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Temperature-specific rates of embryonic development of the European lobster Homarus gammarus (L.)
J. Exp. Mar. Biol. Ecol., 1600 (1992) 61-66
61
© 1992 Elsevier Science Publishers BV. All rights reserved0022-0981/92/$05.00 JEMBE 01787
Temperature-specific rates of embryonic development of the European lobster Homarus gammarus (L.) Guy Charmantier a and Raymond Mounet-Guillaume b aLaboratoire d'l~cophysiologie des lnvertdbr~s. UniversiMdes Sciences, Montpellier 2, Montpelier, France; bD~partement de Biologie Appliqu~e, IUT, Uni'versitddes Sciences, Montpeilier2, Montpellier. France
(Received 12 March 1991; revision received 3 March 1992; accepted 9 April 1992) Abstract: Berried Homarus gammarus (L.) females were reared at three constant temperatures (10, 15 and
20 °C), and the rates of developmentof their embryos monitored by measuring embryoniceye index according to Perkins. An equation relatingthe increase in eye index to temperaturewas established,yielding the estimatedtime to hatch at differenttemperatures. It may be used to approximatetb: ~#te of l-~atchac.cording to rearing temperaturein H. gammarus. Key worfls:Embryo; Homarus; Lobster; Temperature
INTRODUCTION Since Templeman's (1940) study, it has been known that temperature has an influence on the rate of development of early embryos of the American lobster Homarus americanus Milne-Edwards. In his study of the effect of temperature on the developmental rates of embryos of H. americanus, Perkins (i 972) demonstrated that: (1) after the formation of the eye pigment, the rate of develogment of lobster embryos can be monitored by measuring the eye size of the embryos; and (2) water temperature has a direct effect on the developmental rate of lobster embryos. In H. americanus, Perkins found that hatching occurred for an eye index (defined as half the sum of width and length of the eye of the embryo) of 560 #m and he established an equation predicting hatching date, given the eye index and water temperature. These results were used by Waddy & Aiken (1984) to obtain year-round production of H. americanus larvae in the laboratory. Perkins' eye index was also used as an indicator of developmental stage in several studies conducted on embryos of H. americanus (Schuur et al., 1976; Cole & Lang, 1980; Sasaki et al., 1986; Beltz & Kravitz, 1987; Beltz et al., i990; Helluy & Beltz, 1990; Meier & Reichert, 1990; Helluy & Beltz, 1091). Similar temperature-specific equations have not been published for the European lobster H. gammarus (L.). In this species, Branford (1978) studied the incubation period at various temperatures, but did not check embryonic development through the Correspondenceaddress: G. Charmantier, Laboratoire d'l~cophysiologiedes Invertrbr6s, Universit6des Sciences, Montpellier2, PI. E. Bataillon, 34095 MontpellierCrdex 05, France.
62
G. CHARMANTIER AND R. MOUNET-GUILLAUME
evolution of the eye index. Hepper & Gough (1978) followed the eye index on eggs sampled from females caught in the wild at different seasons, but they did not manipulate temperature in rearing conditions. Richards & Wickins (1979) monitored the eye index to determine the time to hatching at only one temperature regime, 13-15 °C. Latrouite et al. (1984) used the eye index to assess the embryonic development in a study of the fecundity of the European lobster. In the present study, the rate of development of embryos of H. gammarus was monitored at different temperatures, using the eye it,alex to establish an equation that will forecast the date of hatching.
