Molluscan Studies

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Journal of

The Malacological Society of London

Molluscan Studies Journal of Molluscan Studies (2015) 0: 1 –8. doi:10.1093/mollus/eyv026 Advance Access publication date: 00 Month 0000 Copy Edited by: P.A. Language used: UK/ize

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Embryonic and larval development of the invasive biofouler Mytilopsis sallei (Rećluz, 1849) (Bivalvia: Dreissenidae) Jian He1, Jian Fei Qi1,2, Dan Qing Feng1 and Cai Huan Ke1

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College of Ocean and Earth Sciences, Xiamen University, Xiamen 361005, China; and 2 Fisheries Research Institute of Fujian, Xiamen 361000, China Correspondence: D. Q. Feng; e-mail: [email protected]

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(Received 22 September 2014; accepted 19 May 2015)

ABSTRACT 20

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The marine dreissenid bivalve Mytilopsis sallei is a fouling organism that has invaded habitats outside its original range. Understanding its early development will be useful for early detection in the environment, for species identification in ballast water and for development of control strategies targeted at early life stages, which will help us better manage this important invader. The processes of embryogenesis, shell formation and larval development of M. sallei are described here for the first time by using light and scanning electron microscopy. Released oocytes are 64 mm in diameter. Fertilized eggs were incubated at 27 + 1 8C. The trochophore, with an apical tuft and a prototroch, developed by 6.0 + 2.3 h postfertilization (hpf). At 16.5 + 4.2 hpf, D-shaped veligers with shell length (SL, mean + SD) of 87.3 + 8.2 mm appeared, each possessing a velum and a calcified shell. At 2 –3 d postfertilization (dpf ), the D-shaped veligers developed into umbonate larvae (SL ¼ 111.9 + 10.7 mm), the last obligate freeswimming veliger stage. Pediveligers (SL ¼ 232.8 + 37.1 mm) observed at 6–8 dpf could either swim using their velum or crawl with their foot. Pediveligers settled by secreting byssal threads and metamorphosed to plantigrades (SL ¼ 298.7 + 45.2 mm) 8–10 dpf. It is noteworthy that the larvae of this invasive bivalve are capable of settlement within 10 d. This is the first detailed study of early shell formation of a species of the family Dreissenidae. Shell field invagination appeared during gastrulation, secreting shell material by expanding over both sides in a saddle-shape during the trochophore stage.

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INTRODUCTION

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Invasive dreissenid molluscs are ecological and economic nuisances in aquatic ecosystems (Aldridge et al., 2008). They have invaded a variety of habitats outside their native range and are also aggressive biofouling organisms, attaching to submerged hard surfaces by their byssal threads. It has been reported that the dreissenid Mytilopsis leucophaeata recently expanded rapidly in Europe (Verween, Vincx & Degraer, 2006) and in some regions has become an economic problem by fouling industrial cooling water systems (Jenner et al., 1998). A population with densities as high as 6.5 million/m2 was found near power station intakes in the Noordzeekanaal in the Netherlands (Rajagopal, van der Velde & Jenner, 1995). Another invasive dreissenid mussel, M. trautwineana, was reported to cause serious fouling problems in shrimp farms on the Caribbean coast of Colombia (Aldridge et al., 2008). The adverse impacts of the invasion of Dreissena species on local aquatic ecosystems and water-related structures (e.g. intake pipes) are also well recognized, particularly with zebra mussels (D. polymorpha) and quagga mussels (D. rostriformis bugensis) (Orlova et al., 2005; Straver, 2009). Early developmental stages are generally accepted as critical to mussel life cycles. Understanding embryonic and larval

development of dreissenid mussels is important for species identification within plankton communities, for investigation of their invasiveness by larval dispersal, settlement and recruitment, and for development of fouling-control strategies. The literature on early development of Dreisseninae has focused on species belonging to the genus Dreissena, such as D. polymorpha (Meisenheimer, 1901; Ackerman et al., 1994; Lucy, 2006; Cohen, 2008; Harzhauser & Mandic, 2010), and D. rostriformis bugensis (Cohen, 2008), while detailed descriptions of early development of Mytilopsis is only available for M. leucophaeata (Siddall, 1980; Verween Vincx & Degraer, 2010; Kennedy, 2011). Furthermore, most observations of early stages used light microscopy, while scanning electron microscopy (SEM) has not been used to study embryonic and larval development in the Dreisseninae. The black-striped mussel M. sallei (Rećluz, 1849), which has a natural Atlantic range, migrated into the Pacific via the Panama Canal (Morton, 1981, 1989). It has now established a firm foothold in the Indo-Pacific Ocean. In China, M. sallei was first found in Hong Kong waters (Tolo Harbour) in 1980 (Morton, 1980). This exotic species has now spread along the southeastern coast of mainland China (Cai et al., 2006). Mytilopsis sallei is a fecund marine fouling organism. Tan & Morton (2006) found

