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Effects of salinity and photoperiod on the burying behavior of brown shrimp Penaeus aztecus Ives
Aquaculture, 8 (1976) 327-336 0 Elsevler Scientific Publiehing Company,
327 Amsterdam
- Printed in The Netherlands
EFFECTS OF SALINITY AND PHOTOPERIOD ON THE BURYING BEHAVIOR OF BROWN SHRIMP PENAEUS AZTECUS IVES
G.J. LAKSHMI, A. VENKATARAMIAH Gulf Coast Research Laboratory,
(Received February
27th, 1976;
and G. GUNTER
Ocean Springs, Miss. 39564 (U.S.A.)
revised April 28th, 1976)
ABSTRACT Lakshmi, G.J., Venkataramiah, A. and Gunter, G., 1976. Effects of salinity and photoperiod on the burying behavior of brown shrimp Penaeus aztecus Ives. Aqua@ture, 8: 327-336. The burying behavior of Penceus catecus WASinvestigated with respect to both shortterm and long-term exposures to combinations of four salinities (8.5, 17.0 and 25.6 and 34.0’/,) and three photoperiods (total darkness, total light or partial light). Exposure either to total dark or total light regimes disturbed the diurnal rhythm and altered the normal body color of the shrimp. However, long-term exposures did not change the salinity-related burying responses. In response to light stimulii, greater numbers of shrimp always buried in lower (8.6 and 17.00/,) than in higher salinities, particularly$lljo”/,. On the basis of the parallel relationship between burying responses and growth rates, burying behavior was interpreted as a means of conserving energy, which in turn may promote greater growth in brown shrimp.
INTRODUCTION
In marine animals locomotion, tube-building, water propulsion or feeding rates are known to change with salinity variations (Kinne, 1971). Burying in brown shrimp, Penaeus aztecus Ives, is such an activity, and it is exhibited diurnally. The shrimp bury for protection from predators (Fuss and Ogren, 1966). They also bury to escape stress from extreme low temperature (Aldrich et al., 1968) or extreme low and high salinity and temperature conditions (Venkataramiah et al., 1974). Brown shrimp were shown to be euryhaline in previous studies (Venkataramiah et al., 1974). Yet when tested in a salinity (S) range of 8.5 to 34.0°/oo, the shrimp presented great variations in growth and survival rates. Over this range the animals displayed no symptoms of stress, and survival rates appeared normal (Venkataramiah et al., 1973 a and b). Burying responses of brown shrimp have been examined to see if salinity within this normal range (8.5 to 34.0°/o~S) could also affect their behavior. Animals were acclimated to
328
combinations of four salinities and three photoperiods, burying was induced with light as the stimulus.
and spontaneous
MATERIALS AND METHODS
Laboratory-reared juvenile brown shrimp, Penaeus u&ecus, of 40 mm mean length were used. The postlarvae were raised to juveniles and later maintained in the laboratory, following procedures described previously (Venkataramiah et al., 1974). Acclimation process Four groups of 30 juvenile shrimp were acclimated respectively to experimental salinities of 8.5,17.0, 25.5 and 34.0°/00 at room temperature (26°C). The shrimp previously had been kept at 17.0°ho S. Salinities were brought to acclimation levels by progressive changes at the rate of 1.5 to 2.0 parts per mille per day. Acclimation salinities were prepared with natural seawater of 32°/oo S either by concentrating with Instant Ocean Synthetic Sea Salt (Aquarium Systems, Inc.) or by diluting with dechlorinated tap water. The acclimation photoperiods were (1) total light, (2) total darkness and (3) partial light (or darkness) conditions. Animals adapted to total light or darkness were exposed to the respective photoperiods on a 24-h basis. The partially light-adapted shrimp were kept in 12-h light (day) and 12-h dark (night) regimes. An incandescent lamp (25 W) was fixed well above the water level (to minimize any temperature effect) of each of four aquaria. These lamps provided light on a 24-h basis for the total light-adapted shrimp. The aquaria used for dark adaptation were all enclosed in black polyethylene coverings. Acclimation and testing were carried out in 60-l glass aquaria. The aquaria were provided with biological filters and constant aeration. Testing procedure Each morning at about 08.00 h shrimp from each of the salinity-photoperiod combinations were exposed, one batch at a time, to normal room light for a 30-min period. Starting from zero time the number of buried individuals was recorded at 5-min intervals. Responses recorded during the 30-min period provided information on the spontaneous reaction to light in relation to salinity level. The studies were continued daily for a maximum of 33 days to determine the effects of acclimation on responses’to the salinityphotoperiod combinations. Shrimp were considered buried if they were completely or partially concealed in the substratum. The criterion employed to define the buried state in the case of partially hidden shrimp was a check for quiescence. Most of the observations w;re continued for a period of 20-33 days. The experiments with dark-adapted shrimp in 34.0°ho S were terminated on the
329
