Annual reproductive cycle and condition index of the New Zealand surf clam Mactra murchisoni (Deshayes, 1854) (Bivalvia: Mactridae).
KEY WORDS: Mactra murchisoni, surf clam, reproduction cycle, condition index. New Zealand
Effective fisheries management is not possible without the study of population dynamics, which can be both directly and indirectly affected by fishing pressure (Jennings et al. 1999). These data are fundamental to quantitative fisheries stock assessments providing a suitable measure to compare different populations (Hilborn & Walters 1992). In spite of this, there are limited data on the demography, reproductive history, and annual biomass/growth estimations for many developing commercial species in New Zealand waters. Between 2012 and 2013 there were large increases in the total allowable commercial catch of four species of New Zealand surf clams (Ministry for Primary Industries 2013), including Mactra murchisoni, which had a national increase in total allowable commercial catch from 180 to 744 t. To date, there is no published information on the reproductive cycle of this species.
Environmental conditions have a significant influence on bivalve reproductive cycles. The most influential are temperature and food availability (Ruiz et al. 1992, Pazos et al. 1997, Darriba et al. 2004, Dridi et al. 2007, Enriquez-Diaz et al. 2009), whereas light, tides, and salinity also play a role (Drummond et al. 2006, da-Costa et al. 2013). In temperate climates, among bivalves, there is a common pattern of gonad development speeding up, plus both gametogenesis and spawning being influenced when temperature and food supply increase (Ohba 1959, Holland & Chew 1974, Xie & Burnell 1994, Ojea et al. 2004, Yan et al. 2009, 2010, Dang et al. 2010). Conversely, the gonadal activity in most bivalves is dormant at temperatures less than 8[degrees]C, with spawning seldom taking place at temperatures below 14[degrees]C (Holland & Chew 1974, Mann 1979, Xie & Burnell 1994).
In the lower North Island and throughout the South Island of New Zealand Mactra murchisoni are commonly found in shallow waters off sandy beaches (Powell 1979), living sympatrically with Mactra discors (Conroy et al. 1993). Protandry is common among bivalves, as demonstrated by studies that have shown unequal sex ratios between different size classes (e.g., Rowell et al. 1990, Gribben et al. 2004), but M. murchisoni are reported to be gonochoric (Cranfield & Michael 2001a, Ministry for Primary Industries 2013).
The aim of this study was to describe the annual reproductive cycle and condition of Mactra murchisoni from Pegasus Bay, New Zealand. This will help determine the reproductive stages that are present each month/season, providing useful information in the management of this commercially fished species. In addition, insight may be gained into what effect the reproductive cycle has on the animal's overall mass and subsequent economic value.
MATERIALS AND METHODS
Around 60 Mactra murchisoni (Table 1) were collected at monthly intervals between March 2013 and February 2014 from Pegasus Bay in the South Island of New Zealand (43.3333[degrees] S, 173.0000[degrees] E). The clams were collected by hydraulic dredge and were shipped overnight to the laboratory. Surface seawater temperatures (7) were measured at the time of collection. Upon arrival at the laboratory each individual was weighed and shell length (L: maximum distance along the anterior-posterior axis) measured.
Soft tissues of each individual were removed from the shells and a section of gonadal tissue was cut from the dorsal region. Following routine histological procedures, embedded sections were cut at 6 [micro]m, stained with Ehrlich's hemtoxylin, counterstained with eosin, and examined to determine the sex and the stage of reproductive development. Reproductive maturity was categorized into six stages (Tables 2 and 3) using a modified version of the maturity scale as described by Drummond et al. (2006). On the occasion that more than one developmental stage was evident within a single individual, that individual was allocated to the reproductive stage that was observed in the majority of follicles.
To determine the condition index ([I.sub.C]), the soft tissues were removed and dried to constant weight at 60[degrees]C (dry flesh weight, [W.sub.D]). The shells were weighed after being cleaned of all tissue (shell weight, The [W.sub.S] was calculated as
[I.sub.C] = [W.sub.D] (g) X 100/[W.sub.S](g)
(Robert et al. 1993, Laruelle et al. 1994, Drummond et al. 2006) and represented the percentage of the individual that was flesh.