# The Author 2015. Published by Oxford University Press on behalf of The Malacological Society of London, all rights reserved

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that in Singapore M. sallei was present at densities of up to 83,000 ind m22 in tidal monsoon canals, forming extensive, densely-populated bands on vertical and sloping concrete walls as well as on the floor of monsoon drains, up to several kilometres inland from the sea. In the Taisi township on the southwestern coast of Taiwan, this fouling mussel forms dense monocultures that exclude most other hard clam populations, leading to potential ecological and commercial damage (Liao et al., 2010). Information on morphology (Morton, 1981; Marelli & Gray, 1983), ecology (Morton, 1989; Lin & Yang, 2006; Tan & Morton, 2006), toxicology (Rao et al., 1988; Rao & Balaji, 1994; Devi, 1996) and taxonomy (Zhou et al., 2006; Wong, Meier & Tan, 2011) of M. sallei has been reported in the literature. However, none of these studies has investigated its embryology and larval development. The aim of this study was to provide a detailed description of the embryonic and larval development of M. sallei, using light microscopy and SEM. Knowledge of early development of M. sallei is necessary to obtain morphological characteristics for species identification in plankton. The findings of this study should be important for monitoring M. sallei recruitment in wild populations and developing antifouling methods to combat this species.

each stage at least 30 individuals were collected, put in FSW (27 + 1 8C, 28 psu) and photographed with an optical microscope (Olympus BX51). Embryos and larvae were measured by Image-Pro Express v. 6.0 (Olympus software). 195

Scanning electron microscopy Samples were first fixed in 2.5% glutaraldehyde in 0.1 M phosphate buffer solution (PBS) for 2 h and then fixed in 1% osmic acid for 1–2 h. After fixation, samples were twice rinsed in PBS and then dehydrated through an ascending series of ethanol (30, 50, 70, 90, 95 and 100%). After drying using the tert-butyl alcohol freeze-drying method, samples were sputter-coated with gold before observation in a LEO 1530 SEM at 20 kV acceleration voltage. For veligers, samples were anaesthetized with 1 M MgCl2 solution before fixing in glutaraldehyde, to prevent the retraction of the velum inside the calcified valves when glutaraldehyde was added.

RESULTS

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Embryogenesis Newly fertilized eggs were brown and spherical with a diameter (mean + SD) of 64.3 + 3.4 mm (n ¼ 30; Fig. 1A). The fertilization membrane could be seen clearly. Fertilization occurred normally and no polyspermy was observed in the several batches obtained in this study. Figure 1B shows the active sperm, characterized by a head with a spherical nucleus (diameter 1.2 + 0.2 mm), a smaller conical acrosome (diameter 220 + 15 nm) and a middle piece with four mitochondria (diameter 200 + 10 nm). The first polar body was detected about 0.3 h after sperm contact with the oocyte, which was visual evidence of successful fertilization (Fig. 1C). The second one appeared a few minutes later. The first segmentation cleavage occurred at about 0.5 h postfertilization (hpf ), developing into the trefoil stage with two blastomeres (conventionally named AB and CD) and a polar lobe (Fig. 1D). The polar lobe was soon absorbed by the larger blastomere CD (0.6 + 0.1 hpf ), which was twice the size of the other one (Fig. 1E). The larger macromere was at the vegetal pole of the embryos. The polar body was located at the intersection of the plane of cleavage. The four-cell stage was reached 0.7 + 0.2 hpf, consisting of three equal A-, B- and C-blastomeres and a larger D-blastomere located at the vegetal pole (Fig. 1F). The large macromere at the vegetal pole of four-cell embryos was roughly twice the diameter of neighbouring micromeres, which averaged 40 mm in diameter. The eight-cell stage was completed at about 0.9 hpf with seven micromeres located at the animal pole over a macromere (Fig. 1G). Subsequently, the 16and 32-cell stages were completed during 1.2 –1.5 hpf (Fig. 1H, I). The morula stage occurred within 1.8 hpf and consisted of multiple micromeres (Fig. 1J). After successive cleavages, with a spiral pattern, more than 70% of embryos had reached the blastula stage by 2.0 hpf (Fig. 1K). Blastocysts consisted of multiple micromeres with short cilia on the surface of the embryo, which rotated in the water. The blastula was the first motile stage of development. A large blastopore, resulting from the invagination located at the vegetal pole, could be seen by 2.5 hpf, indicating the start of gastrulation (Fig. 1L).