MATERIALS AND METHODS
Nine berried H. gammarus females were caught in September near Molene Island, Finist6re, and obtained from the fishery "La Langouste", Roscoff, Finist6re, France. They were transported to the laboratory in ,~20 h in wet wood chips at 5 °C. Mean ( _+SE) weights and carapace lengths of the lobsters were 851 _+48 g and 108 + 3 mm, respectively. The females were carrying recently extruded eggs; none of the embryos had detectable pigmented eyes. In the laboratory, lobsters were randomly transferred to cages (circular bottom, 1-m diameter; height, 1.20 m) made of plastic wire (1-cm mesh) immersed in three 3000-1 tanks, the temperatures of which were set at 10 °C (Females 1-3), 15 °C (Females 4-6) and 20 °C (Females 7-9). Seawater of each tank was recirculated and filtered through Eheim pumps and filters. ~ 1/4 of the volume of each tank was replaced every month with fresh seawater of the appropriate temperature. Salinity ranged between 36 and 37%° during the experiment. An artificial light regime of 12 h L: 12 h D was established and lobsters were fed mussels ad libitum three times a week. All experimental conditions, except temperature (tank shape, water volume, salinity, feeding rate, etc.), were kept identical in all three tanks. Temperature was controlled daily to the nearest 0.1 of a degree. Usual variations of temperature did not exceed + 0.2 °C; the maximum variation was + 1.2 °C during 24 h in the 10 °C tank, due to the failure of a refrigeration unit. The mean daily temperature exposure for each female (sum of the daily temperatures divided by the number of days until the hatch) was: Females 1:10.11 °C; 2:10.12 °C; 3: i0.10°C; 4:14.98 °C; 5:14.97 °C; 6: 15.00°C; 7:19.91 °C; 8:19.95 °C; and 9: 19.94°C. The rate of development of the embryos was determined by measuring the size of their eyes and calculating the eye index according to the method of Perkins (1972). Once every 2 wk, samples of 10-15 eggs.female -~ were removed from the periphery of the egg masses to minimize the disturbance of the female. Although such eggs are the furthest advanced in development in H. americanus (Perkins, 1972), in this experiment one observation.female -~ at mid-development of the embryos yielded differences of < 3 ~o between the eye indices of eggs sampled from the periphery and from the center of the egg masses. Eggs were immersed in seawater and the pigmented part
EMBRYONIC DEVELOPMENT OF HOMARUS GAMMARUS
63
(excluding the less pigmented halo) of one eye of each embryo was measured with an ocular micrometer and a dissecting microscope at 40 × magnification; the precision of the measurement was 12.5/am. For each eye, the maximum width and length were measured, the values were summed and divided by two, yielding the eye index. Regression lines of the variations of the eye index related to time were established. Their slopes were statistically compared two by two according to the standard following method (Diem, 1963; Geller, 1983). The equivalency of the residual variances was preliminarily verified by submitting their ratio to Snedecor's test; tr was the estimated common value of the two residual variances. With b and b' representing the slopes of two compared lines, the statistical variable t was calculated: b-b' t = tT
+ r l ' s '2
in which s2 and s' 2 are the variances of the n and n' abscissa of the points of the first and second lines, l'his variable t follows Student's law at (n + n' - 4) df. After these comparisons, a common slope + SE was calculated at each temperature according to Diem (1963). Predicted dates of hatch evaluated at mid-development were compared with the ~:ctual hatching dates in the nine experimental females and in four other females kept at 15 °C (two) and 19 °C (two).
RESULTS
The lowest eye index (EI) values ranged between 35 and 80 #m. Hatch occurred at E1 values of 640-680/am (~ + SE: 648 _+ 11 #m). A value of 650 #m was used for calculations thereafter. From the data collected from the embryos of each female, regression lines of the eye index related to the time were established (Fig. I). The correlation coefficient of each line was high (r>0.98), demonstrating a linear correlation in each set of data, and their slope b (#m.day -]) increased with temperature: b values were 2.68, 2.58 and 2.59 (common slope + SE: 2.62 + 0.02) in Females 1-3 at 10 °C; 4.45, 4.78 and 5.20 (4.79 + 0.05) in Females 4-6 at 15 °C; and 5.88, 6.57 and 5.50 (5.93 _ 0.13) at 20 °C. Statistical comparisons demonstrated that there was no significant difference between the slopes from the three females maintained at one of the temperatures, which permitted the calculation of a common slope (Diem, 1963) for each temperature. On the contrary, slopes at one temperature were different from those at the two other temperatures (10-15 and 10-20 °C: p<0.01; and 15-20 °C: p<0.05). For e~ch female, the time (t~) required to complete the embryonic period from the onset of eye pigment (EI = 0/am) to hatching was calcala~ed by dividing the EI at hatch by the slope of the regression line. The increase in E1 (/am-wk- ~) was then calculated by dividing the EI at hatch by t~. For instance, in Female 1, the EI at hatch was 653/am
64
G. CHARMANTIER
A N D R. M O U N E T G U I L L A U M E
20oC
'
600
I
I
lO°(:
s/
/
t / •
15°C
"J .
