Aquaculture, 41 (1984) 93-98 93
Elsevier Science
Publishers B.V., Amsterdam - Printed in The Netherlands
AN HYPOTHESIS ON THE LIGHT REQUIREMENTS FOR SPAWNING
PENAEID SHRIMP, WITH EMPHASIS ON PENAEUS SETIFERUS
WILLIAM
A. WURTS1 and ROBERT R. STICKNEY2
Department
of Wildlife and Fisheries Sciences, Texas A & M University, College
Station, TX 77843 (U.S.A.)
1To whom correspondence should be
addressed.
2Present address: Fisheries Research Laboratory, Southern Illinois
University, Carbondale, IL 62901 (U.S.A.)
(Accepted 25 April 1984)
ABSTRACT
Wurts, W.A. and Stickney, R.R., 1984. An hypothesis on the light
requirements for spawning penaeid shrimp, with emphasis on Penaeus setiferus. Aquaculture, 41: 93-98.
Using standard
equations, we have concluded that the light intensities which have been used by
investigators attempting to induce maturation and spawning are up to 4000 times higher than would occur over
the spawning grounds of Penaeus setiferus, a species for which successful controlled spawning has yet to be achieved.
We recommend that future attempts at captive maturation and spawning with that
species be conducted at light levels of no more than 12 µW cm-2 and
that the intensity at which other species of shrimp are matured and spawned be
examined relative to natural light levels on their spawning grounds. It is
further recommended that the need for light of blue or green color be evaluated
for its role as a promotor of maturation and spawning. The intent of these
modifications in current spawning procedures is to eliminate the need for
eyestalk ablation in captively spawned penaeid shrimp.
Viable spawns from unablated white
shrimp, Penaeus
setiferus, matured
in the laboratory, have not been reported. Current methods for inducing
maturation and spawning in other species of penaeid shrimp depend heavily on
the technique of unilateral eystalk ablation (Aquacop, 1975; Wear and Santiago,
1976; Santiago, 1977; Halder, 1978; Primavera, 1978; Beard and Wickins, 1980;
Lawrence et al., 1980; Brown et al., 1980; Emmerson, 1980, 1983). Eyestalk
ablation should be considered a stop-gap measure until less traumatic methods
can be developed. Utilization of environmental rather than surgical stimulation
of maturation may overcome the problems of reduced fecundity and poor egg viability
which frequently follow use of the ablation technique.
Many of the problems which have
led to difficulty in spawning penaeid
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1984 Elsevier
Science Publishers B.V.
94
shrimp in the laboratory appear to relate to the fact that researchers
have not faithfully reproduced natural conditions with respect to light
intensity and color. To date, primary environmental control has been in terms
of temperature and photoperiod.
The importance of light in
controlling phytoplankton blooms in aquatic environments is widely recognized,
as is the tendency of some planktonic species to key their reproductive peaks
to correspond with phytoplankton blooms (Heinrich, 1962). While it has not been
firmly demonstrated experimentally, there is circumstantial evidence to
indicate that many marine animals, particularly those with herbivorous or
omnivorous planktonic larvae, synchronize their hormonal cycles and gonadal
maturation periods with those portions of the annual light cycle which promote
the highest levels of primary productivity. In addition, the presence of an
eyestalk mediated reproductive mechanism in shrimp emphasizes the importance of
light on maturation. It is our purpose in this paper to present support for the
contention that light intensity and color are keys to the induction of gonadal
development in penaeid shrimp and to suggest a light intensity above which
captive spawning of unablated white shrimp (Penaeus setiferus) may not readily occur.
Light is attenuated as it passes through sea water and it
is only wavelengths in the blue and green which penetrate to appreciable
depth, even in clear water. Turbidity affects light penetration as does angle
of incidence. The amount of surface light reaching a given depth can be
determined from the equation
Iz = I0e-kz
where, I0 is surface light intensity; IZ
is the light intensity at depth z (in meters); e is the natural
logarithm; and k is the extinction coefficient.
Light intensities are reported in µW cm -2.
The extinction coefficient is related to wavelength
and the level of suspended particulate matter. In clear coastal waters, k is
about 0.15, while in turbid coastal waters it is around 0.46. In the highly
turbid waters of many harbors and estuaries, values of k may be as high as 1.5 (Clark and
Denton, 1962). In coastal waters, light in the green or yellow-green range
(500--600 nm wavelength) is the least attenuated (Clark and Denton, 1962;
Gross, 1972).
P. setiferus occur in highest concentrations at
a depth of about 20 m off the Texas coast (Darnell et al., 1983), so it was
that depth which was selected for evaluation of light intensity. We calculated
the maximum possible light intensity at the ocean bottom in 20 m of water on a
clear day at latitudes of 25°N and 30°N at solar noon on
22 June, 21 March and 22 December. Since P. setiferus is found close to shore, an
extinction coefficient of 0.46 was adopted. We find that coefficient to be
consistent with an actual value (k = 0.47) calculated from a set of light readings obtained
at 28°59.3'N and 90°0.5'W in the Gulf of Mexico (T.J.
Bright, personal communication, 1983).
95
Surface light
intensity was calculated for the time and dates of interest from tables
presented by List (1951). The data are presented in Table I for 20 m. Of
particular importance is the fact that at each latitude and date the maximum
light intensity at depth Iz is less than 12 µW cm-2. If
maturation actually occurs in shallow water where mature shrimp are most often
taken along the Texas coast (Gallaway and Reitsema, 1981), the calculated value
for 20 m may still be high due to the increased nearshore values of the
extinction coefficient. It is our hypothesis that the values presented in Table
I are maximum light intensities to which maturing white shrimp would be exposed.
