sampled again, although it was mentioned by several authors (Clessin,
1889; Locard, 1886, 1899) and more recendy by Lamy (1937) and Ic
Danois (1948) with probable reference to 'Le Travailleur" collection.
According to early and recent observations in the southeastem bay of Biscay, M. fischeri was exclusively sampled on muddy slope bottoms between
857 and 1190 m. Such a narrow bathymetric zonation of deep benthic
species was also depicted by Rowe & Menzies (1969) and attributed to
along-slope larval dispersal due to bottom currents parallel to isobaths.
The Cantabrian mytilid populations are under the influence of the Mediterranean overflow water that flows northwards around the Iberian Peninsula between 700 and 1300 m (Durrieu de Madron et al., 1999). In
the eastern bay of Biscay, this water mass is characterized by a potential
temperature of approximately 9.8°C, by a salinity maximum (35.7535.76) and an oxygen minimum at about 1000 m water depth. The
Mediterranean water was also detected within the upper part of the Capbreton canvon up to approximately 650 m where a salinitv of 35.73 was
measured in the near-bottom water (Ogawa & Tauzin, 1973). Furthermore, in that part of the canyon (laetween l°50’ antl 1°58’W), low bottom oxygen contents were also measured in November 1972 (minimum
value: 1.96 ml 1 -1 at 20 cm above the sea floor) and attributed to some
water confinement due to canyon morphology (Ogawa & Tauzin,
1973). These peculiar near-bottom environmental conditions have a
structuring influence on the underlying benthic communities and are
probably responsible for the local relative abundance of a few “canyon
indicator species” including M.fischeri (see Marquiegui & Sorbe, 1999).
M. fischeri was apparently absent on the open slope north from the
Capbreton canyon, as sugrested by negative sled samplings at approximately 1000 m depth (Plateau des Landes off Mimizan, Oxybent programme; southern margin of the Cap-Ferret canyon off Arcachon, Elizalde
& Sorbe, unpublished observations) as well as at approximately 2400 m
depth within the Cap-Fenst canyon (ECOFER programme). Finally, according to substratum characteristics (soft bottoms) and byssus examination (presence of mud at the tip of the byssal filaments), this filterfeeding mytilid lies on die mud in an epibenthic habitat as also demonstrated by aquarium video recording on live specimens (Massabuau &
Corbari, pers. comm.).
The 375 specimens sampled at station J were used for biometric measurements and histological studies. /\t the laboratorv, all intact shells were
measured either with a hand-held calliper accurate to 0.03 mm (largest
specimens) or with a micrometer eyepiece with a precision of 0.02 mm
(smallest specimens). Shell measurements (mm) were recorded following
the method given by Maas et ai (1999) for mytilid species : maximum
length 1, maximum height H (perpendicular to L measurement),
maximum width W (greatest dimension through both valves). The sizefrequency histograms drawn from these measurements (fig. 3) depicted
the demographic structure of the rmtilid population, characterized by a
98
1889; Locard, 1886, 1899) and more recendy by Lamy (1937) and Ic
Danois (1948) with probable reference to 'Le Travailleur" collection.
According to early and recent observations in the southeastem bay of Biscay, M. fischeri was exclusively sampled on muddy slope bottoms between
857 and 1190 m. Such a narrow bathymetric zonation of deep benthic
species was also depicted by Rowe & Menzies (1969) and attributed to
along-slope larval dispersal due to bottom currents parallel to isobaths.
The Cantabrian mytilid populations are under the influence of the Mediterranean overflow water that flows northwards around the Iberian Peninsula between 700 and 1300 m (Durrieu de Madron et al., 1999). In
the eastern bay of Biscay, this water mass is characterized by a potential
temperature of approximately 9.8°C, by a salinity maximum (35.7535.76) and an oxygen minimum at about 1000 m water depth. The
Mediterranean water was also detected within the upper part of the Capbreton canvon up to approximately 650 m where a salinitv of 35.73 was
measured in the near-bottom water (Ogawa & Tauzin, 1973). Furthermore, in that part of the canyon (laetween l°50’ antl 1°58’W), low bottom oxygen contents were also measured in November 1972 (minimum
value: 1.96 ml 1 -1 at 20 cm above the sea floor) and attributed to some
water confinement due to canyon morphology (Ogawa & Tauzin,
1973). These peculiar near-bottom environmental conditions have a
structuring influence on the underlying benthic communities and are
probably responsible for the local relative abundance of a few “canyon
indicator species” including M.fischeri (see Marquiegui & Sorbe, 1999).
M. fischeri was apparently absent on the open slope north from the
Capbreton canyon, as sugrested by negative sled samplings at approximately 1000 m depth (Plateau des Landes off Mimizan, Oxybent programme; southern margin of the Cap-Ferret canyon off Arcachon, Elizalde
& Sorbe, unpublished observations) as well as at approximately 2400 m
depth within the Cap-Fenst canyon (ECOFER programme). Finally, according to substratum characteristics (soft bottoms) and byssus examination (presence of mud at the tip of the byssal filaments), this filterfeeding mytilid lies on die mud in an epibenthic habitat as also demonstrated by aquarium video recording on live specimens (Massabuau &
Corbari, pers. comm.).
The 375 specimens sampled at station J were used for biometric measurements and histological studies. /\t the laboratorv, all intact shells were
measured either with a hand-held calliper accurate to 0.03 mm (largest
specimens) or with a micrometer eyepiece with a precision of 0.02 mm
(smallest specimens). Shell measurements (mm) were recorded following
the method given by Maas et ai (1999) for mytilid species : maximum
length 1, maximum height H (perpendicular to L measurement),
maximum width W (greatest dimension through both valves). The sizefrequency histograms drawn from these measurements (fig. 3) depicted
the demographic structure of the rmtilid population, characterized by a
98