10th day, owing to accidental aeration failure and complete mortality. Replacements were made only for those animals that died in the first week. Prior to testing, the shrimp were allowed a Z-day adjustment period in their respective acclimation conditions. Feeding was provided daily at the rate of 8% of the initial body weight of each group. Acclimation salinities were adjusted every 3 days. The data obtained in these experiments were analyzed following methods of Snedecor and Cochran (1967). RESULTS
General obserua tions Under natural conditions brown shrimp are nocturnal animals; they confine their feeding and other activities to the nighttime. They bury in the bottom sediments in the daytime. The characteristic diurnal rhythm of brown shrimp was disturbed in the course of their exposure to total light and total darkness. Most of the shrimp ceased to bury and remained active even in the daytime. Under partial light conditions, however, the animals maintained normal diurnal rhythm. Bluishblack chromatophores developed among the shrimp held in the dark and partial light conditions. The color intensity of the chromatophores decreased in proportion to their exposure to light, with the result that the shrimp in total light were almost translucent. The color of these chromatophores compares with none of the four major types described by Kleinholz (1961). The chromatophores also lacked the usual diurnal rhythmicity, dark by day and pale by night, observed in Astacus and Uca (Brown, 1961). Perhaps such rhythmicity has no protective significance for the benthic shrimp. The development of light bluish-black chromatophores in the partially light-adapted shrimp suggests that totally dark nights may be as unnatural for them as are brightly lighted conditions. Effect of pho toperiod on burying Burying rates in the total light-adapted shrimp (Fig. 1A) show that a 30min exposure to room light failed to induce any further burying activity after zero time. On the other hand, the dark-adapted shrimp buried in greatest numbers (Fig. 1A). This is an indication of the magnitude of their photonegativity. In contrast to these groups and judged from the number of animals buried, the partially light-adapted shrimp displayed a moderate level of photosensitivity (Fig. 1B). The mean burying rates of animals from the three light regimes were compared to determine the effect of light acclimation on these rates. The burying rates essentially are similar at zero time in all the photoperiod-salinity combinations (Figs 1A and B). The similarity is most striking between the darkadapted and partially light-adapted shrimp due to the fact that both groups
DARK
PARTIAL
LIGHT J
I
I
I
0
5
IO
15
I
I
20
25
*.5%.
o------o
V.O%o
A-------O
25.5X0
o---o
34.0%0
x------l
1
J
I
30
0
5
I
IO
1
15
LIGHT
4
202?1
TIME (MINUTES)
Fig. 1. Mean burying rates (* 1 SE) of Penaeus aztecus, exposed to light for 30-min periods at each of four salinities, after previous exposure to darkness (A, upper), light (A, lower) and partial right (B). In the shrimp previously exposed to light and tested in the light there was no change in the number buried (indicated by the straight lines in panel A).
were in darkness for 12 h before being tested the next morning. However, both light-adapted and dark-adapted shrimp reacted differently toward the light stimuli after zero time; the difference must be due to the differences in their acclimation states. Comparison of burying rates between these groups showed highly significant differences (P > 0.01) at 5,15 and 25-min (or 30min) intervals in both 8.5 and 34.0*&, salinities (Table I). The response of the partially light-adapted shrimp was similar to that of the dark-adapted animals in low salinities (8.5 and 17.0%,), and to the light-adapted ones in high salinities, particularly in 34.0’/&, S (Table I). The speed with which shrimp responded to light stimuli varied with the acclimation photoperiod. The dark-adapted shrimp, being the fastest to react from the beginning, showed highly significant variations (P > 0.01) in burying rates within 0 to 5-min and 5 to 15-min intervals (Table II). The slower reacting partially light-adapted shrimp exhibited similar differences only within the 0 to 15-min interval. There was no appreciable change in burying rate in either group after the 18min interval. As shown in Table II,
331 TABLE I Significance of differences between mean burying rates of Penaeus aztecus in light (L), dark (D) and partial light (P) conditions at different time intervals Photoperiod compared
Salinity (“1,)
Intervals (min) 0
5
15
25
Lvs D
8.5 34.0
n.s. n.s.
P > 0.01 P > 0.01
P > 0.01 P > 0.01
P > 0.01 P > 0.01
L vs P
8.5 34.0
. . :.:.
P > 0.01
P > 0.01
F-F 0.01
n.6.
n.s.
8.5 34.0
n.s. P > 0.01
n.s. n.8.
n.s. n.s.
K
PvsD
0.05
n.s. = not significant. TABLE II Significance of differences between mean burying rates of Penaeus aztecus at different time intervals. In those acclimated to light, there was no further increase in burying after zero time Photoperiod
Salinity (“I,)
Intervals (min) compared ovs5
ovs15
15 vs 25 or 30
Dark
8.5 34.0
P> 0.01 P> 0.01
P> 0.01 P> 0.01
n.8. n.8.
Partial light
8.5 34.0
n.8. n.8.
P > 0.01 P > 0.01
n.6. n.s.
