Effects of Low Salinity on Evechinus chloroticus Larvae: Vertical Distribution, Larval Growth, and Egg and Larval Survival
Background
The sea urchin Evechinus chloroticus is endemic to New Zealand waters and inhabit the fiords of the southwestern South Island. Evechinus chloroticus is a grazer found in the shallow reaches of subtidal areas, with greatest abundances above 10 meters depth (Wing et al. 2000). Spawning of E. chloroticus typically occurs annually in the summer months from December to March when planktotrophic larvae are released into the water column and commence development and metamorphosis in 18-30 days (Lamare and Barker 1999). Recruitment of larvae has been proposed to be a significant process in determining invertebrate population structure (Underwood and Fairweather 1989) and in Evechinus chloroticus (Lamare and Barker 1999).
Doubtful Sound is the second largest of the New Zealand fiords (McCully 1996) with a length of 40.4 km and mean width of 1.2 km (Stanton and Pickard 1981). Doubtful Sound has typical fiord features: narrow with steep sides and a high sill that separates its inner basins from the sea (Stanton and Pickard 1981). High volumes of freshwater enter Doubtful Sound due to copious amounts of precipitation in this region. Additionally, the outflow of the Manapouri Power Station, situated in Deep Cove, diverts freshwater from a lake into the fiord (Stanton and Pickard 1981) effectively doubling the freshwater input (Gibbs 2001). Due to extensive freshwater input into the fiord, a low salinity layer (LSL) floats above denser ocean water and are common in New Zealand fiords (Grange et al. 1991). This LSL typically has a salinity of 1-5 psu (Stanton 1984), while surface salinity is typically 34-35 psu off the West Coast of New Zealand (Heath 1984). The LSL generally is thicker at the head of the fiord compared to the mouth (McCully 1996). Due to wind and other effects, this LSL progresses seaward while coastal seawater is drawn into the fiord and displaces bottom waters.
The salinity tolerances of adult Echinoderms are varied. Populations of Echinoderms occur in waters ranging from 0.5-80‰ (Stickle and Diehl 1987). The salinity tolerances of larval Echinoderms have been investigated, and show particular sensitivity at this critical stage of life (Lucas 1973, Hendler 1977). Fenchel (1965) showed that the tolerance of the sea star Luidia sarsi larvae have been shown to be acutely sensitive to low salinities. Salinity tolerances of E. chloroticus larvae are unknown.
Low salinity may have several effects on larvae of invertebrates. Salinity has been determined as a dominant factor influencing the larval development and growth of Echinoderms (Watts et al. 1982) and other invertebrates (Richmond and Woodin 1996). Studies have shown that invertebrate larvae also actively avoid critically low salinities (Metaxas and Young 1998, Scarratt and Raine 1967). Below a critical salinity, larval development may be delayed significantly in all species (Penchenik 1987).
Due to the low salinity layer, populations of E. chloroticus at the head of Doubtful Sound in Deep Cove or Hall Arm may be isolated from those populations at the mouth of the Sound if their larvae cannot survive development in the LSL. Larvae of other marine species have been shown to avoid low salinities (Scarrat and Raine 1967), if this is the case with E. chloroticus, do the larvae aggregate a certain level below the LSL?
Questions and Hypotheses
Does the low salinity layer in Doubtful Sound influence Evechinus chloroticus larvae along the fiord?
· Is the vertical distribution of E. chloroticus larvae influenced by the low salinity layer?
I hypothesize that the distribution of Evechinus chloroticus larvae in the water column in Doubtful Sound will be different due to the low salinity.
· Do laboratory cultures of E. chloroticus actively avoid low salinities similar to those found in the low salinity layer of Doubtful Sound?
I hypothesize that larvae will sink or swim out of lower salinity water, aggregate around the halocline.
· Does prolonged exposure to low salinity cause extensive (over 50%) mortality or a halt in development of E. chloroticus larvae?
I hypothesize that larvae will have greater mortality and develop more slowly in lower salinities.
Expectations
· Larvae sampled from the field will distribute themselves below the LSL, and be densest within 3m of halocline
· Larvae in the halocline experiments will modify their vertical distribution, aggregating below the halocline
· In speaking with other researchers:
· I expect that eggs will not survive in salinities below 30‰, and
· Larvae will not survive in salinities below 25‰
o Larvae may be deformed below 30‰
References
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