,/ /
I
# /
J
J
/
400
/
xl u a
I
I
I
I
I
I
/
ul
2OO
I
I
I
I
I
I
0
50
100
150
200
250
TI/AF (DAYS) Fig. 1. Regression lines of EI related to time at three temperatures. Lines terminate at hatching, at mean value of 650 s m . Dotted and solid lines correspond to individual and common regression lines. See text for values of slopes at each temperature.
and the slope was 2.68 #m'd-t yielding a tc of 244 days necessary for the development of the embryo from El = 0 tO 653 #m; the increase in ~I was thus 18.76 #m.wk -~. The relationship between the increase in El and the rearing temperature was linear (r= 0.9745). Eqn. 1 describing the relationship between increase in E] (y, #m.wk -I) and temperature (x, °C) was: y = 2.5104 x - 5.6240. As proposed by Perkins (1972) for H. americanus, the equation relating the increase in embryonic EI to temperature may be used in H. gammarus to predict the hatching date of larvae. In this intent, El, measured from eggs sampled ti'om a given female, must be substracted from EI at hatch (650 #m in H. gammarus); the result is divided by the value of the increase in EI.wk -] at the rearing temperature given by Eqn. 1, yielding If"
650 - El ll =
2.5104
x
-
5.6240'
in which n is the number of wk to hatch, EI the eye index in #m and x the temperature in °C. In Homarus, hatching is spread from 1 wk to 10 days. Since it is based on
EMBRYONIC DEVELOPMENT OF HOMARUS GAMMARUS
65
a mean value of EI, I"/predicts the date of the peak of hatching, approximately situated midway through the hatching period. Dates of beginning and end of hatching can also be evaluated, using individual maximum and minimum EI values, respectively, from an egg sample. Predicted dates (at mid-development for eye indices close to 300 #m) and actual dates of hatch did not differ by > ~ 10 days. Discrepancies between the two dates were found maximum in females kept at 19-20 °C (+ 10 d) and miniwum at 10 ¢C (_+ 6 d) despite the longer time between prediction and actual hatch in the latter condition.
DISCUSSION
In this study, the El value at hatch in H. gammarus was ~ 650 #m, which is slightly higher than the maximum values previously reported in this species, i.e., 600-620 #m (Richards & Wickins, 1979) and 634 #m (Hepper & Gough, 1978). This difference could originate in the diverse geographical origins of the experimental animals (embryos of different origins could slightly differ in size) or in minute measurement error (e.g., calibration of the ocular micrometer or magnification). The corresponding E1 value for H. americanus is 560 #m (Perkins, 1972) or 570 #m (Helluy & Beltz, 1991). The difference in EI at hatch, and therefore in eye dimension, between the European and American species is certainly related to the larger size of the eggs, embryos and larvae of the former species. Egg diameters and cephalothoracic lengths of first stage larvae reported for H. gammarus are 1.8 and 2.54 mm (Scott, 1903; Pandian, 1970a) and 1.6-1.5 and 1.74 mm for H. americanus, respectively (Herrick, 1895; Pandian, 1970b; Gruffyd et al., 1975). The relationship between the increase in EI and the rearing temperature (10-20 ° C) was linear, according to y = 2.5104x- 5.6240. Branford (1978) also reported an approximately linear relationship between the incubation period and the sum of average temperatures in H. gammarus. This relationship is also linear in H. americanus (i.e., y = 2.6019x - 8.3151) between 5 and 25 °C (Perkins, 1972). The slopes of equations for H. gammarus and H. americanus are nearly identical, indicating similar effects of temperature on embryonic development in the two species. Discrepancies in the intercepts of these equations are due to differences in size.
ACKNOWLEDGEMENTS
We thank Mr. Cabioch, "La Langouste", Roscoff, for his help in providing lobsters; and M. Charmantier-Daures, M.-A. Garcia, H.P. Lin, P. Thuet and J.-P. Trilles for their assistance in the laboratory.