Light
8.6 34.0
No change in burying rates No change in burying rates
n.s. = not significant.
the rates at the 15 and 30-min intervals were not significantly different from one another. Effect of salinity variations Regardless of their light-adapted or dark-adapted acclimation state, more shrimp buried themselves in lower than in higher salinities. This is demonstrated by the consistently higher rate of burying in 8.5 and 17.0°/oo S compared with that in 34.0°/oo S (Figs 1A and B). A comparison of the rates between 8.5 and 34.0yoo S showed significant differences (P > 0.01) at inter-
332 vals
of 5,15 and 25 min (Table III). Although the partially light-adapted shrimp did not show such variations initially, 25 min later the differences were significant (P > 0.01). These variations might have occurred owing to the fact that shrimp in 34.0°/ooS uncovered themselves from the substratum from 15 min onwards, while in 8.5’bo S they did not (Fig. 1B). This behavior would indicate that high salinity ( 34.Oo/oo)may cause more “discomfort” than light to the partially light-adapted shrimp. In contrast, the dark-adapted animals buried throughout the 30-min interval. In this case light may be a more stressful factor than high salinity because of their total darkness acclimation state. TABLE 111 Significance of differences b&ween mean burying rates of Penaeus aztecus at salinities of 8.5 and 34.00/, Photoperiod
Dark Partial light Light
Salinities (“I,) compared
8.5 vs 34.0 8.5 vs 34.0 8.5 vs 34.0
Intervals (min) 5
15
25
P> 0.01
P> 0.01
K
31
P> 0.01 P > 0.01 P > 0.01
0.01
0.01
n.s. = not significant.
Effect of photoperiod
acclimation
Spontaneous burying rates in the various salinities were not altered by acclimation to the three photoperiods. Comparison of daily burying rates showed no positive correlation with respect to acclimation period in most of the salinities; an exception is found in 17°/ooS, a salinity that is near optimal for brown shrimp (Table IV). No significant differences in burying rates occurred between adjacent salinities in the process of light acclimation. However, the partially light-adapted shrimp held in 8.5%, S responded differently from those held in other media (8.5 vs 25.5°/ooS, P < 0.01; 8.5 vs 17.0 and 34.0°/oo S, P < 0.05), a result to which currently we can ascribe no biological significance. DISCUSSION
We conclude from these studies that (1) changes in salinity influence the burying rate in brown shrimp, (2) the acclimation light regime considerably affects burying rate, and (3) salinity-related burying responses are not altered as a result of acclimation to different photoperiods.
333 TABLE IV Correlation coefficient8 with reepect to salinity and burying rate8 of Peneaeus aztecus, under varying condition8 of acclimation. Note the significant correlation in burying rate8 in 17.0’/,, S Photoperiod
Salinity (“I,)
Day8 of observation
Correlation coefficient (r)
Dark
8.5 17.0 25.5 34.0
23 24 15 9
8.5 17.0 25.5 34.0
19 20 14 15
8.5 17.0 25.5 34.0
31 28 33 25
-e.o7 0.44 0.38 0.51 -0.14 0.51 0.41 0.27 0.27 0.45 -0.20 -0.30
Partial light
Light
Level of significance
:“; 0.05 n.s. n.s. :
0.05
n.s.
n.s. F”; n.s. n.s.
0.05
n.8. = not significant.
Brown shrimp occur naturally in a wide range of salinities from 0.22’ho (Gunter and Hall, 1963) to 69.0°/oo (Simmons, 1957). In the laboratory the young shrimp were shown further to withstand wide salinity fluctuations like other euryhaline species (Venkataramiah et al., 1974). Despite their euryhahnity, juvenile shrimp have exhibited variations in burying response within a normal salinity range of 8.5 to 34.0°/oo. The influence of salinity on their behavior evidently is strong enough that it could not be altered by the photoperiods to which the animals were acclimated for 3-4 weeks. Similar changes in burying response were reported in the pink shrimp, Penaeus duorancm, by Fuss and Ogren (1966). These animals were relatively more active in high salinities than in low salinities. Pardi and Papi (1961) cited examples where adult Puluemonetes sp. and nauplii of barnacles responded photonegatively in low salinities and photopositively in high salinities. Burying rates in the present experiments were compared with blood chloride ion concentrations of juvenile shrimp, determined’earlier in the same salinities (Venkataramiah et al., 1974). The comparison failed to provide a satisfactory explanation of the salinity-related behavioral mechanism. However, when burying rates were compared (Fig. 2) with growth rates in the same series of salinities (Venkataramiah et al., 1973 a and b) they were almost parallel to each other. In low salinities (8.5 and 17.0%,), where burying rates were high, growth was also correspondingly high. Conversely in high salinities (25.5 and 34.0°ho), growth rates were relatively low.
334
30
6C
.-----. 0 25 w (L ii
20
% E iii
z I a
‘\
NO. BURIED
\
\
4C
6 Z
15
k 9 g
5c
2
t
30
,
/
5 IO
/
L;I 20
s 5
0
IO -
.
0
1
8.5
I
17.0 SALINITY
I
25.5
34.0
(%c.)
Fig. 2. Comparison of growth rates of Penaeus oztecus in relation to burying and oxygen consumption rates. In compiling the figure, burying rates of dark-adapted shrimp were used at the 16-min interval. Growth rates, from Venkataramiah et al. (1973 a); oxygen consumption values, from Venkataramiah et al. (1974).