Our calculations may lead to values which are actually higher than those
present over shrimp spawning grounds since the calculations assumed cloudless
skies and the absence of a turbidity layer near the bottom of the water column.
The calculations were also made at solar noon, when light penetration is
maximum for the day. The actual value from the Gulf of Mexico obtained on 12
February 1983 at between 14.25 and 14.30 h at 15.8 m depth under 30% cloud
cover was 2.25 µW cm-2 (T.J. Bright, personal communication, 1983).
TABLE I
|
||||
Conditions influencing light penetration and the
intensity of light at 20 m depth at solar noon on three dates and two
latitudes
|
||||
Date
|
Latitude
|
Angle
of sun from horizon
|
Surface
light (µW cm-2)
|
Iz
(µW cm-2)
|
22 December
|
25°N
|
36°33'
|
65 171.7
|
6.6
|
22 December
|
30°N
|
41°33'
|
56 597.7
|
5.7
|
21 March
|
25°N
|
59°55'
|
98 864.1
|
10.0
|
21 March
|
30°N
|
64°55'
|
93 513.2
|
9.4
|
22 June
|
25°N
|
83°27'
|
115 968.1
|
11.7
|
22 June
|
30°N
|
88°27'
|
115 119.8
|
11.6
|
When compared with light
intensities reported by various investigators who have attempted to spawn
penaeid shrimp in the laboratory, we find that our value is as much as 4000
times lower (Table II). Support for our value comes from the work by
Chamberlain and Lawrence (1981) with P. stylirostris which showed that
greatest spawning success was at a light level equivalent to 13 µW cm-2.
No attempts were made to spawn that species at light levels between 0 and 13 µW
cm-2.
If annual seasonal cycles are to
be approximated in the laboratory to induce spawning in shrimp, it is important
to adjust not only temperature and photoperiod, but also to adjust for seasonal
differences in light intensity. As shown in Table I, light intensity is reduced
by more than 40% in the winter as compared with the summer.
There is some experimental evidence to support the
contention that not
96
only should shrimp be spawned
under low light intensity, but that blue or green light should be used.
Emmerson et al. (1983) demonstrated that unablated P. indicus grew and spawned continuously when
held in a 12 h photoperiod at 45 µW cm-2 under green and blue light. However, shrimp of the same
species ceased spawning and lost weight when maintained under the same
photoperiod in diffuse natural light of 50 µW cm-2. Spectral influences on spawning were also examined by
Caillouet (1972) and Primavera (1980).
TABLE II
|
||
Light intensities calculated from reports in the
literature for shrimp maturation facilities
|
||
Shrimp species
|
Light
Intensity (µW cm-2)
|
Ref.
|
Penaeus
merguiensis,
|
11597-46
387
|
Aquacop (1975);
|
P.
japonicus,
|
Lawrence
et al.
|
|
P.
aztecus,
|
(1980)
|
|
P.
semisulcatus,
|
||
P.
setiferus,
|
||
Metapenaeus
ensis
|
||
P.
japonicus
|
130- 544
|
Laubier-Bonichon (1978)
|
P.
stylirostris,
|
0- 320
|
Chamberlain
and
|
P.
vannamei
|
Lawrence
(1981)
|
|
P.
indicus, P. monodon
|
70
|
Emmerson (1980, 1983)
|
P.
monodon
|
39- 68
|
Beard and Wickins (1980)
|
P.
indicus
|
45- 50
|
Emmerson et al. (1983)
|
P.
plebejus
|
3- 17
|
Kelemec
and Smith (1980)
|
Another apparently light-related
phenomenon involves the lack of spawning in small tanks. Many researchers feel
that only relatively large tanks can be used to spawn shrimp. Emmerson (1983)
noted that unablated P. monodon would
not spawn in tanks 2 m X 1.5 m X 0.5 m deep, even under reduced light
intensity. In an earlier study, the same author (Emmerson, 1980) found that P.
indicus held in tanks with black interiors
spawned twice as frequently as those maintained in tanks with white interiors.
The difference would appear to relate to the absorbance of light in black tanks
as compared with reflectance in white tanks. While some shrimp species may
actually require considerable space to affect mating, an argument can be made
that small tanks reflect more light internally than large ones and, because of
generally shallower water, may also attenuate less light. Thus, under the same
surface intensities, the Iz to which the shrimp are exposed
may be higher in a small tank, another reason to use tanks with dark colored
interiors and to measure light intensities within tanks, rather than at the
water surface.
97
In summary, our evaluation has
demonstrated that the levels of light being utilized to spawn penaeid shrimp in the laboratory
are excessively high compared with conditions on the natural spawning grounds
and that the color of light being used does not simulate natural conditions.
Incorporation of light intensities over spawning chambers of no more than 12 µW
cm-2 utilizing blue or green light may obviate the need
for eyestalk ablation. This theory should, at least, be thoroughly tested by
investigators who have the facilities to conduct the studies. The ultimate goal
will be to eliminate the need for eyestalk ablation and to spawn shrimp on
demand through manipulation of environmental conditions alone. While P. setiferus was used as our model,
the approach may have merit in conjunction with other penaeids, other
crustaceans and finfish as well.
ACKNOWLEDGEMENTS
We would like to thank John H.
Wormuth and G.A. Franceschini for their valuable assistance in directing us to
the appropriate information sources with respect to surface light intensities
and to light attenuation in sea water, and for their helpful comments during
preparation of the manuscript. The manuscript was reviewed by George
Chamberlain, whose comments and criticisms are appreciated.
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