Stoner et al,: Reproduction and larval. abundance in queen conch 



165 



the surface at 1.5 m depth for another 10 minutes. At 

 the other three sites where there was considerable ver- 

 tical mixing and shallow depth, the net was towed for 

 20 minutes in the upper 1.5 m of the water column. 

 Water volume sampled was calculated using a cali- 

 brated General Oceanics flowmeter, and larval abun- 

 dance was expressed in numbers of veligers per lOm*^. 

 To identify larvae, samples were refrigerated, sorted 

 live (within 4 hours), and compared with laboratory- 

 cultured larvae of the two most abundant Strombus 

 spp. in the central Bahamas, S. gigas and S. costatus. 

 Two other strombids occur in the Lee Stocking Island 

 area {S. galliis and S. raninus); however, both are very 

 rare relative to S. gigas and neither has been observed 

 on the windward side of the island or in the inlets. Shell 

 length, shell width, and shape of the shell tip were the 

 principle criteria used to identify early-stage larvae. 

 Number and shape of shell whorls and other shell 

 characteristics were used to identify advanced larval 

 stages. Measurement of shell length, from apex to 

 siphonal edge, was made with an ocular micrometer 

 and reported in microns for all intact shells. 



Results 



Conch reproduction 



The reproductive season for Strombus gigas at Lee 

 Stocking Island extended from mid-April to early 

 October. The beginning of the season was marked 

 by a massive migration of conch from hardground 

 (mounds, rubble, and boulder areas) to sand habitats 

 (F'ig. 4) where first copulation, pairing, and spawning 

 were observed on 14, 15, and 25 April 1988, respec- 

 tively. In subsequent months, virtually all reproductive 

 behavior occurred on sand (see later). The number of 

 females engaged in reproductive activity increased 

 gradually from April (9.7% of total sampled population) 

 to July (18%) (Fig. 3B). In August, 13.8% of the popula- 

 tion were reproductively active females; the percentage 

 declined to less than 1.0% in September and October. 

 Last copulation and pairing were seen in August, but 

 egg-laying was observed through September. The last 

 egg mass was discovered on 5 October 1988. 



The number of reproductive conch increased with 

 conch density on sand (Fig. 4) from January and Feb- 

 ruary (0 conch/1000m2) to July (10 conch/1000 m^). 

 Density decreased after the beginning of August and 

 was 0.61 conch/1000 m^ in October. Conch were ag- 

 gregated on some dates and not distributed evenly 

 along the transect lines. Large error bars in Figure 4 

 show that the two transect lines frequently had dif- 

 ferent densities of conch during the primary reproduc- 

 tive season. In August 1987, measurements in areas 

 with high conch densities ranged from 11.1 to 20.7 



conch/1000 m2. Values as high as 29.7 conch/1000 m2 

 (SE 2.0) were found in June 1988. 



Low bottom-water temperatures were observed from 

 early March to early April 1988 (near 23.6°C) (Fig. 3A). 

 First pairing and copulating conch were seen at a 

 temperature of 24 °C in mid-April, and the first egg- 

 laying female was found at 24.5 °C. The number of 

 copulating females increased as a linear function of bot- 

 tom water temperature until the reproductive max- 

 imum (r 0.916, F 15.726, p 0.029; March through July 

 1988). There was no significant correlation between 

 egg-laying and temperature (p 0.061) and pairing and 

 temperature (p 0.285). Bottom temperature was rela- 

 tively constant (28.3-28.8°C) from the end of July 

 through September; the last pairing and copulating 

 conch were observed during this period. Temperature 

 decreased rapidly after September, and the last egg 

 mass was found on 5 October. Water temperature was 

 26.5°C by late November 1988, decreasing to 25.1°C 

 in late December. 



All pairing and copulation were confined to the 

 season with photoperiod greater than 12 hours, while 

 egg-laying was observed until day length declined 

 to 11 hours (Fig. 3A,B). Highest correlation oc- 

 curred between length of day and copulation (r 0.870, 

 F 24.838, p 0.001), but significant correlations were 

 also found between photoperiod and both pairing 

 (r 0.709, F 8.064, p 0.022) and egg-laying (r 0.838, 

 F 18.896, p 0.002). 



A few conch were buried partially in sand in mid- 

 October 1987 and again in January and early February 

 1988. Burrowing was not seen again until mid-Sep- 

 tember 1988. In November, a few conch were buried 