These observations suggest that burying reduces locomotor activity and conserves energy, which is ultimately utilized for growth. Respiratory rates measured under similar conditions seem to confirm this conclusion. Oxygen consumption rates of juvenile shrimp were determined at these salinities, after a 3-week acclimation period (Venkataramiah et al., 1974). The comparatively low oxygen uptake values in low salinities, (8.5 and 17.Oo/oo)and high values in high salinities (25.5 and 34.0%,) reflect the low or high energy demands in the respective salinities. The same pattern has been observed at 31°C. On the basis of oxygen consumption, Egusa (1961) demonstrated that “Kuruma” prawns, Penaeus japonicus, expend less energy when buried than when active. In the process of optimizing shrimp culture techniques, the physiological and behavioral responses of shrimp should be taken into consideration. Photoperiod and salinity are shown in these studies to influence growth in brown shrimp through effects on their behavior. Growth was maximal in the low salinity range (8.5 to 17.0°/ooS) where the shrimp buried most. In fact, this is the salinity range that the young shrimp actually occupy in their
335
natural habitat. Exposure to constant light or darkness is unnatural for brown shrimp adults as opposed to partial light. In both constant light or dark conditions the animals bury less and tend to be consistently active. The first signs of photonegativity were noticed when the postlarval brown shrimp were 14 mm long and 25 days old. The response strengthened as they grew older and larger. Light therefore is judged to be an important factor for brown shrimp in controlling their burying behavior. These animals spend a greater part of their lifetime buried, and uncover themselves at night to feed. Our unpublished diurnal oxygen consumption recordings with juveniles revealed the presence of two major peaks of nocturnal activity between 19.00 and 23.00 h, and 02.00 and 06.00 h. Between these peaks activity is considerably reduced, probably because the animals buried once again to protect themselves from predators. Apparently it is during these peak activity periods that the animals feed. We have observed this to be true during the first peak. We suggest that appreciation of the described salinity-photoperiod relation might help define tactics by which growth rate of shrimps under culture could be increased. ACKNOWLEDGEMENTS
We express our appreciation to MS Patricia Biesiot and MS Ann McCaslin for their help in preparation of the manuscript.
REFERENCES Aldrich, D.V., Wood, C.E. and Baxter, K.N., 1968. An ecological interpretation of low temperature responses in Penaeus aztecus and P. setiferus postlarvae. Bull. Mar. Sci., 18: 61-71. Brown Jr, F.A., 1961. Physiological rhythms. In: T.H. Waterman (Editor), The Physiology of Crustacea, Vol. 2. Academic Press, New York, N.Y., pp. 401-430. Egusa, S., 1961. Studies on the respiration of the “Kuruma” prawn, Penaeus japonicus Bate. II. Preliminary experiments on its oxygen consumption. Bull. Jap. Sot. Sci. Fish., 27: 650-659. Fuss, C.M. and Ogren, L.H., 1966. Factors affecting activity and burrowing habits of the pink shrimp, Penaeus duorarum Burkenroad. Biol. Bull., Woods Hole, Mass., 132: 170-191. Gunter, G. and Hall, G.E., 1963. Biological investigations of the St. Lucie Estuary (Florida) in connection with Lake Okeechobee discharges through the St. Lucie Canal. Gulf Res. Rep., 1: 189-307. Kinne, O., 1971. Salinity, 4-3 Animals. 4-31 Invertebrates. In: 0. Kinne (Editor), Marine Ecology, Vol. 1, Part 2. Wiley-Interscience, New York, N.Y., pp. 821-995. Kleinholz, L.H., 1961. Pigmentary effectors. In: T.H. Waterman (Editor), The Physiology of Crustacea, Vol. 2. Academic Press, New York, N.Y., pp. 133-169. Pardi, L. and Papi, F., 1961. Kinetic and tactic responses. In: T.H. Waterman (Editor), The Physiology of Crustacea, Vol. 2. Academic Press, New York, N.Y., pp. 365-399. Simmons, E.G., 1957. An ecological survey of the upper Laguna Madre of Texas. Publ. Inst. Mar. Sci., Univ. Texas, 4: 156-200.
336 Snedecor, G.W. and Cochran, W.G., 1967. Statistical Methods. Iowa State University Press, Ames, Iowa, 593 pp. Venkataramiah, A., Lakshmi, G.J. and Gunter, G., 1973 a. The effects of salinity and feeding levels on the growth rate and food conversion efficiency of the shrimp Penaeus aztecus. In: J.W. Avault Jr (Editor), Proc. Third Annu. Workshop World Maricult. SOC., La. State Univ., Baton Rouge, La., 3: 267-264. Venkataramiah, A., Lakshmi, G.J. and Gunter, G., 1973 b. The effects of salinity, temperature and feeding levels on the food conversion, growth and survival rates of the shrimp Penaeus aztecus. In: L.R. Worthen (Editor), Food-Drugs from the Sea Proceedings 1972. Mar. Technol. Sot., Washington, D.C., pp. 29-42. Venkataramish, A., Lakshmi, G.J. and Gunter, G., 1974. Studies on the Effects of Salinity and Temperature on the Commercial Shrimp Penaeus aztecus Ives, with Special Regard to Survival Limits, Growth, Oxygen Consumption and Ionic Regulation. U.S. Army Corps of Engineers Waterways Experiment Station, Vicksburg, Miss. Contract Report H-74-2, XII, 134 pp.
