Age and Growth of <I>Astraea Undosa</I> Wood (Mollusca: Gastropoda

BULLETIN OF MARINE SCIENCE. 59(3): 490-497. 1996 AGE AND GROWTH OF ASTRAEA UNDOSA WOOD (MOLLUSCA: GASTROPODA) IN BAJA CALIFORNIA, MEXICO Fabio German Cupul-Magaiia and Guillermo Torres-Moye ABSTRACT Although the marine gastropod Astraea undosa is becoming an important fishery resource in Baja California, limited data are available on its growth. We verified formation of the annual opercular growth rings and compared the age-growth relationship for both sexes of a natural population in Todos Santos Bay. The hialine (low growth) ring was formed during spring-summer and the opaque (high growth) ring during autumn-winter. Growth patterns differed among sexes, the males having higher growth. The marine gastropod Astraea undosa is becoming an important fishery resource on the Pacific coast of Baja California, Mexico. There is little published literature on individual growth of this gastropod or other related species. The work of Schwalm (1973) on Point Lorna, San Diego, California on A. undosa, and Guanes and Torres (1991) in Todos Santos Bay, Baja California, Mexico on A. turbanica, appear to be the only studies on growth of the genus Astraea. The available information for A. undosa is with reference to the habitat in which it is found (McLean, 1962; Bishop and Bishop, 1973), opercular mineralogy and biochemistry (Adegoke, 1973), food preference and stomach content analysis (Leighton, 1966; Aguilar et aI., 1990), predation (Schmitt, 1981) and population dynamics and energetics (Schwalm, 1973). The study of opercular marks has been a useful technique to determine the individual growth rate of gastropods (Kubo and Kondo, 1953; Miranda, 1975; Sire and Bonnet, 1984; Santarelli and Gros, 1985; Kraeuter et aI., 1989; Guanes and Torres, 1991). However, not always do the authors verify the periodical formation of the growth rings and such omission can result in serious underestimates of ages, which its important to understand the biology and stock dynamics (Gef- fen, 1987). This present study reports data on growth of A. undosa in Baja California, Mexico. We verify the periodical formation of opercular growth marks and use them to determine and compare the individual growth patterns of males and females. METHODS AND MATERIALS The study site was the semiexposed rocky sublittoral flat of Punta Banda, located on the south- western side of Todos Santos Bay (Fig. 1). Monthly sampling was carried out with Scuba gear from May 1987 to June 1988 except for the month of August 1987. On each sampling date, samples were obtained at depths between 2 and 6 m on areas of approximate 12 m2, collecting all the individuals of A. undosa present regardless of their size until a sample of approximately 30 organisms was reached. The animals were transported alive to the Marine Science Faculty laboratory, in Ensenada, Baja California, Mexico where the shells were measured, sex was determined and operculi were extracted. The longest diameter of the shell's base (Fig. 2) was the size estimated for each individual, considering this a practical measurement that has recently been considered for fishing regulations. All shell and opercular measurements were done with a vernier caliper to the nearest 0.01 mm. Snails were sacrificed to determine their sex, based on the color of the gonads observed under the stereoscopic microscope. Operculi were separated from the foot, and their length were measured with vernier caliper. Func- 490 CUPUL-MAGANA AND TORRES-MOYE: AGE AND GROWTH OF ASTRAEA UNDOSA 491 II !l0' 31°50' ~ TODOS SANTOS TODOS SANTO~ BAY ISLANDS ~ Figure 1. Study area and sampling station a in the Todos Santos Bay, Ensenada, Mexico. tional regressions (Ricker, 1973) were calculated to relate the opercular length and longest shell diameter. Operculi were polished on the internal side with grinding paper discs of 600, 450 and 240 grit (Kennish et aI., 1980), to remove the periostracum and to make growth rings clearer. Operculi were exposed to transmitted optic fiber light and observed with the aid of a stereoscopic microscope. The opercular rings (Fig. 3) corresponded to slow zones (narrow hialine rings) and rapid growth zones (wide opaque rings). To confirm the annual ring formation within the operculum, we made monthly estimations of the Marginal Increment (M.I.) calculated as follows: M.I. = Ro - rn, where M.I. is Marginal Increment in mm, Ro is operculum length in mm and rn is distance to the most recently formed hialine ring in mm. Monthly mean values for the M.I. and their 95% confidence interval were estimated with a "t" distribution (Sokal and Rohlf, 1979), and multiple comparisons were obtained to test the significance 492 BULLETIN OF MARINE SCIENCE. VOL. 59. NO.3. 