MATERIAL AND METHODS 150

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Spawning induction and fertilization Adults of Mytilopsis sallei with anteroposterior shell length (SL) of 20 –30 mm were collected by hand from the submerged ropes of a fish farm in Maluan Bay, Xiamen, China (248330 N, 1188010 E). Mussels were cleaned by scrubbing and rinsing their shell to remove epifaunal organisms. For spawning induction, about 300 individuals were exposed in air for 8 h and then placed into a 20-l plastic tank containing warm seawater (UV-treated, temperature 32 8C) with addition of gonadal extracts. The gonadal extracts were prepared as follows: gonads were dissected from 30 –50 adults of M. sallei, ground in 300 ml of 0.22-mm filtered seawater (FSW, 28 psu) and then filtered through a 120-mm mesh net. The seawater in the tank was aerated vigorously. After about 6 h, eggs and sperm were released. Breeding mussels were removed from the tank after 0.5 h of spawning. Fertilized eggs were washed with a 100-mm mesh net to remove extraneous tissue and then sieved through a 30-mm mesh net to eliminate excess sperm. Embryos were incubated in a 10-l tank filled with UV treated FSW, at a temperature of 27 + 1 8C. The density of the embryos was adjusted to 30 ml21. The seawater was gently aerated. The antibiotics streptomycin sulphate or chloramphenicol were added (5 mg l21) to reduce the proliferation of harmful bacteria.

Larval culture After 24 h of incubation, veliger larvae were collected from the tank with a 50-mm mesh net and transferred at a density of 3– 5 larvae ml21 to three replicate 15-l incubation tanks containing FSW. Larvae were fed daily with a diet of Dicrateria zhanjiangensis (Chrysophyta) at 0.5–1 105 cells ml21. The water was gently aerated and changed daily with FSW. The water temperature was maintained at 27 + 1 8C and salinity was maintained at 28 psu.

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Early shell formation Late gastrulation (4.2 + 0.8 hpf ), was characterized by two depressions, a small round blastopore and a shell-field invagination (Fig. 2A). At 6 hpf, more than 80% of the late gastrulas had differentiated into typically pyriform trochophores, 72.5 + 9.6 mm in size, swimming actively in the water column (Fig. 2B). The trochophore had a ciliary ring ( prototroch), used

Optical microscopy Samples were collected every 10 min, from fertilized egg to trochophore stage and every hour from trochophore to D-shaped larva. Once the D-stage was reached, larvae were sampled daily until settlement and metamorphosis occurred. In 2

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Please do not annotate this PDF with corrections - use the unmarked copy provided EARLY DEVELOPMENT OF MYTILOPSIS SALLEI

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Figure 1. Embryonic development of Mytilopsis sallei. SEM views from egg to early gastrula. A. Spawned egg. B. Sperm. C. Fertilized egg. D. Trefoil stage. E. Two-cell stage. F. Four-cell stage. G. Eight-cell stage. H. Sixteen-cell stage (1.2 + 0.3 hpf). I. Thirty-two-cell stage (1.5 + 0.2 hpf). J. Morula. K. Blastula. L. Early gastrula. Abbreviations: f, flagellum; h, head; pb, polar body; pl, polar lobe. Scale bars: A, C–L ¼ 20 mm; B ¼ 1 mm.

