286 M
. Huber, D.A. Bengtson J. Exp. Mar. Biol. Ecol. 240 1999 285 –302
accumulated significantly more visceral fat than those exposed to the 15-h photoperiod. In females, the amount of visceral fat accumulated was inversely proportional to the hours of light.
These findings suggest that this species has evolved mechanisms that enable it to anticipate the coming of winter as well as the coming of suitable breeding conditions and ensure that it exhibits
the appropriate response at the appropriate time reserve accumulation for the winter or gonad maturation in the spring.
1999 Elsevier Science B.V. All rights reserved.
Keywords : Photoperiod; Temperature; Reproduction; Gonadosomatic index; Hepatosomatic index;
Visceral fat; Spawning cues; Silversides
1. Introduction
In temperate regions conditions suitable for the development of larval stages of organisms occur during restricted times of the year. Although these conditions vary
among species they usually include an abundance of food organisms, a predator- minimized environment, and or temperatures that can optimize the rate of growth and
development before the cold and food-limited conditions typical of the winter occur Balon, 1975; Hubbs, 1976. The occurrence of these conditions in a given habitat is
affected by a multitude of factors and thus their exact timing may vary from year to year. Nevertheless, fish must be able to anticipate these conditions and begin the
maturation process beforehand to ensure that reproduction occurs at a time when offspring can survive DeVlaming, 1974.
Munro 1990 proposed a tentative classification for proximate controlling factors responsible for the regulation of teleost reproductive cycles. The first category includes
predictive cues responsible for the initiation of maturation by triggering the reproductive axis of the fish and initiating the sequestration of resources to reproductive tissues. The
second category includes synchronizing cues which signal the arrival of suitable breeding conditions and induce final maturation and spawning. The final category
contains terminating cues, which end the breeding season by inducing gonadal regression. Modifying factors can alter the response of individuals to any of the cues
altering the various stages of the reproductive cycle.
The present study focuses on the effect of predictive cues on the regulation of the initiation of the reproductive cycle of the inland silverside Menidia beryllina Cope.
This species is found in coastal estuaries extending from Cape Cod, Massachusetts, USA, to Tampico, Veracruz, Mexico Bigelow and Schroeder, 1953. It is also found in
freshwater systems in the United States, such as the Mississippi River drainage in Tennessee Gosline, 1948, Lake Texoma in Oklahoma Mense, 1967; Hubbs et al.,
1971; Hubbs, 1982, and has been introduced into lakes in Northern California Cook and Moore, 1970; Elston and Bachen, 1976. The duration of the spawning interval of
M
. beryllina decreases with increasing latitude. Rhode Island, USA, is near the northernmost extreme of the distribution of M
. beryllina. The breeding season is very short Huber, 1990 and the resulting year class has a very narrow age and size range
Huber, 1990; Gleason and Bengtson, 1996. Thus, distinct changes in life history events e.g. preparation for overwintering, maturation, spawning, and death tend to be
M . Huber, D.A. Bengtson J. Exp. Mar. Biol. Ecol. 240 1999 285 –302
287
remarkably synchronized. These populations have, at any given time, only one year class and usually only one age cohort alive and provide a very useful model for the study of
environmental induction of life history events. This study was carried out in the ecological and evolutionary context that M
. beryllina appears to have evolved different maturation times from those of its sympatric congener Menidia menidia Linnaeus, so
that competition between the two species is minimized Bengtson, 1984; Huber and Bengtson, 1999.
In two Rhode Island estuaries the timing and duration of M . beryllina’s spawning
interval varies among years and also between estuaries within the same year Huber, 1990. However, timing of the beginning of gonadal weight increase early May
appears to be more consistent than the timing of peak gonad weights and or peak gonadosomatic indices Huber and Bengtson, 1999 suggesting that initiation of
maturation is regulated by more consistent environmental cues than those regulating spawning. Temperature and photoperiod are some of the more consistent and influential
environmental cues for regulating reproduction of temperate estuarine and intertidal teleost species Scott, 1979; Wootton, 1982; Lam, 1983; Bye, 1984; Munro, 1990;
Taylor, 1990. Consequently we decided to study the effects of photoperiod and temperature on the regulation of the onset of maturation of M
. beryllina in two laboratory studies. In addition field studies were conducted to determine the pattern of
resource allocation of this species during most of its life cycle in a Rhode Island estuary, so that natural changes experienced by the fish in the wild could be compared with the
results of the experiments.
2. Methods
