MORGAN: ASPECTS OF LARVAL ECOLOGY Of SQUILLA EMPUSA 



made from Instant Ocean Synthetic Sea Salts^ 

 (Aquarium Systems, Inc., Eastlake, Ohio) and 

 tapwater. 



Larvae representing all nine developmental 

 stages of S. empusa were reared in 16 combina- 

 tions of temperature and salinity, each having a 

 similar composition of larval stages. The experi- 

 mental temperatures used were 10°, 15°, 20°, and 

 25° C, and salinities were 10, 15, 25, and 35%o, 

 chosen because they represent the range of condi- 

 tions the larvae might be expected to encounter in 

 the lower Chesapeake Bay. The salinities of 20 and 

 30%o were omitted from the experimental regime 

 because insufficient numbers of larvae were ob- 

 tained to determine their tolerances to all inter- 

 mediate salinities as well as to the more extreme 

 salinities. Thirty-six larvae were subjected to each 

 temperature-salinity combination. Because the 

 larvae were not hatched in the laboratory under 

 the temperature-salinity combination at which 

 they would be reared, some larvae were subjected 

 to changes as great as 5° C and 10%o per day until 

 the experimental value was attained. No light 

 cycle was used in the experiment, the larvae being 

 maintained in total darkness except for 10-min 

 periods when the larvae were given fresh food and 

 water. 



Each larva was reared in 25 ml of water and 

 given freshwater and approximately 30 Artemia 

 salina nauplii/ml daily. Great increases in size 

 from the first to the last stage necessitated ad- 

 justments in food size and quantity. At about the 

 fifth stage of development food was switched from 

 A. salina nauplii to decapod zoeae or A. salina 

 larvae grown on a yeast or algal culture. While 

 changing the culture medium, observations were 

 recorded on the progress of each larva regarding 

 the frequency of molting, duration of larval devel- 

 opment, survival, and the stage of development. 



Percent survival and molting frequency are 

 often used as measures of success of larvae under 

 different temperature-salinity regimes, but were 

 not meaningful in this experiment because the 

 larvae were captured at different stages of devel- 

 opment and different places in the molt cycle. 

 Therefore, the length of survival and number of 

 molts were used as the standards of success. The 

 temperature and salinity combinations which 

 promoted the greatest number of molts and the 



longest periods of survival among the larvae were 

 considered to be most conducive to the larval de- 

 velopment of S. empusa, because the larvae were 

 not only surviving best but were also maturing 

 fastest. The mean number of ecdyses and days of 

 survival were calculated for each larva and then 

 collective means were figured for each 

 temperature-salinity combination. In this way a 

 general indication of success of populations under 

 varying temperature and salinity conditions could 

 be determined. 



RESULTS 



Seasonal Occurrence 



The RANN survey extended from 16 August 

 1971 until 25 July 1973. and S. empusa larvae were 

 found in the Chesapeake Bay only from late July 

 to mid-September or late October (Figure 2). Dur- 

 ing these months in 1971 the RANN study sampled 

 on 16-19 August, 21-23 September, and 26-29 Oc- 

 tober, while in 1972 samples were taken on 24-27 

 July, 15, 17, 18, 21 August, 12-14 September, and 16, 

 18, 24 October. In 1973, sampling was conducted on 

 23-25 July. 



The monthly sampling program used by the 

 RANN program left the larval occurrence of S. 

 empusa somewhat unclear In both years of the 

 survey, larvae were found on the first day of sam- 

 pling in July, 24 July 1972 and 23 July 1973; since 

 a month elapsed between the June and July sam- 

 plings, however, the earliest appearance of the 



ASONDJ FMAMJ JASON DJ FMAM, 

 1971 1972 1973 



A SON 



1976 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



Figure 2. — Lar\'al abundance ofSquilla empusa collected from 

 the lower Chesapeake Bay from August 1971 to July 1973 and 

 from June to November 1976. 



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