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for swimming (Fig. 2C, D). Cilia at the apical plate continued to elongate (40 –60 mm long) and thicken during the following 2 h to form an apical tuft (Fig. 2C –E). The late trochophore (8.0 + 0.7 hpf ) began to secrete shell material in the shell-field (Fig. 2F). At 9.0 + 0.7 hpf, the shell material spread into a saddle shape with bilateral symmetry (Fig. 2G). Subsequently, the shell material expanded over the body, forming valves covering the entire body and compressing the trochophore, at 11– 13 hpf (Fig. 2H, I). The prototroch was pushed aside in the ventral direction, near the mouth, developing into the velum. 3

Larval development

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Typical straight-hinged D-shaped larvae developed from the trochophore by 15.5 hpf, with a SL (mean + SD) of 87.3 + 8.2 mm (Fig. 3A). Early D-shaped larvae had a bilaterally symmetrical bivalve shell, still uncalcified as suggested by the wrinkled aspect. At this stage the prototroch developed into the velum with a few rows of cilia, which could extend and retract freely. The veliger larvae could use their vela to swim actively through the water column. Later, D-shaped veligers (16.5 + 4.2 hpf) were

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435 Figure 2. Early shell formation of Mytilopsis sallei. SEM views from late gastrula to late trochophore. A. Late gastrula. B. Apical view of late gastrula.

C. Early trochophore. D. Apical view of early trochophore. E. Apical tuft located in the apical plate, with a spiral pattern. F. Dorsal view of late trochophore. G. Saddle-stage trochophore. H. Anterior-lateral view of a trochophore. I. Late trochophore. Abbreviations: at, apical tuft; mv, microvilli; p, prototroch; t, telotroch; v, valve. Scale bars: A– D, F–I ¼ 20 mm; E ¼ 2 mm.

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enclosed within the fully calcified prodissoconch I (Fig. 3B). Prodissoconch II appeared in veliger larva by 30.0 + 4.2 hpf (Fig. 3C) and prodissoconchs I and II were separated by a visible boundary. The prodissoconch I had a smooth outer surface, whereas the prodissoconch II had an outer surface with commarginal growth lines. In addition, the digestive system formed gradually (Fig. 3D), consisting of a mouth, a foregut and a digestive gland followed by an intestine, all of which became more distinct with increasing shell size. After 2–3 dpf, the umbo larvae (SL ¼ 119.9 + 10.7 mm) were oval and slightly umbonate (Fig. 3E). Because the anterior side of the umbo was slightly elongated and less rounded compared with the posterior side, umbo larvae developed an asymmetrical shell shape, which became increasingly pronounced as the larvae approached settlement (Fig. 3G). The rim of the velum bore concentric bands of cilia (Fig. 3F). There were two rings of preoral cirri, which were the most visible cilia on the velum. Below the band of preoral cirri there was a band of smaller cilia, the adoral band. On the velum closest to the shell, a single line of randomly short cilia appeared, which is referred to as the postoral band. Most larvae exhibited settlement behaviour when they reached 6 –8 dpf (SL ¼ 232.8 + 37.1 mm) (Fig. 3H, I). In this

pediveliger stage, the feet of larvae were fully developed, but gill filaments were not. The larvae swam and crawled for short intervals. In a free-swimming phase, larval locomotion was effected by cilia of the velum, with the foot extended; in a crawling phase, the pediveliger crawled on the substratum using its foot, with the velum retracted. Pediveligers settled and metamorphosed to plantigrades at 8 – 10 d posthatching (SL ¼ 298.7 + 45.2 mm; Fig. 3J, K). Byssus threads were secreted and the plantigrade settled on the substrate. By this time, the shell was morphologically similar to that of adult mussels as a result of the secretion of the dissoconch. During metamorphosis the velum was shed completely and gill filaments matured. After 2 weeks postsettlement, the plantigrades developed into juveniles (SL ¼ 660.0 + 157.8 mm; Fig. 3L) and siphons were differentiated in these postlarvae of at least 250 mm SL. A summary of developmental times and sizes of M. sallei is provided in Table 1.

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DISCUSSION The egg size of Mytilopsis sallei (64.3 mm) is similar to another Mytilopsis species, M. leucophaeata (61.1 mm; Kennedy, 2011). This 4

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Figure 3. Larval development of Mytilopsis sallei. A– C, E, F, H, J. Light micrographs. D, G, I, K, L. Scanning electron micrographs. A. Intermediate stage between typical trochophore and early D-shaped veliger. B. Early D-shaped veliger. C. Late D-shaped veliger. D. Late D-shaped veliger. E. Early umbone larva. F. Lateral view of velum. G. Late umbone larva. H. Ventral view of pediveliger. I. Pediveliger. J. Lateral view of plantigrade. K. Apical view of plantigrade. L. Juvenile. Abbreviations: ad, adoral ciliary band; at, apical tuft; b, byssus; d, dissoconch; dg, digestive gland; es, exhalant siphon; f, foot; g, gill filaments; i, intestine; is, inhalant siphon; m, mouth; pro, preoral ciliary band; pI, prodissoconch I; pII, prodissoconch II; pso, postoral ciliary band; t, telotroch; v, valve; ve, velum. Scale bars: A– E, G ¼ 20 mm; F ¼ 3 mm; H–K ¼ 50 mm; L ¼ 100 mm.

