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Journal of Experimental Marine Biology and Ecology 248 2000 225–238
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Light induced larval release of a colonial ascidian
a , b
c
Richard B. Forward Jr. , James M. Welch , Craig M. Young
a
Duke University Marine Laboratory , School of Environment, 135 Duke Marine Lab Road, Beaufort,
NC 28516, USA
b
Biology Department , Wittenberg University, P.O. Box 720, Springfield, OH 45501-0720, USA
c
Division of Marine Sciences , Harbor Branch Oceanographic Institution, 5600 US 1 North, Fort Pierce,
FL 34946, USA
Received 7 October 1999; received in revised form 15 January 2000; accepted 31 January 2000
Abstract
Larval release and photobehavior were studied in the colonial ascidian Polyandrocarpa zorritensis. The test hypothesis was that if larval release is induced by light, then larvae should be
attracted to settlement areas where light is sufficient for larval release. Light induced larval release but the time course varied with light intensity. As the intensity of either sunlight or blue-green
light decreased 1 the time until the beginning of larval release latency became longer, 2 the mean time of larval release increased, and 3 the time interval over which larvae were released
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increased. The threshold light intensity to induce larval release in blue-green light 8.75 3 10
22 21
13 22
21
photons cm s
was lower than that in sunlight 3.6 3 10 photons cm
s . Light induced
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larval release was not affected by currents up to 15 cm s . Larvae aggregate in light when given
a choice between light and dark. This response did not vary with larval age. The lowest light
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intensity, at which larvae could distinguish between light and dark was 5.0 3 10 photons cm
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22 21
s in blue-green light and 2.9 3 10
photons cm s
in sunlight. Thus, the hypothesis is supported because larvae are attracted to areas where light intensity is sufficient for larval release.
2000 Elsevier Science B.V. All rights reserved.
Keywords : Ascidian; Behavior; Larvae; Light; Polyandrocarpa zorritensis; Settlement
1. Introduction
Colonial ascidians release fully developed larvae upon exposure to light following a period of dark adaptation Svane and Young, 1989. Actual larval release could result
Corresponding author. Tel.: 1 1-252-504-7610; fax: 1 1-252-504-7648. E-mail address
: rforwardduke.edu R.B. Forward Jr. 0022-0981 00 – see front matter
2000 Elsevier Science B.V. All rights reserved.
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.B. Forward et al. J. Exp. Mar. Biol. Ecol. 248 2000 225 –238
from musculature contractions of the adult or active swimming of larvae upon perception of light Svane and Young, 1989 and depends upon aspects of light exposure
and prior dark adaptation. The duration of light exposure before the onset of larval release is termed the dormant Huus, 1939 or latency period Lampert et al., 1981 and
varies with species and lighting conditions Svane and Young, 1989. In addition, in some species the number of larvae released increases as the length of the dark adaptation
period increases Watanabe and Lampert, 1973.
The present study considered light induced larval release by the polystyelid ascidian Polyandrocarpa zorritensis, which occurs on hard substrate in shallow water areas. After
egg incubation in the atrial cavity, this species releases fully developed free swimming larvae about 1.4 mm in length Vazquez and Young, 1996. Larvae become less active
over time and settle and metamorphose a number of hours after release Vazquez and Young, 1998. Larvae have a photolith, which is a compound sensory structure
composed of a statocyst and ocellus Berrill, 1947; Torrence, 1980. Although many invertebrate larvae are positively phototactic early in development and become negative
near the end of the larval phase Thorson, 1964, phototaxis of ascidian larvae is variable throughout development Svane and Young, 1989. For example, P
. zorritensis became positively phototactic near the end of development, which would be useful for
locating suitable shallow water habitats for settlement Vazquez and Young, 1998. If larval release is induced by light, then it is reasonable to hypothesize that larvae
should not be photonegative but rather should be attracted to settlement areas where light is sufficient for inducing larval release. To evaluate this hypothesis, the present
study determined 1 the minimum light intensity necessary to initiate larval release by P
. zorritensis, 2 the preference of larvae for light and dark areas, and 3 the minimum light intensity necessary for this preference. It was assumed that a preference for light
represented attraction to lighted areas for settlement. The hypothesis was supported, as larvae were attracted to areas where the light intensity was sufficient for larval release.
2. Material and methods
