Swimming rate and responses of larvae of three mactrid bivalves to salinity discontinuities

Abstract

Straight-hinge, umbo and pediveliger larvae of the mactrid bivalves Spisula solidissima, Mulinia lateralis and Rangia cuneata were exposed to intense salinity discontinuities of 0, 5, 10 and 15 O/w in vertically oriented swimming chambers. Larvae concentrated in the region of highest gradient, i. e. at the salinity discontinuity, regardless of species, stage of development or larval brood. S. solidisima larvae, initially swimming at 30 O/w salinity, crossed discontinuities of both 5 and 10 %O but not of 15 %o. M. lateralis larvae, initially swimming at 25 %O salinity, also crossed a discontinuity of 5 %O but not of 10 or 15 %o. R. cuneata larvae, initially swimming at 10 %O salinity, generally preferred to remain at that salinity. Swimming and passive sinking velocities, defined as vertical distance traversed per unit time, were measured in different salinities under constant temperature and light. For all species, swimming rate changed with larval stage, highest velocity occurring at the umbo stage. Upward swimming rate of S. solidissima larvae ranged from 0.18 to 0.49 mm S-' and increased with increasing salinity. Upward swimming rate of M. lateralislarvae ranged from 0.25 to 0.50 mm S-', but was not consistently related to salinity. Upward swimming rate of R. cuneata larvae ranged from 0.18 to 0.53 mm S-'; swimming rate of pediveliger larvae increased consistently as salinity decreased. Downward swimming rates were similar to upward rates. No significant differences in downward swimming rate were detected in relation to salinity. Passive sinking was more frequent than active downward swimming in umbo and pediveliger larvae. Sinking rate increased with larval size of S. solidissima and M. lateralis larvae; however, R. cuneata straight-hinge larvae sank faster than umbo and pediveliger larvae. Species-specific differences in larval sinking and swimming are related to the different habitats occupied by adults. Larvae of S. solidissima, a marine stenohaline species, remained in high salinity water. Larvae of M. lateralis, a euryhaline species, use their preference for discontinuities or higher salinity water to assist retention in partially mixed estuaries. High sinking rate and short larval period of R, cuneata may offset the behavioural characteristic and aid in the retention of R. cuneata larvae in the low salinity zone of most partially mixed estuaries.

Ocorrência de larvas Pedivéliger de ostras do genero Crassostrea Sacco, 1897 no setor euhalino do Complexo Estuarino de Paranaguá – PR.

“…Este processo sugere uma interação entre o comportamento de natação das larvas e as características de circulação do ambiente (SILVA; ABSHER, 1996; BOEHS; ABSHER, 1997). A retenção de larvas pelágicas estuarinas também é influenciada pelo transporte ativo e passivo das mesmas (MANN et al, 1991;SILVA;ABSHER, 1996;ABSHER, 1997). Alguns estudos sugerem que a larva pode manter sua posição na coluna d`água independentemente de fatores ambientais (WOOD;HARGES, 1971;HIDU;HASKIN, 1978).…”

Relationship between feeding and vertical distribution of bivalve larvae in stratified and mixed waters

“…Sensitive depth regulatory responses for bivalve veligers have been described by several authors. Different stimuli have been demonstrated to modify the swimming behaviour of veligers, such as light intensity (Bayne 1964, Kaartvedt et al 1987, salinity discontinuities (Mann et al 1991) and pressure variations (Bayne 1964, Cragg 1980. Evolution of depth regulation, which leads to larval aggregation, could be the result of selection pressures initiated by the avoidance of demersal predators as well as visual predators in the surface layer and/or the necessity to feed in the upper rich food layer in stratified water masses.…”

Pelagic conditions affect larval behavior, survival, and settlement patterns in the Caribbean coral Montastraea faveolata

“…Variations in salinity due to large-scale hydrodynamic processes or rainfall are likely to be a potential natural stressor during this life-phase, as members of the lower invertebrate groups, including the cnidarians, are not capable of osmoregulation (HoeghGuldberg & Smith 1989). Salinity affects the biology of many marine organisms (Lyster 1965, Mann et al 1991, Metaxas & Young 1998, Simpson & Hurlbert 1998, Garrison 1999 and most corals have a narrow salinity tolerance (Muthiga & Szmant 1987, Porter et al 1999, Mundy & Babcock 2000. Although salinity stress does occur, it is unlikely that it creates altered environmental conditions for prolonged periods of time, since hydrodynamic processes constantly mix large bodies of seawater.…”