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study provides a high-magnification image of the mature spermatozoon in M. sallei, characterized by a head with a spherical nucleus and a small conical acrosome, a middle piece containing four spherical mitochondria and a flagellum. This head morphology is different from those of other dreissenid species, such as Dreissena polymorpha and D. bugensis (Walker, Edwards & Black, 1996), but is similar that in the Donacidae (Donax gemmula; Introini et al., 2013) and Pinnidae (Atrina pectinata; Kang et al., 2012).

The development from fertilization to D-shaped larva in M. sallei requires 12 –20 h at 27 8C, while in the previously analysed dreissenid species, 21–24 h at 23 8C is needed for M. leucophaeata, 24 h at 24 8C for D. polymorpha and 29 h at 24 8C for D. rostriformis bugensis (Kennedy, 2011). The relatively rapid rate of embryogenesis in M. sallei reported here may be due to the higher rearing temperature. It has been demonstrated that temperature affects larval development time in the Pectinidae (Cragg, 2006). 5

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This is the first study of early shell formation in a dreissenid to be documented with high-resolution photomicrographs. The initial shell-field invagination can occur at various developmental stages in molluscs and there has been disagreement on the order of appearance of the shell-field invagination and the prototroch (Waller, 1981). Our study indicates that in M. sallei the shell-field invagination first becomes evident during gastrulation, preceding the formation of the prototroch. Such a pattern has been previously described in other bivalves (e.g. Moue¨za, Gros & Frenkiel, 1999, 2006; Aranda-Burgos et al., 2014). Observations of shell secretion in Pectinidae (Cragg, 2006), Pteriidae (Wassnig & Southgate, 2012), Veneridae (Moue¨za et al., 1999, 2006; Aranda-Burgos et al., 2014) and Ostreidae (Kakoi et al., 2008) suggest that many bivalves develop shells by expanding their shell field over both sides of the trochophore in the form of a saddle. This pattern has also been observed in the present work, confirming its occurrence in at least one member of the Dreissenidae.

In many bivalves, shell calcification is associated with the formation of the prodissoconch I (Moue¨za et al., 1999, 2006; Silberfeld & Gros, 2006; Da Costa Darriba & Martinez-Patino, 2008; Wassnig & Southgate, 2012; Aranda-Burgos et al., 2014). In M. sallei, prodissoconch I secretion begins during the transition from trochophore to D-shaped veliger and a calcified shell can be observed at the D-shaped veliger stage (Fig. 3). This was confirmed by element analysis of larval valves using an Energy Dispersive Spectrometer (data not shown), which revealed that there was no calcium present until prodissoconch I appeared in the early D-shaped veliger (Fig. 3B). In M. sallei, the velum appeared in the early D-shaped larval stage and was shed completely during metamorphosis. The composition of the ciliary bands is similar to that in other bivalves, such as in Ruditapes decussatus (Aranda-Burgos et al., 2014), Chione cancellata (Moue¨za et al., 2006), Tivela mactroides (Silberfeld & Gros, 2006) and Pecten maximus (Cragg, 1989). The apical tuft, another common structure in numerous bivalve species, is conspicuous from the trochophore to the veliger stage (Moue¨za et al., 1999). Here it was confirmed that the apical tuft appeared at the trochophore stage and remained until pediveliger metamorphosis occurred, which was also observed in R. decussatus (Aranda-Burgos et al., 2014). The apical tuft has been suggested to function as a sense organ (Tardy & Dongard, 1993). According to investigations in some gastropod larvae, the apical tuft is the site of cell-surface receptors for inducers of settlement and metamorphosis (Baxter & Morse, 1992; Hadfield, Meleshkevitch & Boudko, 2000). Mytilopsis sallei was introduced into the Indo-Pacific Ocean by transportation as a fouling organism on ship hulls or as pelagic larvae in ballast water (Morton, 1981; Chu et al., 1997). A detailed description of its early development, as presented here, will help to better understand and control this nuisance invasive mussel. Knowledge of the morphological features of its larvae is required for early detection and identification in water samples, which is important to give water managers warning to make adjustments before the populations of mussels becomes large enough to foul facilities severely (Hosler, 2011). However, it can be difficult to unambiguously distinguish M. sallei larvae from larvae of other molluscan species that bear some resemblance and coexist in plankton samples, especially those of other dreissenid species. Table 2 provides a comparison of the