1996 a) b) B L.D.S.B Figure 2. Astraea undosa. a) maximum height and b) longest diameter of the shell base. of the monthly mean differences using a Bonferroni method with 0: = 0.05 and k = 78 (0: = level of significance, k = number of comparisons) (Martin-Andres and Luna del Castillo, 1992) after normality and variance homogenity of data were verified. Once the annual ring formation was confirmed, the back calculation method (Chugunova, 1963; Bagenal, 1978) was used to estimate each year's opercular length by measuring the width of each hialine growth ring. Considering the mean opercular length for each year class and using the functional regression equations, we estimated the year class of each mean shell diameter size. Finally, following Kimura (1980) we estimated the parameters of the von Bertalanffy's growth model to describe and compare the growth patterns of both sexes. RESULTS The total sample size of gastropods was 384, of which 201 were males and 183 females. The functional regressions and R2 values for opercular length and shell size are given on Table 1. The regression equation parameters were the same for both sexes, and hight R2 values suggest an evident proportionality among the measured variables. The M.l. monthly mean values are presented in Figure 4. Lower values were obtained from May to September 1987 and from April to June 1988, and higher rn rn-l rn-2 rn-3 rn-4 rn-5 rn-6 Figure 3. Astraea undosa. Diagramatic representation of opercular growth rings: hialine (slow growth) and opaque (rapid growth) rings. CUPUL-MAGANA AND TORRES-MOYE: AGE AND GROWTH OF ASTRAEA UNDOSA 493 Table I. Functional regression equation and R2 values for the operculum length and the longest diameter of the shell's base from male, female and both sexes R' Sex Equation Determination coefficient Male Y = log-J(0.526 + 0.904 logX) 0.936 Female Y = log-J(0.526 + 0.905 logX) 0.927 Both Y = log-J(0.555 + 0.833 logX) 0.934 values appeared from October 1987 to March 1988. Significant (95% confidence) differences were found between low and high growth months (Table 2). Table 3 shows the opercular mean lengths for each year class of both sexes with the corresponding 95% confidence interval (C.r.). The general non-overlap- ping c.r. values between successive year classes for males and females, indicate their clear differentiation and the low c.r. values suggest a low variation in individual growth. Growth curves of males and females and their von Bertalanffy's equations are shown on Figure 5. Patterns differed with consistently higher growth by the males. The age structure of the total sampled population (Fig. 6) was dominated by snails 4 to 10 years of age. Those outside this age spectrum were rare, presenting less than 10% frequency values. A clear domination by males existed for age classes 3 to 6, followed by a female domination on ages 7 to 10. The global sex proportion (male: female) was 1.0:0.91. DISCUSSION A. undosa presented two different growing periods during our one year study. A slow growth period (low M.r. values) and hialine formation, ocurred mainly during the spring and summer months. The high growth period in which the opaque ring was formed, ocurred during the autumn and winter months. Such growth differences appear to be caused by gastropod's its reproductive cycle. During our study period, A. undosa presented three different reproductive phases; 82.5e -.•.. ~ 2 e f Col 1.5 .5 '; c '6iJ l. C':l ~ 0.5 o MAY .u.I JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN 87 88 Months Figure 4. Marginal increments, monthly mean values. 494 BULLETIN OF MARINE SCIENCE, VOL. 59. NO.3. 1996 0; =o 00 tl: .••..00 '2 •....• .~ o'" c:: II Z ..c =o 8 00 B ~ I zz ..c:: o =8 00 8 00 ClClCl o S; {/H/HIl ..= •..•~ ~ r- ~lzzzSlSlz r- ~lzzzzSl~z r- ~IZZZZZ~~~ r- {f.)~ I ~zzzSl~zzz r- ~ I I I I I I I I I I I r- ~ I zz I ZZZZZZ~ZZZ CUPUL-MAGANA AND TORRES-MOYE: AGE AND GROWTH OF ASTRAEA UNDOSA 495 Table 3. Average opercular length at various ages of male and female of Astraea undosa; SD: standard deviation; C.I.: 95% confidence interval Male Female Age Mean length SD C.I. Mean length SD C.l. (years) (mm) (mm) (mm) (mm) (mm) (mm) 0+ 3.51 0.41 3.44-3.63 3.13 0.10 3.08-3.17 1+ 6.17 0.31 6.10-6.23 5.22 0.20 5.16-5.27 2+ 9.21 0.31 9.15-9.26 8.45 0.56 8.37-8.52 3+ 13.01 0.41 12.94-13.07 11.49 0.15 11.45-11.52 4+ 17.95 0.41 17.88-18.01 14.15 0.31 14.10-14.19 5+ 21.36 0.10 21.35-21.38 17.38 0.15 17.34-17.41 6+ 23.65 0.20 23.60-23.69 19.09 0.15 19.05-19.12 7+ 25.93 0.15 25.87-25.98 20.99 0.20 20.95-21.02 8+ 27.45 0.15 27.37-27.52 23.46 0.15 23.42-23.49 9+ 28.97 0.10 28.92-29.01 26.69 0.41 26.61-26.76 10+ 30.68 0.15 30.60-30.75 30.87 0.36 30.71-31.02 11+ 33.15 0.10 33.06-33.26 33.63 0.10 33.36-33.69 12+ 35.43 0.10 34.51-36.34 35.81 0.25 33.51-38.10 a maturing period (May-July 1987) which ended with a spawning in September and followed by sexually inmature condition (October-March 1988) and another maturation period from April to June 1988 (Almanza and Almanza, 1988).

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