327 Amsterdam
- Printed in The Netherlands
EFFECTS OF SALINITY AND PHOTOPERIOD ON THE BURYING BEHAVIOR OF BROWN SHRIMP PENAEUS AZTECUS IVES
G.J. LAKSHMI, A. VENKATARAMIAH Gulf Coast Research Laboratory,
(Received February
27th, 1976;
and G. GUNTER
Ocean Springs, Miss. 39564 (U.S.A.)
revised April 28th, 1976)
ABSTRACT Lakshmi, G.J., Venkataramiah, A. and Gunter, G., 1976. Effects of salinity and photoperiod on the burying behavior of brown shrimp Penaeus aztecus Ives. Aqua@ture, 8: 327-336. The burying behavior of Penceus catecus WASinvestigated with respect to both shortterm and long-term exposures to combinations of four salinities (8.5, 17.0 and 25.6 and 34.0’/,) and three photoperiods (total darkness, total light or partial light). Exposure either to total dark or total light regimes disturbed the diurnal rhythm and altered the normal body color of the shrimp. However, long-term exposures did not change the salinity-related burying responses. In response to light stimulii, greater numbers of shrimp always buried in lower (8.6 and 17.00/,) than in higher salinities, particularly$lljo”/,. On the basis of the parallel relationship between burying responses and growth rates, burying behavior was interpreted as a means of conserving energy, which in turn may promote greater growth in brown shrimp.
INTRODUCTION
In marine animals locomotion, tube-building, water propulsion or feeding rates are known to change with salinity variations (Kinne, 1971). Burying in brown shrimp, Penaeus aztecus Ives, is such an activity, and it is exhibited diurnally. The shrimp bury for protection from predators (Fuss and Ogren, 1966). They also bury to escape stress from extreme low temperature (Aldrich et al., 1968) or extreme low and high salinity and temperature conditions (Venkataramiah et al., 1974). Brown shrimp were shown to be euryhaline in previous studies (Venkataramiah et al., 1974). Yet when tested in a salinity (S) range of 8.5 to 34.0°/oo, the shrimp presented great variations in growth and survival rates. Over this range the animals displayed no symptoms of stress, and survival rates appeared normal (Venkataramiah et al., 1973 a and b). Burying responses of brown shrimp have been examined to see if salinity within this normal range (8.5 to 34.0°/o~S) could also affect their behavior. Animals were acclimated to
328
combinations of four salinities and three photoperiods, burying was induced with light as the stimulus.
and spontaneous
MATERIALS AND METHODS
Laboratory-reared juvenile brown shrimp, Penaeus u&ecus, of 40 mm mean length were used. The postlarvae were raised to juveniles and later maintained in the laboratory, following procedures described previously (Venkataramiah et al., 1974). Acclimation process Four groups of 30 juvenile shrimp were acclimated respectively to experimental salinities of 8.5,17.0, 25.5 and 34.0°/00 at room temperature (26°C). The shrimp previously had been kept at 17.0°ho S. Salinities were brought to acclimation levels by progressive changes at the rate of 1.5 to 2.0 parts per mille per day. Acclimation salinities were prepared with natural seawater of 32°/oo S either by concentrating with Instant Ocean Synthetic Sea Salt (Aquarium Systems, Inc.) or by diluting with dechlorinated tap water. The acclimation photoperiods were (1) total light, (2) total darkness and (3) partial light (or darkness) conditions. Animals adapted to total light or darkness were exposed to the respective photoperiods on a 24-h basis. The partially light-adapted shrimp were kept in 12-h light (day) and 12-h dark (night) regimes. An incandescent lamp (25 W) was fixed well above the water level (to minimize any temperature effect) of each of four aquaria. These lamps provided light on a 24-h basis for the total light-adapted shrimp. The aquaria used for dark adaptation were all enclosed in black polyethylene coverings. Acclimation and testing were carried out in 60-l glass aquaria. The aquaria were provided with biological filters and constant aeration. Testing procedure Each morning at about 08.00 h shrimp from each of the salinity-photoperiod combinations were exposed, one batch at a time, to normal room light for a 30-min period. Starting from zero time the number of buried individuals was recorded at 5-min intervals. Responses recorded during the 30-min period provided information on the spontaneous reaction to light in relation to salinity level. The studies were continued daily for a maximum of 33 days to determine the effects of acclimation on responses’to the salinityphotoperiod combinations. Shrimp were considered buried if they were completely or partially concealed in the substratum. The criterion employed to define the buried state in the case of partially hidden shrimp was a check for quiescence. Most of the observations w;re continued for a period of 20-33 days. The experiments with dark-adapted shrimp in 34.0°ho S were terminated on the
329