All statistical analysis was performed using R statistical software (www.r-project.org/). Normality was assessed for all datasets using the Shapiro-Wilk normality test, histograms, and Q-Q Plots. An analysis of variance model was applied to test for significant differences in [I.sub.C] between months. Significant differences between monthly [I.sub.C] means were identified with pairwise comparisons using a Tukey's honestly significant difference post hoc test. A two-way analysis of variance was used to test for an interaction between month and female and male shell lengths. The Chi-square test was used to test whether the overall sex ratio was equal to 1:1, and the two-sample Student's Mest was used to test if the mean number of females found each month was larger than the number of males and whether the mean length of females was different to males.
A total of 619 individuals were collected, of which, 397 (Table 1) were used in histological analysis and 241 were dried for the calculation [I.sub.C]. The number of individuals varied by month due to difficulty in obtaining samples. The monthly ratio between the numbers of individuals between the two sections of the study was, however, set at 2:1 in favor of those being analyzed histologically to maximize the chances that the number of males and females matched the calculated number of monthly [I.sub.C].
The average shell length was 83.42 [+ or -] 2.98 cm, ranging from 74.2 cm to 95.4 cm. Shell length did not differ significantly between seasons (7>>0.05) and the average shell length of males (83.67 [+ or -] 3.15 cm) was not significantly different from the average shell length of females (83.37 [+ or -] 2.96 cm; P = 0.1556). Additionally, there was no interaction between month with the male and female shell lengths (P = 0.5275), indicating that the ratios of their lengths were not significantly different over the study (Fig. 1).
Of the 397 individuals that were examined histologically, 207 (52%) were identified as female and 190 (48%) were identified as male, resulting in an overall sex ratio of 1:0.91. This sex ratio was not significantly different from 1:1 (Chi-square = 0.7280; df = 1; P = 0.6065). In addition, the mean monthly numbers of females was not significantly different than males (P = 0.2782; Table 4, Fig. 2).
Figure 3 shows the developmental stages 1-5 for both male and female individuals observed during the study.
All of the gametogenic stages described by Drummond et al. (2006), except the resting stage (stage 0), were observed (Fig. 4). In March, females were either spawning (45%) or spent (55%). April was then dominated by both spent (65%) and spawning (35%) individuals. Only three females were observed in May and they were all in late development, contrasting the males who in May were predominantly in early development. In June, individuals were in either early (39%) or late (61%) developmental stage. Ripe females were again observed in July (48%), whereas 22% remained in early development and 30% in late development. In August, 7% of females were in early development, 47% were in late development, and 47% were ripe. September saw an equal split between individuals in the late development and ripe stages. October was dominated by ripe individuals (72%) and the first spawning individuals were observed (22%). In November, there were no longer any females observed in development stages 1 and 2, and 67% of females were observed to be spawning. December saw a decrease in the percentage of spawning females (13%) with 87% observed in the ripe stage. In January and February, the majority were spawning (56% and 79%, respectively), with the return of spent individuals being observed (13%) in February (Fig. 4).
In March, most males were either spawning (86%) or spent (14%). April was then equally split between spent (50%) and spawning (50%) individuals. In May, 71% were in early development and 29% were in late development. The appearance of ripe males was first recorded in June (13%); however, the majority were in early and late developmental stages (50% and 38%, respectively). In July, 35% of the males were in early development, 45% were in late development, and 20% were ripe. Likewise, in August individuals in early development, late development, and ripe stages were observed (13%, 30%, and 57%, respectively). September produced males that were either in late development (23%) or ripe (77%). October was dominated by ripe individuals (80%), and like the females the first individuals that were in the spawning stage were observed (15%); there was also a small number of individuals in late development (5%). In November, there were no longer any males observed in developmental stages 1 and 2, and 89% of males were observed to be spawning. The percentage of spawning males decreased to 53% in December with 47% observed in the ripe stage. In January and February, the majority were spawning (60% and 94%, respectively), with the remainder of individuals being observed in the ripe stage (Fig. 4).
Surface Seawater Temperature
During the study the monthly T was at its lowest at 10.6[degrees]C in June 2013, which steadily rose until it peaked in January 2014, and according to the temperatures in March, April, and May 2013, it appears that it would then decrease until again reaching a trough around June.
