Freeman, John A.

Kilgus, Gayle

Laurendeau, Diane

Perry, Harriet M.

Northeast Gulf Science

English

Aquila Digital Community

Northeast Gulf ScienceVolume 8Number 2 Number 2 Article 1311-1986Delayed Hardening in Soft Shell Blue CrabsJohn A. FreemanUniversity of South AlabamaDiane LaurendeauUniversity of South AlabamaGayle KilgusUniversity of South AlabamaHarriet M. PerryGulf Coast Research LaboratoryDOI: 10.18785/negs.0802.13Follow this and additional works at: https://aquila.usm.edu/gomsThis Article is brought to you for free and open access by The Aquila Digital Community. It has been accepted for inclusion in Gulf of Mexico Scienceby an authorized editor of The Aquila Digital Community. For more information, please contact Joshua.Cromwell@usm.edu.Recommended CitationFreeman, J. A., D. Laurendeau, G. Kilgus and H. M. Perry. 1986. Delayed Hardening in Soft Shell Blue Crabs. Northeast Gulf Science 8(2).Retrieved from https://aquila.usm.edu/goms/vol8/iss2/13Northeast Gulf Science Vol. 8, No. 2, November 1986 177 DELAYED HARDENING IN SOFT SHELL BLUE CRABS The freshly molted (soft shell) blue crab, Callinectes sapidus, is consumed in its entirety and forms the basis for the soft shell blue crab industry in the western Atlantic and northern Gulf of Mexico regions (Otwell and Cato, 1982; Perry eta/, 1982). Recent innovations and improvements in the design of the biological filter for recirculating seawater systems have made the crab shedding process an economically feasible operation for fishermen and non-fishermen alike and have provided a means of holding crabs away from a natural source of seawater (Manthe eta/, 1984). In such systems the crabs are maintained until ecdysis and then removed for preparation as a soft shell crab. Since the crabs often molt at night, and can harden to the paper shell condi· tion in 3-4 hours, someone must monitor the holding tanks for freshly molted crabs throughout the evening and early morning hours, thus making the shed-ding operation labor intensive. In this paper, we report the use of low calcium seawater (LCW) to retard the initial stages of hardening in laboratory-held blue crabs. METHODS Blue crabs of 40-120mm carapace width (CW) were collected at Dauphin Island, Alabama and maintained indivi-dually in the laboratory in disposable animal cages in artificial seawater at 5-6 ppt and 24·26 °C. The water was changed daily. The crabs were fed four times per week on fish and beef liver until the onset of premolt and then were not fed again. Crabs were considered to be in premolt if apolysis was observed at the lateral edge of the paddle. The normal (control) seawater consisted of Rila® seasalts dissolved in well water with crushed oyster shell added to each tank. Low calcium water was the same except that the oyster shell was not added. The calcium level in the water was measured with an Orion calcium electrode attached to a computer aided titrimeter calibrated with calcium standards from 10-1000ppm according to the manufacturers recom-mended methods. The calcium levels were found to be 253ppm in control seawater (pH= 8.2 · 8.3) and 68-90ppm in LCW (pH= 7.4-7.6). Crabs were checked daily for shed cuticles between 8:00 and 9:00 AM. Molted crabs were removed from the seawater and the phase of postmolt and the texture of the cuticle determined. Crabs that had not hardened were observed several times per day for up to a week, or until the hard crab state was observed. The cuticle was determined to be either soft, leathery, or paper shell as determined by gently pressing the tip of the lateral spins. The crab was cate-gorized as soft if the cuticle was defor-mable and pressure on the lateral spine resulted in the spine curving or bending anywhere along its length. If the spine bent only at the base and the cuticle had a rough texture, the crab was determined to be in the leathery condition. A crab was considered to be in the paper shell condition if no bending occured in the lateral spine. The carapace width of the old and new exoskeleton was measured, and compared, to give the percent increase in CW (% inc CW = [CW new cuticle-CW old cuticle]CW old cuticle). RESULTS AND DISCUSSIONS Blue crabs that molted while in LCW (n = 24) were always found to be in the soft shell or leathery condition several hours after shedding, when examined 1Freeman et al.: Delayed Hardening in Soft Shell Blue CrabsPublished by The Aquila Digital Community, 1986178 Short Papers and Notes the morning after ecdysis. The crabs remained in this condition for at least eight hours after the initial observation time (8:00-9:00 AM). Crabs in control seawater (n 517), however, had reached the paper shell