10th day, owing to accidental aeration failure and complete mortality. Replacements were made only for those animals that died in the first week. Prior to testing, the shrimp were allowed a Z-day adjustment period in their respective acclimation conditions. Feeding was provided daily at the rate of 8% of the initial body weight of each group. Acclimation salinities were adjusted every 3 days. The data obtained in these experiments were analyzed following methods of Snedecor and Cochran (1967). RESULTS
General obserua tions Under natural conditions brown shrimp are nocturnal animals; they confine their feeding and other activities to the nighttime. They bury in the bottom sediments in the daytime. The characteristic diurnal rhythm of brown shrimp was disturbed in the course of their exposure to total light and total darkness. Most of the shrimp ceased to bury and remained active even in the daytime. Under partial light conditions, however, the animals maintained normal diurnal rhythm. Bluishblack chromatophores developed among the shrimp held in the dark and partial light conditions. The color intensity of the chromatophores decreased in proportion to their exposure to light, with the result that the shrimp in total light were almost translucent. The color of these chromatophores compares with none of the four major types described by Kleinholz (1961). The chromatophores also lacked the usual diurnal rhythmicity, dark by day and pale by night, observed in Astacus and Uca (Brown, 1961). Perhaps such rhythmicity has no protective significance for the benthic shrimp. The development of light bluish-black chromatophores in the partially light-adapted shrimp suggests that totally dark nights may be as unnatural for them as are brightly lighted conditions. Effect of pho toperiod on burying Burying rates in the total light-adapted shrimp (Fig. 1A) show that a 30min exposure to room light failed to induce any further burying activity after zero time. On the other hand, the dark-adapted shrimp buried in greatest numbers (Fig. 1A). This is an indication of the magnitude of their photonegativity. In contrast to these groups and judged from the number of animals buried, the partially light-adapted shrimp displayed a moderate level of photosensitivity (Fig. 1B). The mean burying rates of animals from the three light regimes were compared to determine the effect of light acclimation on these rates. The burying rates essentially are similar at zero time in all the photoperiod-salinity combinations (Figs 1A and B). The similarity is most striking between the darkadapted and partially light-adapted shrimp due to the fact that both groups
DARK
PARTIAL
LIGHT J
I
I
I
0
5
IO
15
I
I
20
25
*.5%.
o------o
V.O%o
A-------O
25.5X0
o---o
34.0%0
x------l
1
J
I
30
0
5
I
IO
1
15
LIGHT
4
202?1
TIME (MINUTES)
Fig. 1. Mean burying rates (* 1 SE) of Penaeus aztecus, exposed to light for 30-min periods at each of four salinities, after previous exposure to darkness (A, upper), light (A, lower) and partial right (B). In the shrimp previously exposed to light and tested in the light there was no change in the number buried (indicated by the straight lines in panel A).
were in darkness for 12 h before being tested the next morning. However, both light-adapted and dark-adapted shrimp reacted differently toward the light stimuli after zero time; the difference must be due to the differences in their acclimation states. Comparison of burying rates between these groups showed highly significant differences (P > 0.01) at 5,15 and 25-min (or 30min) intervals in both 8.5 and 34.0*&, salinities (Table I). The response of the partially light-adapted shrimp was similar to that of the dark-adapted animals in low salinities (8.5 and 17.0%,), and to the light-adapted ones in high salinities, particularly in 34.0’/&, S (Table I). The speed with which shrimp responded to light stimuli varied with the acclimation photoperiod. The dark-adapted shrimp, being the fastest to react from the beginning, showed highly significant variations (P > 0.01) in burying rates within 0 to 5-min and 5 to 15-min intervals (Table II). The slower reacting partially light-adapted shrimp exhibited similar differences only within the 0 to 15-min interval. There was no appreciable change in burying rate in either group after the 18min interval. As shown in Table II,
331 TABLE I Significance of differences between mean burying rates of Penaeus aztecus in light (L), dark (D) and partial light (P) conditions at different time intervals Photoperiod compared
Salinity (“1,)
Intervals (min) 0
5
15
25
Lvs D
8.5 34.0
n.s. n.s.
P > 0.01 P > 0.01
P > 0.01 P > 0.01
P > 0.01 P > 0.01
L vs P
8.5 34.0
. . :.:.
P > 0.01
P > 0.01
F-F 0.01
n.6.
n.s.
8.5 34.0
n.s. P > 0.01
n.s. n.8.
n.s. n.s.
K
PvsD
0.05
n.s. = not significant. TABLE II Significance of differences between mean burying rates of Penaeus aztecus at different time intervals. In those acclimated to light, there was no further increase in burying after zero time Photoperiod
Salinity (“I,)
Intervals (min) compared ovs5
ovs15
15 vs 25 or 30
Dark
8.5 34.0
P> 0.01 P> 0.01
P> 0.01 P> 0.01
n.8. n.8.
Partial light
8.5 34.0
n.8. n.8.
P > 0.01 P > 0.01
n.6. n.s.
