Reproductive Cycles of Cerastoderma Glaucum (Bruguiere) and C. Edule (L.) with Special Reference to the Effects of the 1981-82 Severe Winter
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/. Moll. Stud. (1986), 52, 6-14. REPRODUCTIVE CYCLES OF CERASTODERMA GLAUCUM (BRUGUIERE) AND C. EDULE (L.) WITH SPECIAL REFERENCE TO THE EFFECTS OF THE 1981-82 SEVERE WINTER KOBINA YANKSON* Department of Zoology, University College of Swansea, Singleton Park, Swansea SA2 8PP, Wales, U. K. (Received, revised, 20 March 1985) ABSTRACT As part of an extended study on Cerastoderma glaucum and C. edule in discrete populations in Gonadal changes in two species of Cerastoderma South Wales, stereological analysis was used to occurring in discrete populations in South Wales were monitor the gonad cycles of the two species. monitored by stereological analysis during the breed• The sampling period was punctuated by the ing seasons of 1981 and 1982. Fecundity and spawning severe winter of 1981-82. This afforded an efficiencies were determined quantitatively. The relationship between reproductive activity and opportunity to compare the patterns of gonadal changes in meat condition are discussed. activity of the two species of cockle during the Both species are opportunistic breeders. Their nor• breeding seasons prior to and immediately after mal patterns of reproduction are similar, consisting of a severe winter. The results of this comparative minimum activity in winter and peak activity during analysis are presented here and an attempt is spring/early summer. The two species were affected made to explain the observed heavy spatfalls differently by the severe winter of 1981-82—C. glau• that occurred in 1982. cum exhibited a single, 'epidemic' spawning and resumed gametogenesis after a 4-month resting period. C. edule adopted a 'polycyclic' pattern without a resting period. Both of these unusual reproductive MATERIALS AND METHODS strategies, augmented by reduced predation resulted in heavy spatfalls. Histology The population of C. edule studied was at the Burry Inlet in South Wales (National Grid Reference SS INTRODUCTION 527962) while the C. glaucum population was at the Aberthaw Power Station lagoon (ST 035662), the Sastry (1979) reviewed the reproductive pat• only known site for the latter species in Wales. Ten to fifteen specimens of each species were examined at terns of marine bivalves and demonstrated their approximately monthly intervals from November 1980 great sensitivity to environmental fluctuations. to December 1982 for C. edule, and from May 1981 According to Newell et al. (1982), the manner in to January 1983 for C. glaucum. The specimens were which the exogenous variables and endogenous fixed for processing within 3 h of collection or else conditions interact to determine the patterns is kept in a freezer (~10°C) overnight before fixation only partially understood. There may not be a after thawing. single reproductive strategy for a species, but Because of the diffuse nature of the gonad the rather a variety of different patterns depending material for processing was obtained by trimming off on the conditions in a given season. There are, the gills and the anterior and posterior ends of the for instance, several records of heavy spatfalls viscera and foot, retaining the central piece. Such samples were obtained from animals aged between 1 following severe winters (e.g. Thamdrup, 1935; and 3 years (based on winter rings) and fixed in Bouin's Thorson, 1946; Savage, 1956; Boyden, 1972; fluid for 24 h. Standard techniques of dehydration and Franklin, 1972; Verway, 1981). To the best of wax embedding were employed before sectioning at my knowledge, explanation of such a phenom• 6-8 pm. From each animal 6 'serial' sections were enon based on concurrent observations on gon• obtained by cutting 2 sections at 3 different depths adal activity is lacking. approximately 1 mm apart. Duplicates of such sections were prepared to ensure against procedural damage. • Present Address: Department of Zoology, University of Cape Coast, The sections were stained in haematoxylin and Cape Coast, Ghana. counter-stained in eosin. SEVERE WINTER AND REPRODUCTION IN COCKLES 7 Stereological analysis of the gonad prior to fixation. The displacement method employed by Hancock & Franklin (1972) was used. The samples The gonadal activity of the two species of Cer• were pooled to minimise errors involved in the reading astoderma was assessed quantitatively by means of of the water levels in the measuring cylinder. The stereological analysis of the histologically prepared condition index (CI) was calculated as sections using a Weibel graticule as described for Mytilus edulis L. and C. edule by Bayne et al, (1978), _ Meat Volume x 100 Newell & Bayne (1980), Newell et al. (1982). The Whole volume - shell volume graticule was superimposed on the gonad area (i.e. the region of the visceral mass adjacent to the foot) where meat, whole, and