66
G. CHARMANTIER AND R, MOUNET-GUILLAUME
REFERENCES Beltz, B. S. & E.A. Kravitz, 1987. Physiological identification, morphological analysis, and development of identified serotonin-proctolin containing neurons in the lobster ventral nerve cord. J. Neurosci., Vol. 7, pp. 533-546. Beltz, B.S., M.S. Pontes, S.M. Helluy & E.A. Kravitz, 1990. Patterns of appearance of serotonin and proctolin immunc,reactivities in the developing nervous system of the American lobster. J. NeurobioL, Vol. 21, pp. 521-542. Branford, J.R., 1978. Incubation period for the lobster Homarus gammarus at various temperatures. Mar. Biol., Vol. 47, pp. 363-388. Cole, L.L. & F. Lang, 1980. Spontaneous and evoked postsynaptic potentials in an embryonic neuromuscular system of the lobster, Homan~s americanus. J. Neurobiol., Vol. 11, pp. 459-470. Diem, K., 1963. Statistique. In, Tables scientifiques, J. R. Geigy, Geigy D6partement Pharmaceutique, Basel, pp. 146-203. Geller, S., 1983. Statistique. Masson, Paris, 229 pp. Gruffyd, L. D., R. A. Rieser & D. Machin, 1975. A comparison of growth and temperature tolerance in larvae of the lobsters Homanas gammanls and Homaras americmms. Crustaceana, Vol. 28, pp. 23-32. Helluy, S. & B. Beltz, 1990. Stages in the embryonic de" elopment of the American lobster with special emphasis on its nervous system. In, Frontiers #: cnlstacemz he, biology,edited by K. Wiese et al., Birkhauser Verlag, Basel, pp. 530-536. Helluy, S. & B. Beltz, 1991. Embryonic development of the American iobster (Homarus americanus): quantitative staging and characterization of an embryonic molt cycle. Biol. Bull., Vol. 180, pp. 355-371. Hepper, B.I. & C.J. Gough, 1978. Fecundity and rate of embryonic development of the lobster, Homants gammarus (L.), off the coast of North Wales. J. Cons. Int. Explor. Mer, Vol. 38, pp. 54-57. Herrick~ F.H., 1895. The American lobster, a study of its habits and development. Bull. U.S. Fish. Comm., Vol. 15, pp. 1-252, 54 pl. La~rouite, D., Y. Morizur & G. Raguenes, 1984. F6condit6s individuelles et par recrue du homard europ6en Homarus gammarus (L.) des c6tes fran~aises. Cons. h:t. Explor. Met., C.M. 1984/K:38, 9 pp., 4 Tabl., 7
Fig, Meier, T. & tl. Reichert, 1990. Neuronal development in the crustacean nervous system studied by neuronspecific antibody labelling. In, Frol2tiers in cntstacean neurobiology, edited by K. Wiese et al., girkh,'tuser Vcrlag, Basel, pp. 523-529. Pandian, T.J., 1970a. Ecophysiological studies on the developing eggs and embryos of the European lobster Homarus gt~mmarus. Mar. Biol., Vol. 5, pp. 154-167. Pandian, T.J., 1970b. Yolk utilization and hatching in the Canadian lobster Homarus american,ts. Mar. Biol., Vol. 7, pp. 24)-254. Perkins, H.C., 1972. Developmental rates at various temperatures of embryos of the northern lobster (Homarl~s americamls Milne-Edwards). Fish. Bull., Vol. 70, pp. 95-99. Richards, P. R. & J. F. Wickins, 1979. Lobster culture research. Lab. Leafl. MAFFDirect Fish. Res. Lowestoft, No. 47, pp. 1-33. Sasaki, G.C., J.M. Capuzzo & P. Biesiot, 1986. Nutritional and bioenergetic considerations in the development of the American lobster Homarus americamls. Can. J. Fish. Aquat. Sci., Vol. 43, pp. 2311-2319. Schuur, A., W. S. Fisher, J. C. van OIst, J. Carlberg, J. T. Hughes, R. A. Shleser & R, F. Ford, 1976. Hatchery methods for the production of juvenile lobsters (Homams americanus). Inst. Mar. Res. Calif., Vol. 48, pp. 1-20. Scott, A., 1903. On the spawning of the common lobster. Proc. Trans. LiverpoolBiol. Soc., VoL 17, pp. 106113. Templeman, W., 1940. Embryonic developmental rates and egg-laying of Canadian lobsters. J. Fish Res. Board Can., Vol. 5, pp. 71-83. Waddy, S.L. & D.E. Aiken, 1984. Broodstock management for year-round production of larvae for culture of the American lobster. Can. Tech. Rep. Fish. Aquat. Sci., No. 1272, pp. 1-18.