Changes in [I.sub.C] through the year were significant (P < 0.05; Fig. 5). Homogenous months with no significant difference between indices included March, April, and May; March, June, and July; June, August, and February; August, September, November, and February; October and December; and September, November, and December (P > 0.05). April and May had the lowest mean indices of 9.85 [+ or -] 1.25 and 9.64 [+ or -] 1.43, respectively. The indices rose in a linear fashion from May to October, when it peaked with a mean of 15.77 [+ or -] 1.74. The indices then remained relatively static until it decreased significantly in February (and likely January). In addition, a polynomial relationship was shown between the mean monthly observed reproductive stage and the mean monthly [I.sub.C] (Fig. 6). Around stage 3 (ripe), [I.sub.C] was shown to peak and decreased symmetrically as it rose or dropped either side of stage 3.
Currently, there is a need for more information on many of New Zealand's commercial fisheries species. This information would help to ensure sustainable management and could prove useful for the commercial sector. The current study assessed the gametogenic cycle of the surf clam Mactra murchisoni by utilizing a commonly used scale with five stages of reproductive maturity (Holland & Chew 1974, Xie & Burnell 1994, Drummond et al. 2006). In addition, the monthly mean [I.sub.C] was calculated and compared with the monthly mean gametogenic stage to assess whether the tissue-to-shell ratio was higher at different times of the year/gametogenic cycle.
It is well documented that temperature coupled with food availability are likely to be the most influential exogenous factors affecting the gametogenic cycle in bivalves (Ruiz et al. 1992, Pazos et al. 1997, Darriba et al. 2004, Dridi et al. 2007, Enriquez-Diaz et al. 2009). In New Zealand, there is a common pattern of gonadal development in infaunal bivalves that shows gametogenic development begins in autumn, followed by a period of maturation, culminating in spawning that occurs in spring/early summer. New Zealand bivalves that have exhibited this pattern include, Paphies australis (Hooker & Creese 1995), Paphies subtriangulata (Grant & Creese 1995), Ruditapes largillierti (Gribben et al. 2001), Panopea zelandica (Gribben et al. 2004), and Zenatia acinaces (Gribben 2005). The results of the current report are consistent with the literature in that they showed the same seasonal patterns of gametogenic development in Mactra murchisoni as in those species mentioned above.
In females, gonadal development began in May (early winter/late autumn) and continued to the beginning of spring with the first ripe individuals being observed in July. It is noteworthy that in May there was, however, only females observed in late development. Spawning began in October (mid-spring) and peaked in February (late summer) until resorption began taking place in early autumn. Male gametogenic development was relatively synchronous with the females. Gonadal development began in May (early winter/late autumn) and continued into the beginning of spring. The first ripe males were, however, observed in June (1 mo earlier than the females). Spawning began in October (mid-spring) and peaked in February (late summer). Finally, resorption began taking place 1 mo later in males (March) than in females. It is likely the differences between the cycles can be explained by random sampling error, and particularly in the case of May because of the small sample size (only three females were observed).
As gametogenic cycles can vary between locations, as shown by Breber (1980) and Xie and Burnell (1994) who both reported one single period of gamete release during summer in Venerupis decussata (in Italy and Ireland); compared with Borsa and Millet (1992), Laruelle et al. (1994), and Shafee and Daoudi (1991) reported two spawning periods during spring and summer (in France and Morocco). Studies should be carried out on different geographical populations of Mactra murchisoni to tailor fisheries management plans to respective geographical populations.
The condition index can reveal a lot about the gametogenic cycle in bivalves because the production of gametes and their subsequent release show respective increases and decreases in the tissue weight, and therefore the [I.sub.C] (Dang et al. 2010, Uddin et al. 2012). The monthly mean [I.sub.C] peaked in October (when the majority of individuals were observed as ripe) and when plotted against the monthly mean development stage, it likewise peaked around the ripe stage. When spawning in late spring/early summer the condition indices saw a step decline. These [I.sub.C] results suggest that there is a significantly higher flesh-to-shell ratio during the period between late winter and early summer, compared with the period between late summer and early winter. This information may be useful to the commercial sector because it provides a good indication when the individuals are potentially at their most valuable (highest flesh-to-shell ratio) stage.
Protandry is common among bivalves and numerous authors have demonstrated this by recording unequal sex ratios between different size classes (e.g., Rowell et al. 1990, Gribben et al. 2004). If larger individuals are targeted by fisheries and they are predominantly female (protandrous) the fishery will likely be unsustainable. The current study found parity between the sexes, both overall and on a monthly basis; however, the size range shown in the histological section of the present study was limited to shell lengths between 74.2 and 95.4 mm. Thus, to make a comprehensive assessment of sexual development and to allow for the possibility of seeing sequential hermaphroditism, individuals would need to be collected over a broader size/age range from numerous populations over a number of different years. This was not possible in this study because gonad size has been shown to be significantly influenced by age (Ojea et al. 2004), thus it was important to sample individuals over a small size range to get meaningful [I.sub.C] results.
