Armando J. Almeida

A review of the distribution of hydrothermal vent communities along the northern Mid-Atlantic Ridge: dispersal vs. environmental controls

Until 1985, seven vent fields were described from the Mid-Atlantic Ridge (MAR). An eighth field, Mount Saldanha (36° N), discovered in 1998, showed unusual geological and biological settings. Vent sites on the MAR exhibit varied environmental conditions, resulting from depth variation of the axis and associated physical parameters, and different source rocks. These could be considered as first order (i.e. most dominant) factors affecting the composition of vent communities on the MAR, in contrast to the East Pacific Rise (EPR) where geographical isolation appears to be a major determinant of faunal differences. In this paper, the geological setting and vent fluid composition of the fields are considered together with their community composition to tentatively ascertain the order of a hierarchy between dispersal and environmental control. The deepest fields (>3000 m) are rather stable systems. The shallower fields, especially Rainbow and Menez Gwen, present some evidence of instability in time and space. The variability in fluid composition is related to phase separation processes (boiling/distillation of subsurface vent fluids) and to the nature of the basement rocks. Depending on depth, phase separation produces gas-enriched and metal-depleted fluids (Menez Gwen, Lucky Strike) or metal-enriched brines (Rainbow, TAG). In addition, high methane content characterises the fluids formed in ultramafic rocks (Rainbow, Logatchev) compared to basaltic rocks. The discrepancy in mineral particulate fluxes at Lucky Strike and Menez Gwen, on one hand, and TAG and Rainbow, on the other, is correlated to the predominance of the vapour or brine phase. The semi-quantitative description of the faunal composition of the different vent fields displays a continuum from Rimicaris-dominated to Bathymodiolus-dominated assemblages. Rather than geographic or bathymetric zonation, this gradation appears to be related to the metal content of the fluids. In addition, the penetration of non vent species into the vent environment increases with decreasing hydrostatic pressure and/or metal content in the fluids. Similarity analysis between vent communities shows that similarity is strongest between Menez Gwen and Lucky Strike (the shallowest fields), less significant between these sites and Rainbow, and weakest for Snake Pit. The inverse relationship between filter feeding organisms and metal concentration in vent fluids could result from a hindrance of mussel bed development by particulate or toxic metal fluxes, and has to be further investigated. Conversely, high metal and particulate content would less affect the more mobile Rimicaris populations. Considering specific similarities of endemic fauna between the four best known hydrothermal vents, the distance between vent fields appears to be a first order parameter. Nevertheless, within the proximity of the Azores Triple Junction area, and in the absence of geographical discontinuity, the similarity between fields stays rather low suggesting faunal islands that have distinct composition and habitat requirements.

NEW RECORDS OF GREY CUTTHROAT, SYNAPHOBRANCHUS AFFINIS (ACTINOPTERYGII: ANGUILLIFORMES: SYNAPHOBRANCHIDAE) FROM THE EASTERN-CENTRAL ATLANTIC OCEAN

Following a series of surveys with bottom longlines and fish traps along the island slopes off the arch- ipelagos of Madeira, Canaries and Cape Verde, and with bottom trawls along the coast of Morocco and Western Sahara between 2004 and 2006 many specimens of Synaphobranchus were caught, revealing the presence of two species. Based on these specimens and Museum preserved specimens from this area grey cutthroat, Synaphobranchus affinis Gunther, 1877, is recorded for the first time from off Portugal, Madeira archipelago, Great Meteor Seamount, Canary Islands, Morocco, Western Sahara, Cape Verde Islands, and Senegal. It is con- firmed that S. kaupii is also conspecific in this area. Meristic and morphometric data of the specimens studied are also presented.

New Species of Pachycara Zugmayer (Pisces: Zoarcidae) from the Rainbow Hydrothermal Vent Field (Mid-Atlantic Ridge)

Abstract A new species of Pachycara is described from the Rainbow hydrothermal vent field (Mid-Atlantic Ridge, 36°13′N; 33°54′W). Pachycara saldanhai n. sp. is closely related to Pachycara thermophilum Geistdoerfer, 1994, from which it differs in having a lateral line with two branches, and a higher interorbital width to head length ratio, as well as higher vertebral, dorsal- and anal-fin rays counts.

First record of hydrolagus affinis (Holocephali: Chimaeriformes: Chimaeridae) from Madeira and the seine seamount (North Atlantic Ocean)

The smalleyed rabbitfish, Hydrolagus affinis (de Brito Capello, 1868) (Chimaeridae), is recorded from the archipelago of Madeira for the first time, based on seven specimens caught between 1200 and 2500 m depth. This record extends the previously known range of distribution of this species, and also confirms its vertical dis - tribution down to 2500 m. Additional material collected from the Canary Islands and the Azores was used for comparison.

Size does matter: An assessment of reproductive potential in seahorses

In most animals, the mother plays the key role in reproduction, but male pregnancy in seahorses raises the question of whether the female still is the only determinant of offspring size or if she shares some responsibility with the male. This study evaluates the effects of both male and female size on the reproductive output of the long-snouted seahorse, Hippocampus guttulatus. Results demonstrated that, with regard to reproductive potential, the bigger the better. Seahorses preferred similar-sized or larger mates. Larger females produced bigger eggs with larger yolk reserves. Larger males had larger brood pouches, but did not produced larger broods. Male size was negatively correlated with embryo density and positively correlated with juvenile size. Both parents proved to play a decisive role in the reproductive output of this species. Newborn juveniles from the same parents were 15% bigger and 30% heavier when incubated in smaller and lower-density broods. This trade-off between the number and size of embryos inside the brood pouch clearly indicates a limited carrying capacity of the male, and demonstrates that the size of newborn seahorses can be, in part, paternally determined.