61
© 1992 Elsevier Science Publishers BV. All rights reserved0022-0981/92/$05.00 JEMBE 01787
Temperature-specific rates of embryonic development of the European lobster Homarus gammarus (L.) Guy Charmantier a and Raymond Mounet-Guillaume b aLaboratoire d'l~cophysiologie des lnvertdbr~s. UniversiMdes Sciences, Montpellier 2, Montpelier, France; bD~partement de Biologie Appliqu~e, IUT, Uni'versitddes Sciences, Montpeilier2, Montpellier. France
(Received 12 March 1991; revision received 3 March 1992; accepted 9 April 1992) Abstract: Berried Homarus gammarus (L.) females were reared at three constant temperatures (10, 15 and
20 °C), and the rates of developmentof their embryos monitored by measuring embryoniceye index according to Perkins. An equation relatingthe increase in eye index to temperaturewas established,yielding the estimatedtime to hatch at differenttemperatures. It may be used to approximatetb: ~#te of l-~atchac.cording to rearing temperaturein H. gammarus. Key worfls:Embryo; Homarus; Lobster; Temperature
INTRODUCTION Since Templeman's (1940) study, it has been known that temperature has an influence on the rate of development of early embryos of the American lobster Homarus americanus Milne-Edwards. In his study of the effect of temperature on the developmental rates of embryos of H. americanus, Perkins (i 972) demonstrated that: (1) after the formation of the eye pigment, the rate of develogment of lobster embryos can be monitored by measuring the eye size of the embryos; and (2) water temperature has a direct effect on the developmental rate of lobster embryos. In H. americanus, Perkins found that hatching occurred for an eye index (defined as half the sum of width and length of the eye of the embryo) of 560 #m and he established an equation predicting hatching date, given the eye index and water temperature. These results were used by Waddy & Aiken (1984) to obtain year-round production of H. americanus larvae in the laboratory. Perkins' eye index was also used as an indicator of developmental stage in several studies conducted on embryos of H. americanus (Schuur et al., 1976; Cole & Lang, 1980; Sasaki et al., 1986; Beltz & Kravitz, 1987; Beltz et al., i990; Helluy & Beltz, 1990; Meier & Reichert, 1990; Helluy & Beltz, 1091). Similar temperature-specific equations have not been published for the European lobster H. gammarus (L.). In this species, Branford (1978) studied the incubation period at various temperatures, but did not check embryonic development through the Correspondenceaddress: G. Charmantier, Laboratoire d'l~cophysiologiedes Invertrbr6s, Universit6des Sciences, Montpellier2, PI. E. Bataillon, 34095 MontpellierCrdex 05, France.
62
G. CHARMANTIER AND R. MOUNET-GUILLAUME
evolution of the eye index. Hepper & Gough (1978) followed the eye index on eggs sampled from females caught in the wild at different seasons, but they did not manipulate temperature in rearing conditions. Richards & Wickins (1979) monitored the eye index to determine the time to hatching at only one temperature regime, 13-15 °C. Latrouite et al. (1984) used the eye index to assess the embryonic development in a study of the fecundity of the European lobster. In the present study, the rate of development of embryos of H. gammarus was monitored at different temperatures, using the eye it,alex to establish an equation that will forecast the date of hatching.
