Manus Basin, has now been obtained from a hydrothermal site in the North Fiji Basin.