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Table 1. Embryonic and larval stages of Mytilopsis sallei reared at a temperature of 27 + 1 8C and a salinity of 28 psu. 690 Developmental stage

Time postfertilization

Fertilization

0h

Polar-body extruding

0.3 + 0.1 h

2-cell 695 4-cell

0.6 + 0.1 h

8-cell

0.9 + 0.3 h

Morula

1.8 + 0.3 h

Blastula

2.0 + 0.2 h

700 Gastrula Trochophore

6.0 + 2.3 h

Size (mm; n ¼ 30) 64.3 + 3.4

0.7 + 0.2 h

2.5 + 0.3 h 72.5 + 9.6

D-shaped larva

16.5 + 4.2 h

Umbonate larva

2 –3 d

111.9 + 10.7

6 –8 d

232.8 + 37.1

8 –10 d

298.7 + 45.2

18 –22 d

660.0 + 157.8

Pediveliger 705 Plantigrade Juvenile

87.3 + 8.2

Data expressed as mean + SD.

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Table 2. Comparison of developmental sizes and characteristics of early life stages of four dreissenid bivalves. 780

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Mytilopsis leucophaeata ¶

Dreissena polymorpha ¶

Dreissena bugensis ¶

Egg

64.3 + 3.4

61.1 + 1.7

40 –96

64 – 82

Trochophore

72.5 + 9.6

–

57 –121

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D-shaped larva

87.3 + 8.2

78.8 + 3.5

70 –160

89 – 129

Dimensions (mm)

Umbonate larva

111.9 + 10.7

98.4 + 8.5

120 – 180

111 – 198

Pediveliger

232.8 + 37.1

157.3 + 24.3

167 – 330

141 – 168

Plantigrade

298.7 + 45.2

283.0 + 42.0

158 – 500

136 – 232

Juvenile

660.0 + 157.8

.580.0

.500

–

Shape of sperm head

Rounded

Rounded

Elongated and tapered

Elongated and cured

Number of mitochondria in sperm

4

3

4

4

Shape of veliger

Rounded

Rounded

Ovoid

Rounded

Velar pigment

Absent

Absent

Present

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Umbo

More pronounced

More pronounced

Less pronounced

Less pronounced

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725

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Mytilopsis sallei* ¶

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795

Dash indicates data unavailable. *This study. †

Denson & Wang (1998), Verween et al. (2010), Kennedy (2011).

¶ ‡ Ackerman et al. (1994), Denson & Wang (1998), Verween et al. (2010). ¶ § Nichols & Black (1994), Denson & Wang (1998). ¶

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morphological features of early life stages in the four invasive dreissenid mussels, M. sallei, M. leucophaeata, D. polymorpha and D. rostriformis bugensis, as an aid to differentiating these closely related species. Knowledge of M. sallei early development provides information that can also be useful for preventing biofouling. Control strategies can be developed to target its settlement or planktonic life stages. For example, deterring settlement or killing early life stages of dreissenid mussels may need lower doses of biocides than killing their adults (Verween, Vincx & Degraer, 2009), which has ecological and economic benefits.

ACKNOWLEDGEMENTS This research was supported by the National Natural Science Foundation of China under Grants 41276127; the Public Science and Technology Research Funds Projects of Ocean of China under Grant 201305016; the Regional Demonstration Projects for Innovation and Development of Marine Economy in Xiamen under Grant 12PZB001SF09; the Open Fund of Key Laboratory of Marine Environmental Corrosion and Bio-fouling, Institute of Oceanology, Chinese Academy of Sciences under Grant MCKF201413; the Program for New Century Excellent Talents in Fujian Province University and the Fundamental Research Funds for the Central Universities of China under Grant 2013121042.

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