MATERIALS AND METHODS
Nine berried H. gammarus females were caught in September near Molene Island, Finist6re, and obtained from the fishery "La Langouste", Roscoff, Finist6re, France. They were transported to the laboratory in ,~20 h in wet wood chips at 5 °C. Mean ( _+SE) weights and carapace lengths of the lobsters were 851 _+48 g and 108 + 3 mm, respectively. The females were carrying recently extruded eggs; none of the embryos had detectable pigmented eyes. In the laboratory, lobsters were randomly transferred to cages (circular bottom, 1-m diameter; height, 1.20 m) made of plastic wire (1-cm mesh) immersed in three 3000-1 tanks, the temperatures of which were set at 10 °C (Females 1-3), 15 °C (Females 4-6) and 20 °C (Females 7-9). Seawater of each tank was recirculated and filtered through Eheim pumps and filters. ~ 1/4 of the volume of each tank was replaced every month with fresh seawater of the appropriate temperature. Salinity ranged between 36 and 37%° during the experiment. An artificial light regime of 12 h L: 12 h D was established and lobsters were fed mussels ad libitum three times a week. All experimental conditions, except temperature (tank shape, water volume, salinity, feeding rate, etc.), were kept identical in all three tanks. Temperature was controlled daily to the nearest 0.1 of a degree. Usual variations of temperature did not exceed + 0.2 °C; the maximum variation was + 1.2 °C during 24 h in the 10 °C tank, due to the failure of a refrigeration unit. The mean daily temperature exposure for each female (sum of the daily temperatures divided by the number of days until the hatch) was: Females 1:10.11 °C; 2:10.12 °C; 3: i0.10°C; 4:14.98 °C; 5:14.97 °C; 6: 15.00°C; 7:19.91 °C; 8:19.95 °C; and 9: 19.94°C. The rate of development of the embryos was determined by measuring the size of their eyes and calculating the eye index according to the method of Perkins (1972). Once every 2 wk, samples of 10-15 eggs.female -~ were removed from the periphery of the egg masses to minimize the disturbance of the female. Although such eggs are the furthest advanced in development in H. americanus (Perkins, 1972), in this experiment one observation.female -~ at mid-development of the embryos yielded differences of < 3 ~o between the eye indices of eggs sampled from the periphery and from the center of the egg masses. Eggs were immersed in seawater and the pigmented part
EMBRYONIC DEVELOPMENT OF HOMARUS GAMMARUS
63
(excluding the less pigmented halo) of one eye of each embryo was measured with an ocular micrometer and a dissecting microscope at 40 × magnification; the precision of the measurement was 12.5/am. For each eye, the maximum width and length were measured, the values were summed and divided by two, yielding the eye index. Regression lines of the variations of the eye index related to time were established. Their slopes were statistically compared two by two according to the standard following method (Diem, 1963; Geller, 1983). The equivalency of the residual variances was preliminarily verified by submitting their ratio to Snedecor's test; tr was the estimated common value of the two residual variances. With b and b' representing the slopes of two compared lines, the statistical variable t was calculated: b-b' t = tT
+ r l ' s '2
in which s2 and s' 2 are the variances of the n and n' abscissa of the points of the first and second lines, l'his variable t follows Student's law at (n + n' - 4) df. After these comparisons, a common slope + SE was calculated at each temperature according to Diem (1963). Predicted dates of hatch evaluated at mid-development were compared with the ~:ctual hatching dates in the nine experimental females and in four other females kept at 15 °C (two) and 19 °C (two).
RESULTS
The lowest eye index (EI) values ranged between 35 and 80 #m. Hatch occurred at E1 values of 640-680/am (~ + SE: 648 _+ 11 #m). A value of 650 #m was used for calculations thereafter. From the data collected from the embryos of each female, regression lines of the eye index related to the time were established (Fig. I). The correlation coefficient of each line was high (r>0.98), demonstrating a linear correlation in each set of data, and their slope b (#m.day -]) increased with temperature: b values were 2.68, 2.58 and 2.59 (common slope + SE: 2.62 + 0.02) in Females 1-3 at 10 °C; 4.45, 4.78 and 5.20 (4.79 + 0.05) in Females 4-6 at 15 °C; and 5.88, 6.57 and 5.50 (5.93 _ 0.13) at 20 °C. Statistical comparisons demonstrated that there was no significant difference between the slopes from the three females maintained at one of the temperatures, which permitted the calculation of a common slope (Diem, 1963) for each temperature. On the contrary, slopes at one temperature were different from those at the two other temperatures (10-15 and 10-20 °C: p<0.01; and 15-20 °C: p<0.05). For e~ch female, the time (t~) required to complete the embryonic period from the onset of eye pigment (EI = 0/am) to hatching was calcala~ed by dividing the EI at hatch by the slope of the regression line. The increase in E1 (/am-wk- ~) was then calculated by dividing the EI at hatch by t~. For instance, in Female 1, the EI at hatch was 653/am
64
G. CHARMANTIER
A N D R. M O U N E T G U I L L A U M E
20oC
'
600
I
I
lO°(:
s/
/
t / •
15°C
"J .