Light
8.6 34.0
No change in burying rates No change in burying rates
n.s. = not significant.
the rates at the 15 and 30-min intervals were not significantly different from one another. Effect of salinity variations Regardless of their light-adapted or dark-adapted acclimation state, more shrimp buried themselves in lower than in higher salinities. This is demonstrated by the consistently higher rate of burying in 8.5 and 17.0°/oo S compared with that in 34.0°/oo S (Figs 1A and B). A comparison of the rates between 8.5 and 34.0yoo S showed significant differences (P > 0.01) at inter-
332 vals
of 5,15 and 25 min (Table III). Although the partially light-adapted shrimp did not show such variations initially, 25 min later the differences were significant (P > 0.01). These variations might have occurred owing to the fact that shrimp in 34.0°/ooS uncovered themselves from the substratum from 15 min onwards, while in 8.5’bo S they did not (Fig. 1B). This behavior would indicate that high salinity ( 34.Oo/oo)may cause more “discomfort” than light to the partially light-adapted shrimp. In contrast, the dark-adapted animals buried throughout the 30-min interval. In this case light may be a more stressful factor than high salinity because of their total darkness acclimation state. TABLE 111 Significance of differences b&ween mean burying rates of Penaeus aztecus at salinities of 8.5 and 34.00/, Photoperiod
Dark Partial light Light
Salinities (“I,) compared
8.5 vs 34.0 8.5 vs 34.0 8.5 vs 34.0
Intervals (min) 5
15
25
P> 0.01
P> 0.01
K
31
P> 0.01 P > 0.01 P > 0.01
0.01
0.01
n.s. = not significant.
Effect of photoperiod
acclimation
Spontaneous burying rates in the various salinities were not altered by acclimation to the three photoperiods. Comparison of daily burying rates showed no positive correlation with respect to acclimation period in most of the salinities; an exception is found in 17°/ooS, a salinity that is near optimal for brown shrimp (Table IV). No significant differences in burying rates occurred between adjacent salinities in the process of light acclimation. However, the partially light-adapted shrimp held in 8.5%, S responded differently from those held in other media (8.5 vs 25.5°/ooS, P < 0.01; 8.5 vs 17.0 and 34.0°/oo S, P < 0.05), a result to which currently we can ascribe no biological significance. DISCUSSION
We conclude from these studies that (1) changes in salinity influence the burying rate in brown shrimp, (2) the acclimation light regime considerably affects burying rate, and (3) salinity-related burying responses are not altered as a result of acclimation to different photoperiods.
333 TABLE IV Correlation coefficient8 with reepect to salinity and burying rate8 of Peneaeus aztecus, under varying condition8 of acclimation. Note the significant correlation in burying rate8 in 17.0’/,, S Photoperiod
Salinity (“I,)
Day8 of observation
Correlation coefficient (r)
Dark
8.5 17.0 25.5 34.0
23 24 15 9
8.5 17.0 25.5 34.0
19 20 14 15
8.5 17.0 25.5 34.0
31 28 33 25
-e.o7 0.44 0.38 0.51 -0.14 0.51 0.41 0.27 0.27 0.45 -0.20 -0.30
Partial light
Light
Level of significance
:“; 0.05 n.s. n.s. :
0.05
n.s.
n.s. F”; n.s. n.s.
0.05
n.8. = not significant.
Brown shrimp occur naturally in a wide range of salinities from 0.22’ho (Gunter and Hall, 1963) to 69.0°/oo (Simmons, 1957). In the laboratory the young shrimp were shown further to withstand wide salinity fluctuations like other euryhaline species (Venkataramiah et al., 1974). Despite their euryhahnity, juvenile shrimp have exhibited variations in burying response within a normal salinity range of 8.5 to 34.0°/oo. The influence of salinity on their behavior evidently is strong enough that it could not be altered by the photoperiods to which the animals were acclimated for 3-4 weeks. Similar changes in burying response were reported in the pink shrimp, Penaeus duorancm, by Fuss and Ogren (1966). These animals were relatively more active in high salinities than in low salinities. Pardi and Papi (1961) cited examples where adult Puluemonetes sp. and nauplii of barnacles responded photonegatively in low salinities and photopositively in high salinities. Burying rates in the present experiments were compared with blood chloride ion concentrations of juvenile shrimp, determined’earlier in the same salinities (Venkataramiah et al., 1974). The comparison failed to provide a satisfactory explanation of the salinity-related behavioral mechanism. However, when burying rates were compared (Fig. 2) with growth rates in the same series of salinities (Venkataramiah et al., 1973 a and b) they were almost parallel to each other. In low salinities (8.5 and 17.0%,), where burying rates were high, growth was also correspondingly high. Conversely in high salinities (25.5 and 34.0°ho), growth rates were relatively low.
334
30
6C
.-----. 0 25 w (L ii
20
% E iii
z I a
‘\
NO. BURIED
\
\
4C
6 Z
15
k 9 g
5c
2
t
30
,
/
5 IO
/
L;I 20
s 5
0
IO -
.
0
1
8.5
I
17.0 SALINITY
I
25.5
34.0
(%c.)
Fig. 2. Comparison of growth rates of Penaeus oztecus in relation to burying and oxygen consumption rates. In compiling the figure, burying rates of dark-adapted shrimp were used at the 16-min interval. Growth rates, from Venkataramiah et al. (1973 a); oxygen consumption values, from Venkataramiah et al. (1974).
