To what extent can specialized species succeed in the deep sea? The biology and trophic ecology of deep-sea spiny eels (Notacanthidae) in the Mediterranean Sea

Abstract

The population structure, reproductive biology and feeding ecology of the two notacanthids inhabiting the deep Mediterranean, Notacanthus bonapartei and Polyacanthonotus rissoanus, were analyzed in the Balearic Basin at depths from 579 to 2233m (mainly pristine depths or subjected to low fishing impact), including seasonal variations. Preferred average depths (Centres of Gravity, CoG) of Notacanthusbonapartei were situated over the middle slope (942m) and of P. rissoanus on the lower slope (1680m). For both species, bigger individuals collected at the deepest depths had the highest values of a gonadosomatic index (GSI), suggesting that in reproductive periods adults migrate into deeper waters. The reproductive season for N. bonapartei was late summer and autumn; that of P. rissoanus was narrower, restricted to summer. N. bonapartei exploited benthic prey, among identified prey mainly echinoderms (e.g., Penilpidia ludwigi, Hymenodiscus coronata) and sessile benthic organisms (e.g., actinians and polyps of the bamboo coral Isidella elongata). Consumption of bamboo coral polyps by N. bonapartei constitutes a unique specialized trophic strategy and a direct link with such corals. Some differences in the diet composition related to depth were observed, as were a few differences related to periods of water-column stratification and homogenization. Gut fullness (F) of this species was mainly correlated with surface Chlorophyll a recorded 2–3 months before sampling date and somewhat but less so with river discharges 1 month before sampling. That suggests that vertical flux of organic matter was the food source for prey exploited by N. bonapartei. Diet of P. rissoanus was based on epibenthic-suprabenthic crustaceans, e.g. tanaids (Apseudes spp.), isopods (Munnopsurus atlanticus) and amphipods (Rhachotropis caeca) and on polychaetes (Polynoidae, Harmothoe spp.), all these prey more mobile than consumed by N. bonapartei. No significant differences in diet composition were found related with either depth or homogenization/stratification of the water column. This lack of changes in diet is probably attributable to the greater stability of the lower slope where P. rissoanus lives. Gut fullness was mainly correlated with surface Chlorophyll a recorded simultaneously with the fish sampling. Lactate dehydrogenase (LDH) activity was similar in the muscle of the two notacanthids (N. bonapartei=3.72–8.75µmol/min/mg prot; P. rissoanus=7.56µmol/min/mg prot). Values for N. bonapartei were the highest found compared to other deep-sea fish in the deep Mediterranean. This could be related with the special feeding behaviour of this species when it removes sessile prey from substrate.