,/ /
I
# /
J
J
/
400
/
xl u a
I
I
I
I
I
I
/
ul
2OO
I
I
I
I
I
I
0
50
100
150
200
250
TI/AF (DAYS) Fig. 1. Regression lines of EI related to time at three temperatures. Lines terminate at hatching, at mean value of 650 s m . Dotted and solid lines correspond to individual and common regression lines. See text for values of slopes at each temperature.
and the slope was 2.68 #m'd-t yielding a tc of 244 days necessary for the development of the embryo from El = 0 tO 653 #m; the increase in ~I was thus 18.76 #m.wk -~. The relationship between the increase in El and the rearing temperature was linear (r= 0.9745). Eqn. 1 describing the relationship between increase in E] (y, #m.wk -I) and temperature (x, °C) was: y = 2.5104 x - 5.6240. As proposed by Perkins (1972) for H. americanus, the equation relating the increase in embryonic EI to temperature may be used in H. gammarus to predict the hatching date of larvae. In this intent, El, measured from eggs sampled ti'om a given female, must be substracted from EI at hatch (650 #m in H. gammarus); the result is divided by the value of the increase in EI.wk -] at the rearing temperature given by Eqn. 1, yielding If"
650 - El ll =
2.5104
x
-
5.6240'
in which n is the number of wk to hatch, EI the eye index in #m and x the temperature in °C. In Homarus, hatching is spread from 1 wk to 10 days. Since it is based on
EMBRYONIC DEVELOPMENT OF HOMARUS GAMMARUS
65
a mean value of EI, I"/predicts the date of the peak of hatching, approximately situated midway through the hatching period. Dates of beginning and end of hatching can also be evaluated, using individual maximum and minimum EI values, respectively, from an egg sample. Predicted dates (at mid-development for eye indices close to 300 #m) and actual dates of hatch did not differ by > ~ 10 days. Discrepancies between the two dates were found maximum in females kept at 19-20 °C (+ 10 d) and miniwum at 10 ¢C (_+ 6 d) despite the longer time between prediction and actual hatch in the latter condition.
DISCUSSION
In this study, the El value at hatch in H. gammarus was ~ 650 #m, which is slightly higher than the maximum values previously reported in this species, i.e., 600-620 #m (Richards & Wickins, 1979) and 634 #m (Hepper & Gough, 1978). This difference could originate in the diverse geographical origins of the experimental animals (embryos of different origins could slightly differ in size) or in minute measurement error (e.g., calibration of the ocular micrometer or magnification). The corresponding E1 value for H. americanus is 560 #m (Perkins, 1972) or 570 #m (Helluy & Beltz, 1991). The difference in EI at hatch, and therefore in eye dimension, between the European and American species is certainly related to the larger size of the eggs, embryos and larvae of the former species. Egg diameters and cephalothoracic lengths of first stage larvae reported for H. gammarus are 1.8 and 2.54 mm (Scott, 1903; Pandian, 1970a) and 1.6-1.5 and 1.74 mm for H. americanus, respectively (Herrick, 1895; Pandian, 1970b; Gruffyd et al., 1975). The relationship between the increase in EI and the rearing temperature (10-20 ° C) was linear, according to y = 2.5104x- 5.6240. Branford (1978) also reported an approximately linear relationship between the incubation period and the sum of average temperatures in H. gammarus. This relationship is also linear in H. americanus (i.e., y = 2.6019x - 8.3151) between 5 and 25 °C (Perkins, 1972). The slopes of equations for H. gammarus and H. americanus are nearly identical, indicating similar effects of temperature on embryonic development in the two species. Discrepancies in the intercepts of these equations are due to differences in size.
ACKNOWLEDGEMENTS
We thank Mr. Cabioch, "La Langouste", Roscoff, for his help in providing lobsters; and M. Charmantier-Daures, M.-A. Garcia, H.P. Lin, P. Thuet and J.-P. Trilles for their assistance in the laboratory.
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G. CHARMANTIER AND R, MOUNET-GUILLAUME
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