These observations suggest that burying reduces locomotor activity and conserves energy, which is ultimately utilized for growth. Respiratory rates measured under similar conditions seem to confirm this conclusion. Oxygen consumption rates of juvenile shrimp were determined at these salinities, after a 3-week acclimation period (Venkataramiah et al., 1974). The comparatively low oxygen uptake values in low salinities, (8.5 and 17.Oo/oo)and high values in high salinities (25.5 and 34.0%,) reflect the low or high energy demands in the respective salinities. The same pattern has been observed at 31°C. On the basis of oxygen consumption, Egusa (1961) demonstrated that “Kuruma” prawns, Penaeus japonicus, expend less energy when buried than when active. In the process of optimizing shrimp culture techniques, the physiological and behavioral responses of shrimp should be taken into consideration. Photoperiod and salinity are shown in these studies to influence growth in brown shrimp through effects on their behavior. Growth was maximal in the low salinity range (8.5 to 17.0°/ooS) where the shrimp buried most. In fact, this is the salinity range that the young shrimp actually occupy in their
335
natural habitat. Exposure to constant light or darkness is unnatural for brown shrimp adults as opposed to partial light. In both constant light or dark conditions the animals bury less and tend to be consistently active. The first signs of photonegativity were noticed when the postlarval brown shrimp were 14 mm long and 25 days old. The response strengthened as they grew older and larger. Light therefore is judged to be an important factor for brown shrimp in controlling their burying behavior. These animals spend a greater part of their lifetime buried, and uncover themselves at night to feed. Our unpublished diurnal oxygen consumption recordings with juveniles revealed the presence of two major peaks of nocturnal activity between 19.00 and 23.00 h, and 02.00 and 06.00 h. Between these peaks activity is considerably reduced, probably because the animals buried once again to protect themselves from predators. Apparently it is during these peak activity periods that the animals feed. We have observed this to be true during the first peak. We suggest that appreciation of the described salinity-photoperiod relation might help define tactics by which growth rate of shrimps under culture could be increased. ACKNOWLEDGEMENTS
We express our appreciation to MS Patricia Biesiot and MS Ann McCaslin for their help in preparation of the manuscript.
REFERENCES Aldrich, D.V., Wood, C.E. and Baxter, K.N., 1968. An ecological interpretation of low temperature responses in Penaeus aztecus and P. setiferus postlarvae. Bull. Mar. Sci., 18: 61-71. Brown Jr, F.A., 1961. Physiological rhythms. In: T.H. Waterman (Editor), The Physiology of Crustacea, Vol. 2. Academic Press, New York, N.Y., pp. 401-430. Egusa, S., 1961. Studies on the respiration of the “Kuruma” prawn, Penaeus japonicus Bate. II. Preliminary experiments on its oxygen consumption. Bull. Jap. Sot. Sci. Fish., 27: 650-659. Fuss, C.M. and Ogren, L.H., 1966. Factors affecting activity and burrowing habits of the pink shrimp, Penaeus duorarum Burkenroad. Biol. Bull., Woods Hole, Mass., 132: 170-191. Gunter, G. and Hall, G.E., 1963. Biological investigations of the St. Lucie Estuary (Florida) in connection with Lake Okeechobee discharges through the St. Lucie Canal. Gulf Res. Rep., 1: 189-307. Kinne, O., 1971. Salinity, 4-3 Animals. 4-31 Invertebrates. In: 0. Kinne (Editor), Marine Ecology, Vol. 1, Part 2. Wiley-Interscience, New York, N.Y., pp. 821-995. Kleinholz, L.H., 1961. Pigmentary effectors. In: T.H. Waterman (Editor), The Physiology of Crustacea, Vol. 2. Academic Press, New York, N.Y., pp. 133-169. Pardi, L. and Papi, F., 1961. Kinetic and tactic responses. In: T.H. Waterman (Editor), The Physiology of Crustacea, Vol. 2. Academic Press, New York, N.Y., pp. 365-399. Simmons, E.G., 1957. An ecological survey of the upper Laguna Madre of Texas. Publ. Inst. Mar. Sci., Univ. Texas, 4: 156-200.
336 Snedecor, G.W. and Cochran, W.G., 1967. Statistical Methods. Iowa State University Press, Ames, Iowa, 593 pp. Venkataramiah, A., Lakshmi, G.J. and Gunter, G., 1973 a. The effects of salinity and feeding levels on the growth rate and food conversion efficiency of the shrimp Penaeus aztecus. In: J.W. Avault Jr (Editor), Proc. Third Annu. Workshop World Maricult. SOC., La. State Univ., Baton Rouge, La., 3: 267-264. Venkataramiah, A., Lakshmi, G.J. and Gunter, G., 1973 b. The effects of salinity, temperature and feeding levels on the food conversion, growth and survival rates of the shrimp Penaeus aztecus. In: L.R. Worthen (Editor), Food-Drugs from the Sea Proceedings 1972. Mar. Technol. Sot., Washington, D.C., pp. 29-42. Venkataramish, A., Lakshmi, G.J. and Gunter, G., 1974. Studies on the Effects of Salinity and Temperature on the Commercial Shrimp Penaeus aztecus Ives, with Special Regard to Survival Limits, Growth, Oxygen Consumption and Ionic Regulation. U.S. Army Corps of Engineers Waterways Experiment Station, Vicksburg, Miss. Contract Report H-74-2, XII, 134 pp.