shell volumes are the displa• and the points coinciding with gonad material were cements (ml) caused by the meat (wiped dry with counted and expressed as the 'gamete volume fraction' blotting paper), the intact animal with the valves (GVF). This is the proportion of the gonad area that closed, and the empty shell valves respectively. is composed of follicles containing the cells at various stages of gametogenesis. For each animal 12 separate counts were obtained (i.e. 2 from each of the 6 sections) and the averages computed as GVF%. A RESULTS standard magnification of x30 was chosen for .convenience. Seasonal changes in the gonad and meat conditions Estimation of fecundity and spawning efficiency In both species of Cerastoderma the cyclical changes in the gonadal activity during the study Various methods have been used to estimate the fec• period were similar in the two sexes. In order undity of bivalves: (1) gonad weight before and after to compare the reproductive cycles of the two spawning (e.g. Fox & Coe, 1943); (2) soft tissue weight just prior to and after spawning (e.g. Griffiths, 1977; species from their typical natural habitats the Bayne & Worrall, 1980; Kautsky. 1982); (3) egg combined means of all the GVF values of males counts in squash preparations (e.g. Oertzen. 1972; and females were plotted with 95% confidence Ivell. 1981). The first method is inapplicable to Cer• limits for each sampling data (Fig. 1). The astoderma because of lack of discrete gonad. The seasonal changes in condition index (CI) are second is inappropriate because changes in meat con• also illustrated in the same figure to demonstrate dition as a result of spawning may be masked by the effects of the reproductive activity on the rapid growth during the same period (see Hancock meat condition. Inspection reveals some dif• & Franklin, 1972). The third method precludes the ferences in the reproductive patterns of the two inclusion of males in the estimation. According to Prof. A. Lucas of Laboratoire de Zoologie, Universite de Bretagne Occidentale (pers. comm.. 1982) the reproductive output of male bivalves is higher than females. Table 1. C. glaucum. Density variations indicating Since the gamete volume fraction (GVF) is a direct recruitment (spatfall) patterns in the Aberthaw measure of the proportion of the gonadal tissue which Power Station lagoon during the breeding is actually devoted to gamete production, the dif• seasons of 1981 and 1982. (Notice the heavy spat• ference in the GVF immediately before and after the fall in June 1982 due to epidemic spawning.) spawning period was considered as a better estimate of fecundity. The spawning efficiency, i.e. the percentage Density (No. m-2) proportion of the gametes which the animals are able to discharge during the spawning period, was Spatfall expressed as: Date Increase in GVF, - GVF,( = Fecundity) x 100 1981 Total 0-Group 0-Group GVF, 14 May 360 0 0 where GVF, and GVF, are the gamete volume frac• 10 June 451 120 120 tions before and after spawning respectively. This 15 July 1626 1337 1217 approach was used to compare the gamete production 12 August 1931 1621 284 and spawning efficiencies, where possible, of the two 16 Sept. 2022 1776 155 species during the breeding seasons of 1981 and 1982. 14 Oct. 481 392 0 1982 Condition index 12 May 166 0 0 16 June 32,290 32,130 32,130 The condition index (meat condition) of the specimens 22 July 28,666 28,500 0 used for the histological preparations was determined 8 K. YANKSON species during the successive breeding seasons spawning decline in GVF from August reaching which will therefore be described separately. 0.5% in November is probably due to the cytoly- sis or reabsorption of unspawned gametes. By November there was no sign of revival of C. glaucum gametogenesis. The GVF value of 59.2% recorded in May 1981 In 1982 the GVF again peaked in May at (Fig. la) may be regarded as the maximum 53.8% but decreased rapidly to a minimum of pre-spawning gonad condition since specimens 0.4% in July. This was followed by a 4-month collected in April that year did not spawn upon resting period and re-initiation of gonadal stimulation with temperature shock while some activity in November. The GVF then increased of the specimens collected in May did. Eighty from 8.1% in November 1982 to only 11.1% in percent of the May sample were at their final January 1983 indicating a rather slow game- stages of maturity while only 20% showed signs togenic activity over the winter months. of spawning. The progressive decline in GVF The pattern of changes in the meat condition from 59.2% in May to 8.0% in August of that (CI) followed that of the gonad condition in a year may be attributed to a protracted spawning similar manner. In both years the CI peaked in which was evidenced by continuous recruitment May and attained minimum values during the during the same period (Table 1). The post- post-spawning resting period. The lower values (a) C.