condition by the initial observation period, or, if not, were always found to be in paper shell by noon of that day. Although the paper shell con-dition was finally reached in some crabs held for extended periods in LCW, a truly hardened condition was never attained and in some cases the crabs died within one week after ecdysis. These results show that the initial stages of hardening (paper shell condition) can be delayed by placing the crabs in LCW. Although the effect of the low calcium seawater was considered to be the result of low levels of calcium, it is also possible that the pH of the seawater may have also con-tributed to the reduced hardening. Another result of maintaining the crabs in LCW was that the increase in size obtained after ecdysis was significantly greater (p< 0.01, F-test) in animals kept in LCW than in normal seawater (Fig. 1). These findings suggest that the cuticular event that physically . .... as 20 () . 10 0 c: "#. 0 I"" f---1-1-CA -CA Figure 1. A comparison of postmolt increase in carapace width (spine tip to spine tip) (ordinate) in crabs held in normal (CA) (n = 17) and low calcium (·CA) (n = 24) seawater. Each bar equals mean + 1- 95% confidence intervals. halts the expansion of the soft cuticle is perturbed in an environment that has low calcium levels. Whether this is a result of: 1) the lack of calcium for enzymatic or cellular activities associated with postmolt tanning, calcification or inter-nal osmotic regulation or, 2) whether it reflects a level of calcium that is too low to support calcification cannot be deter-mined from these data. However, the fact that expansion of the cuticle did stop in the crabs in LCW suggests that it was the initial events of postmolt that were affected by the low calcium levels. Further research will have to be done before we can determine whether one or more of the integumental events of early postmolt are dependent on the presence of calcium. Since Callinectes stores little calcium internally and must absorb almost all of the calcium from the am-bient seawater (Vigh and Dendinger, 1982; Cameron, 1985), the LCW probably caused a reduction in the calcium levels in the hemolymph, and the epidermal cells. The findings reported here make the use of LCW a potential labor reduction device in the soft shell shedding opera-tion. The fact that the crabs do not enter the paper shell condition for at least eight hours may obviate the need to check the crab shedding tanks at hourly intervals for soft, freshly molted crabs. It is possible, however, that the use of LCW in the recirculating systems may re-quire some modification of the filtering capacity of the system and that poten-tial water quality problems may arise as a result of the lack of the oyster shell to buffer alkalinity (Perry et a/, 1982; Manthe eta/, 1984). ACKNOWLEDGEMENTS The authors wish to thank Dr. S. Sikes for measuring the calcium 2Gulf of Mexico Science, Vol. 8 [1986], No. 2, Art. 13https://aquila.usm.edu/goms/vol8/iss2/13DOI: 10.18785/negs.0802.13Northeast Gulf Science Vol. 8, No. 2, November 1986 179 levels of the normal and low calcium seawater. This research was supported by Grant No. NA85AA·D-SG005 from the Mississippi-Alabama Sea Grant Consortium. LITERATURE CITED Cameron, J. 1985. Post-molt calcification in the blue crab. Proc. of the National Symposium On The Soft-Shelled Blue Crab Fisher, February 12-13, 1985, Biloxi, MS (in press). Manthe, D.P., R.F. Malone and S. Kumar. 1984. Limiting factors associated with nitrification in closed blue crab shed-ding systems. Aquacultural Engineer-ing 3: 119-140. Otwell, W.S. and J.C. Cato. 1982. Review of the soft shell crab fishery in the United States. Proc. of the Blue Crab Colloqium, 1979. (H. Perry and W.A. Van Engle, eds). pp 129·136. Perry , H.M., J.T. Ogle, and L.C. Nicholson. 1982. The fishery for soft crabs with emphasis on the develop-ment of a closed recirculating seawater system for shedding crabs. Proc. of the Blue Crab Colloquim, 1979 (H. Perry and W.A. Van Engle, eds) pp 137-150. Vigh, D.A. and J.E. Dedinger. 1982. Tem-poral relationships of postmolt deposi-tion of calcium, magnesium, chitin and protein in the cuticle of the Atlantic blue crab, Callinectes sapidus Rathbun. Comp. Biochem. Physiol. 72A: 365-369. John A. Freeman, Diane Laurendeau, Gayle Kilgus, Department of Biology, University of South Alabama, Mobile, AL 36688, and Harriett M. Perry, Gulf Coast Research Laboratory, Ocean Springs, MS 39564. 3Freeman et al.: Delayed Hardening in Soft Shell Blue CrabsPublished by The Aquila Digital Community, 1986

Delayed Hardening in Soft Shell Blue Crabs

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