This study demonstrated that the surf clam Mactra murchisoni from Pegasus Bay, New Zealand was dioecious and gametogenesis occurred at similar times in males and females. Gametogenic development began in late autumn, followed by a period of maturation. This led up to spawning that began in mid-spring and continued through to early autumn. 7C results showed that during the period individuals were ripe they had a higher flesh-to-shell ratio compared with all other times of the year. The present study provides useful information for the fisheries management and commercial development of the surf clam M. murchisoni.
We would like to express our gratitude to Isaac Piper and Mike Ponder from Cloudy Bay Clams Ltd. for supplying us with the samples each month. We were assisted in the laboratory by several staff and students of the School of Applied Sciences as AUT. We would especially like to acknowledge, Jim Clarke, Dr. Armagan Sabetian, Professor Andrea Alfaro, Nuha Al Jadani, Tomas Wakefield, and Rob Nottingham.
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CHRISTOPHER DAVID NOTTINGHAM AND W. LINDSEY WHITE *
Institute of Applied Ecology New Zealand, School of Applied Sciences, Auckland University of Technology, Private Bag 92006, Auckland 1142, New Zealand
* Corresponding author. E-mail: email@example.com
TABLE 1. Number of monthly samples analyzed histologically or dried for the calculation of condition index ([I.sub.c]). Month Year Histology [I.sub.c] March 2013 27 27 April 2013 31 20 May 2013 10 20 June 2013 34 25 July 2013 43 26 August 2013 38 23 September 2013 42 20 October 2013 38 20 November 2013 42 20 December 2013 33 20 January 2014 19 NA February 2014 40 20 TABLE 2. Reproductive stages for male Mactra murchisoni. Stage Description Stage 0: resting Gonad predominantly composed of connective tissue: sex hard to distinguish. Stage 1: early Gonad proliferation started, many follicles developing with numerous follicle cells, spermatogonia centripetal to follicle walls, spermatocytes present, no spermatids or spermatozoa. Stage 2: late Spermatogonia, spermatocytes, spermatids, and developing spermatozoa coexisted in follicles: in less developed specimens, there was no dominant cell type: in more developed specimens, the majority of the follicle was filled by spermatids and spermatozoa. Stage 3: ripe Follicles predominantly composed of mature spermatozoa; spermatozoa bands close to the follicle wall in very ripe specimens; follicles neat and orderly in appearance. Stage 4: spawning Spermatozoa clearly visible in a swirling shape and accounting for the greatest portion of cells in the follicle; empty space in some follicles due to release of mature spermatozoa. Stage 5: spent Follicles appear broken, scattered and relatively empty; in advanced spent individuals, only residual spermatozoa found, with resorption occurring; presence of phagocytes. Modified from Drummond et al. (2006). TABLE 3. Reproductive stages for female Mactra murchisoni. Stage Description Stage 0: resting Gonad predominantly composed of connective tissue: sex hard to distinguish. Stage 1: early Gonad proliferation started: increasing numbers developing of discernible oocytes in follicle walls; oocytes small; few free oocytes present in the lumen. Stage 2: late Free oocytes present in the lumen but developing accounting for less than half of the total oocytes present in the follicles; attached oocytes equally abundant. Stage 3: ripe Gonad filling large surface area; most oocytes free in the lumen with a polygonal configuration; follicle wall thin. Stage 4: spawning Number of free oocytes per follicle reduced; some follicles empty having released their gametes; follicle walls breaking down. Stage 5: spent Follicles appear broken, scattered and relatively empty; only residual oocytes found in follicles, most undergoing resorption; numerous phagocytes present. Modified from Drummond et al. (2006). TABLE 4. Sex ratios for monthly samples that were analyzed histologically. Month Year Female Male March 2013 20 7 April 2013 17 14 May 2013 3 7 June 2013 18 16 July 2013 23 20 August 2013 15 23 September 2013 20 22 October 2013 18 20 November 2013 24 18 December 2013 16 17 January 2014 9 10 February 2014 24 16
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|Author:||Nottingham, Christopher David; White, W. Lindsey|
|Publication:||Journal of Shellfish Research|
|Date:||Dec 1, 2015|
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