INTRODUCTIONThe occurrence of dense beds of the brittle-star Ophiothrix fragilis (Abildgaard) in certain localized areas around the British Isles has been reported by several authors (Allen, 1899; Vevers, 1952; Cabioch, 1961; Holme, 1961; Brun, 1969), with population densities varying from 340/m2 (Vevers, 1952) to 2196/m2 (Brun, 1969). These populations seem to be permanent features of the sea bed since the aggregation sampled by Allen was in the same position as that photographed by Vevers fifty years later.

Aratus pisoni (Milne Edwards) breeds throughout the year according to a lunar rhythm; the hatching of the eggs occurs at both full and new moon. Females become mature at about six months old or 12 mm carapace width and from then on, ovulate, on the average, once every intermoult; the number of eggs laid is directly proportional to the volume of the crab. By the time a female is eighteen months old at a size of 18 mm carapace width it will have laid 27,500 eggs and passed through seven moults. Mortality is greatest during the larval life and is caused mainly by predation.

We present new constraints on the opening of the South Atlantic Ocean from a joint interpretation of marine magnetic anomaly grids and forward modelling of conjugate profiles. We use 45,000 km of recently collected commercial ship track data combined with 561,000 km of publically available data. The new data cover the critical ocean–continental transition zones and allow us to identify and downgrade some poorly navigated older ship tracks relied upon in earlier compilations. Within the final grids the mean cross-over error is 14 nT computed from 8,227 ship track intersections. The forward modelling used uniformly magnetised bodies whose shapes were constrained from coincident deep-seismic reflection data. We find the oldest magnetic anomalies to date from M10r (134.2 Ma, late Valanginian) north of the Falkland-Agulhas Fracture Zone and M3 (129.3 Ma, Barremian) south of the Rio Grande Fracture Zone. Hence, assuming the GPTS used is correct, continental breakup was contemporaneous with the Parana and Etendeka continental flood basalts. Many of the landward linear anomalies overlap seismically mapped Seaward Dipping Reflectors (SDRs). We interpret this to mean that a significant portion of the SDRs overlay crust formed by subaerial seafloor spreading. Here crustal accretion is envisaged to be similar to that at mid-ocean ridges, but sheet lava flows (that later form the SDRs) rather than pillow basalts form the extrusive component. Segmentation of the linear anomalies generated implies that this stage of continental breakup is organised and parallels the seafloor spreading centre that follows. Our results call into question the common assumption that at volcanic continental margins the first linear magnetic anomalies represent the start of conventional (submarine) oceanic crustal generation

The chelae of Cancer pagurus and Macropipus depurator were examined with respect to mechanical advantage. The closer muscles were investigated with respect to sarcomere length in the constituent fibres and to the force developed by the whole muscle during isometric contraction. Cancer chelae have a relatively high mechanical advantage, 0.329 ± 001. Cancer closer muscles contain a high proportion of fibres with long sarcomeres, mean lengths mostly falling between 12 and 15 μm, and develop a maximum stress of about 496 kN.m−2 during contraction. These figures are typical for “slow” crustacean muscle. The chelae of M. depurator are dimorphic. In one, the strong chela, the mechanical advantage is 0.248 ± 0.066 while in the other, the fast chela, the mechanical advantage is 0.177 ± 0.006. M. depurator closer muscles contain fibres with mean sarcomere lengths mostly falling between 6 and 10 μm. The muscle develops a maximum stress of about 145 kN.m−2 during contraction. These figures are typical of “intermediate” crustacean muscles. “Fast” muscle fibres with short sarcomeres (about 30 um) were found in the chelae of both Cancer and M. depurator but were much commoner in the latter. Thus in Cancer a high mechanical advantage is correlated with slow muscle while in M. depurator lower mechanical advantages are broadly correlated with faster muscle. Consistent correlation between mechanical advantage and muscle type in the dimorphic chelae of M. depurator, however, is lacking. No consistent regionation of fibres with similar properties was found in the muscles.

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Basket-stars are suspension-feeders and suspension-feeding by other brittle-stars was suspected long ago on circumstantial grounds, but it is only in the last twenty years that it has been actually demonstrated, and detailed descriptions of suspension-feeding behaviour are still available for only a handful of species. Blegvad (1914) pointed out that Ophiothrix fragilis (Abildgaard) and other species lead sedentary lives and feed largely on detritus on, or near, the sea-floor. Wintzell (1918) explicitly suggested tha and Ophiopholis aculeata (L.) are suspension-feeders, primarily on the grounds that they are commonly found in current-swept localities where plankton is rich, in association with known suspension-feeders such as Modiolus, Lima and Alcyonium. The same suggestion was made by Vevers (1952) who noted the occurrence of dense aggregations of O. fragilis in offshore, current-washed areas near Plymouth, and who later observed suspension-feeding behaviour in that species and in Ophiocomina nigra (Abildgaard) (Vevers, 1956). This was only the second time that suspension-feeding had been documented for any (ophiurid) brittle-star, but now it has been described in about a dozen species and suspected in others, mostly members of the Amphiuridae, Ophiactidae, Ophiothricidae and Ophiocomidae (MacGinitie, 1949; Kuznetsov & Sokolova, 1961; Moore, 1962; Magnus, 1964; Buchanan, 1964; Fontaine, 1965; Fricke, 1970; Pentreath, 1970).

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