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Annual reproductive cycle and condition index of the New Zealand surf clam Mactra murchisoni (Deshayes, 1854) (Bivalvia: Mactridae).

ABSTRACT The reproductive cycle of the large trough shell Mactra murchisoni (Deshayes, 1854) from Pegasus Bay, New Zealand, was examined between March 2013 and February 2014. Histological analysis indicated that M. murchisoni is dioecious (there were no observed hermaphrodites) and gametogenic development is synchronous between the sexes. Gonadal development began in May in both male and females, with the first ripe individuals observed in June and July for male and females, respectively. Spawning began in spring, peaking in late summer with resorption taking place around autumn. No resting samples were observed. Condition indices were calculated each month and were at their highest during the period individuals were ripe. These indices were significantly different (P < 0.05) between the majority of months and indicate that a high portion of individuals' mass was gonadal tissue during ripe periods. Finally, the results displayed parity between the sex ratio (P = 0.2782), reinforcing that the species are dioecious.

KEY WORDS: Mactra murchisoni, surf clam, reproduction cycle, condition index. New Zealand

INTRODUCTION

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

Samples

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.

Histological Techniques

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.

Condition Index

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.

Statistics

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.

RESULTS

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].

Shell Lengths

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).

Sex Ratio

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).

Reproductive Cycle

Figure 3 shows the developmental stages 1-5 for both male and female individuals observed during the study.

Female

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).

Male

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.

Condition Index

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.

DISCUSSION

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.

Reproductive 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.

Sex Ratio

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.

ACKNOWLEDGMENTS

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: lwhite@aut.ac.nz

DOI: 10.2983/035.034.0306

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
Article Type:Report
Geographic Code:8NEWZ
Date:Dec 1, 2015
Words:4458
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