No abstract

No abstract

Feeding habits of blackmouth catshark Galeus melastomus and velvet belly lantern shark Etmopterus spinax were studied throughout the Spanish Mediterranean, from the Alboran Sea to the Gulf of Lion, between 400 and 790 m depth. Diets were studied taking into account size and depth differences. Included within the trophic guild of non-migratory macroplankton feeders, both species preferably exploited mesopelagic resources (mainly natantian decapods, euphausiids, mesopelagic fish and cephalopods). G. melastomus mostly preyed on decapod crustaceans (46% in terms of IRI), with cephalopods, euphausiids and mesopelagic fish as a secondary prey item. The diet of E. spinax was composed primarily of mesopelagic fish (61.4% in terms of IRI), with decapod crustaceans and cephalopods of secondary importance. Both species showed ontogenetic changes in their diets: small blackmouth catshark specimens (between 150 and 350 mm total length) mainly consumed cephalopods, medium size individuals (351-450 mm TL) consumed decapod crustaceans, while larger specimens (larger than 451 mm TL) seemed to be more generalist-feeders. Smaller specimens of E. spinax (150-250 mm TL) mostly fed on small crustaceans and cephalopods, whilst an increase in the consumption of mesopelagic fish (mycthophids and Stomiiformes) was detected in larger individuals (251-450 mm TL). Diet of G. melastomus also changed throughout the narrow depth range explored, mainly consumed euphausiids and mesopelagic fish between 400 and 500 m depth, whilst preferably exploiting natantian decapods and cephalopods below 500 m of depth. However, this trend may be correlated to the larger-deeper trend found for this species. Slight but not significant differences were found in E. spinax diet by depth, with euphausiids mainly consumed at lower depths (400-500 m). In a multispecies MDS analysis, diets of G. melastomus and E. spinax were separated and the ANOSIM ANOSIM test proved evidence for significant differences in the diets of the two species (R = 0.25; P = 0.05), mainly attributed to the stronger pelagic habits of E. spinax in comparison with G. melastomus. Low overlap (by Schoener Index) also occurred when comparing specimens of the same size range. In general, the higher occurrence of benthic prey in the diet of G. melastomus (i.e. the brachyuran crab Geryon longipes, the thalassinid shrimp Calocaris macandrae) than in E. spinax pointed to a stronger pelagic behavior for the velvet belly lantern shark. Both multivariate analysis and the Levins Index pointed to a narrow niche breadth for the two sharks. A trend of increasing fullness was found for both species in the highly productive areas of the Alboran Sea and Vera Gulf, probably related to higher resource availability, enhanced by local upwellings.

Deception Island has traditionally been considered as a collapse caldera formed by subsidence into a magma chamber of a group of overlapping volcanoes along arcuate and radial faults. In fact, the morphological features of Deception Island (horseshoe shape, location of post-caldera vents apparently along concentric faults, existence of a depression in the centre of the island, concentration of post-caldera activity along the ‘ring fault’, etc.) support this idea. However, a detailed revision of the structure of Deception Island combining field geology, high resolution seismic profiles and analysis of local and regional seismicity, indicates that most of the structural features identified as evidence for supporting the model of caldera formed by collapse of a magma chamber have been misunderstood. Post-caldera volcanic activity is not restricted to the border of the depression, but appears both inside (submarine volcanism) and outside (subaerial volcanism) the hypothetical ring fault. Nearly all the post-caldera vents are located on linear faults, and seismic profiles indicate that most of these linear faults, which are parallel or normal to the spreading axis of Bransfield Strait, can be traced outside of the island. The postulated ring fault results from these intersecting linear faults in the interior of the island where they define a depression. No arcuate faults were identified on the border of the depression. The structure of the depressed sector of the island is also defined by several blocks limited by a nearly orthogonal network of normal faults. This geometry is also indicated by the epicentral location of more than 100 seismic events, which define a linear distribution of earthquakes following the main regional normal faults across the entire island. This contrasts with the circular pattern found in collapse calderas with well defined ring fault systems (i.e. Long Valley, Rabaul, Campi Flegrei). Moreover, the presence of radial faults, postulated by previous authors, has not been proved. The existence of a well developed system of radial faults reflects the pushing action of a shallow magma chamber and should normally imply the intrusion of dykes through these fractures. Very few dykes, always of basaltic composition, have been identified in Deception Island and were intruded along linear regional faults. Because the central depression exists we cannot deny the existence of the caldera depression at least in a morphological sense. However, we propose a mechanism of caldera formation contrasting to the one proposed by previous authors. The tectonic features of Deception Island indicate the existence of a tensional stress field in all directions at the surface, compatible with the regional stress field which has characterized the Bransfield Strait at least during the last 4 Ma. This favoured the normal faulting of the island and the subsidence of the central blocks without the formation of any ring fault system. After the formation of the depression at the centre of the island submarine and subaerial volcanic activity has continued along the nearly orthogonal network of normal faults

No abstract