Ambient Spring Research Articles

Patterns of metamorphosis in summer flounder, Paralichthys dentatus

Summer flounder, Paralichthys dentatus, spawn over the continental shelf off the east coast of the United States from September to January with the peak in October–November. Based on plankton collections, mid‐metamorphic larvae (stages G‐H; mean s.l. 13.1 mm) enter Great Bay–Little Egg Harbor estuary in southern New Jersey as early as October with continued ingress through April. In the laboratory, mortality during metamorphosis ranged from 17 to 83% among treatment groups, and was significantly greater in flounder maintained at approximately 4°C relative to those maintained at ambient temperatures (daily average temperature 10.l°C). Laboratory‐reared summer flounder averaged 24.5 days (range 20 to 32 days) to complete metamorphosis (from Stage F– to Stage I) at ambient spring temperatures (daily average temperature =16.6° C). The time to completion of metamorphosis in wild‐caught flounder maintained in the laboratory was clearly temperature dependent. Both cold and ambient temperature treatments resulted in delayed metamorphosis such that, at ambient winter temperatures (daily average=6.6°C), partial metamorphosis (from Stage H – to Stage I) required as much as 92.9 days (range 67 to 99 days). There was no apparent effect of starvation on either mortality or time to completion of metamorphosis at cool water temperatures (< 10° C). It appears that prevailing temperature conditions influence the duration of metamorphosis in summer flounder, and that mortality during metamorphosis may play a significant role in the population dynamics of this species.

Photoperiodic regulation of parturition in the self-fertilizing viviparous polychaete Neanthes limnicola from central California

Seasonally-changing photoperiod controls the timing of parturition in the viviparous, self-fertilizing polychaete Neanthes limnicola (Johnson, 1901) from Watsonville Slough, a central California estuary. During 1987 to 1989, worms in the field gave birth mainly in the spring. Those born in late February from field-collected parents and maintained in the laboratory under in-phase photoperiodic conditions reproduced in 12 to 13 mo, under spring light-regimes. When maintained under light conditions 6 mo out of phase,they required only about 6 to 8 mo to reproduce, giving birth in the fall, but under spring light-regimes. Worms born in the laboratory in fall and then maintained in phase reproduced in the ambient spring, at 6 to 8 mo of age; those maintained out of phase took 12 to 13 mo, giving birth the following fall under spring light-regimes. Photoperiod treatments had no consistent effect on the number of young produced, and age and fecundity were only weakly correlated. Highest fecundities were in salinities of 15 to 20‰,with lower fecundities at higher salinities. Worms maintained in fullstrength sea water(33‰ S) showed abnormal development and produced very few or no young. Salinity did not affect timing of parturition. Temperature differences of 3 to 7 C° between treatments had no effect on timing of parturition or number of young produced, and marginal effects on life span. These results indicate that photoperiod regulates the timing of reproduction in N. limnicola in central California, while salinity mainly influences fecundity.

Fluid evolution during uplift of the Annapurna Himal, central Nepal

Fluid inclusions from post-metamorphic quartz show that there has been considerable CO 2-rich fluid flux during uplift of greenschist-amphibolite facies rocks along the Main Central Thrust (MCT). Maximum fluid CO 2 content increased with metamorphic grade of the host rock, ranging from ∼ 20–30 mole% in the greenschist facies to both the north and south of the MCT, to 70 mole% in the amphibolite facies. The highest CO 2 content is in late-stage veins which cross-cut uplift-related schistosity in the sillimanite zone. This contrasts with the syn-metamorphic fluid which was probably dominantly aqueous. Fluid immiscibility occurred in quartz veins near the brittle-ductile transition, at 280–310°C and 700±300 bar fluid pressure. An upper limit of 1600 bar lithostatic pressure was determined from a CO 2-rich isochore. The high thermal gradient implied by P- T data is due to rapid uplift along the MCT. MCT movement ceased in the Miocene, and the rocks have experienced slower uplift along a lower thermal gradient. During this slower uplift CaNa brines, some with freezing point depression below −20°C, have traversed the rock. At first the brines mixed with CO 2, then brines dominated until diluted by incursion of lowsalinity, presumably meteoric water near the surface. Brines and CO 2 may be reaching the surface today, in diluted from as numerous warm and ambient springs. Brines were probably derived from connate fluids in the Tibetan carbonate/clastic sedimentary succession to the north of the MCT. CO 2 was derived from metamorphosed equivalents of these sediments, and perhaps from similar sediments beneath the MCT.