BULLETIN OF MARINE SCIENCE, 61(2): 225–240, 1997

LARVAL AND POSTLARVAL DEVELOPMENT OF THE NEW ZEALAND PIPI, AUSTRALIS (: )

Simon H. Hooker

ABSTRACT Pipi () were conditioned, spawned and the resultant larvae reared to settlement in the laboratory. Adult pipi were successfully conditioned after 23 d at 22° C. Spawning was induced using a combination of increased temperature and a dilute sperm solution. Settlement occurred 18-22 d after spawning at a mean shell length of 264 µm. After settlement and metamorphosis pipi gradually began to take on the adult shape. Cultured juveniles were grown to 37 mm shell length both in the laboratory and later in the wild. Once in the wild, cultured pipi grew from 13 to 37 mm in 17 mo with a strong seasonal component to their growth. Microscopic examination revealed slight differ- ences in shell morphology between pipi larvae and the closely related (Paphies subtriangulata) and toheroa (). Pipi were more rounded and smaller than both tuatua and toheroa at a similar stage of development. The results of scanning electron microscopic examination of larval shell hinge structure confirmed previous pre- liminary findings for pipi. The larval shell ligament is posterior to the center of the provinculum in pipi but central in both the tuatua and toheroa. These differences are sufficient to enable larval pipi to be distinguished from larval toheroa and tuatua.

The pipi (Paphies australis) is a common burrowing bivalve of the Family Mesodesmatidae, one of the most familiar of New Zealand beaches. The family is less common in other parts of the world. Pipi are found throughout New Zealand, the Chatham Islands and in the Auckland Islands (Powell, 1979). There are four species in the family; Paphies ventricosa, (toheroa) Paphies subtriangulata (tuatua), Paphies donacina (deep water tuatua) and Paphies australis. Pipi are restricted to channels and sand banks at the mouths of harbors and occur in the top few centimeters of coarse sediments. The other Paphies species are most abundant on open coasts (Morton and Miller, 1968). Pipi have long been an important part of the diet of New Zealanders. They are an important part of the recreational harvest, and support a small commercial with an average annual harvest of around 120 t (Haddon, 1989). The pipi is also a potential candidate for (Hayden, 1988). Despite its popularity and the widespread distribution of the species there is no published scientific information available on pipi. Some information, usually resulting from short-term environmental studies, is contained in a number of unpublished reports (Creese, 1988; Richie, 1974; Richie and Mason, 1977; Mason and Richie, 1979; Dickie, 1986a; 1986b; 1986c; 1987a; 1987b; Venus, 1984), and an unpublished thesis (Grace, 1972). Bivalves show the greatest diversity of larval development in the (Chanley, 1969). Their larvae are an important part of marine plankton communities around the world (Thorson, 1946). Identification of bivalve larvae by shell characteristics, such as morphometric shell measurements, hinge structures and overall size, is well documented (Rees, 1950; Loosanoff et al., 1966; Chanley and Andrews, 1971; Lutz et al., 1982). It has been successfully applied to many species around the world. In New Zealand, provi-

225 226 BULLETIN OF MARINE SCIENCE, 61(2): 225–240, 1997 sional larval identification of many of the common bivalve larvae in the plankton was made by Booth (1977; 1979a; 1979b; 1983). However, positive larval identification usu- ally requires laboratory culture of the species (Loosanoff et al., 1966; Chanley and Andrews, 1971; Booth, 1983). Larval identification of New Zealand bivalves has been confirmed so far by laboratory culture for the rock (Dinamani, 1973; 1976; Chanley and Dinamani, 1980), the gigas (Chanley and Dinamani, 1980), the Tiostrea chilensis (Chanley and Dinamani, 1980), two commensal bivalve molluscs Arthritica crassiformis and Arthritica bifurca (Chanley and Chanley, 1980), the New Zealand Chione stutchburyi (Stephenson and Chanley, 1979), four species Mytilus edulis aoteanus, , Xenostrobus pulex, Modiolarca impacta (Redfearn et al., 1986), and two surf clams, the toheroa P. ventricosa (Redfearn, 1982), and tuatua P. subtriangulata (Redfearn, 1987). This paper describes the larval and early post-larval development of the pipi by larval culture techniques. This information is useful for identification of pipi larvae from plankton samples to help in the prediction of settlement patterns. Pipi larval culture and informa- tion on the growth of juvenile pipi may also help to establish the feasibility of pipi aquac- ulture.

MATERIALS AND METHODS

The following research was done in two batches (batch 1 and batch 2). All pipi used in this study were collected from the Whangateau Harbor, in northeastern New Zealand. Batch 1.—The following experiment was designed to investigate the feasibility of conditioning adult pipi and rearing their larvae, and to gather preliminary information on the possibility of sus- pended culture and growth rates of hatchery pipi. Conditioning.—In May 1991, 92 adult pipi (mean shell length = 61.0 mm, SE = 1.31), were collected from the Whangateau Harbor. Thirty were fixed in Bouin’s solution for histological ex- amination and the remaining 62 pipi were placed in 300-liter tanks of filtered (10 µm) seawater at ambient temperature. The water temperature was gradually increased over a period of 5 d to a constant temperature of 22°C ± 1°C. Each day the tank was emptied, cleaned and refilled with 10 µm filtered seawater at 22°C. were fed daily with approximately 40 liters of live, near axenic microalgae, (Isochrysis galbana “Tahitian” CS-177 and Thalassiosira pseudonana, clone 3H CS-173; obtained from CSIRO, Australia). Histology.—All tissue sections were blocked in paraffin, cut at 5-7 µm thickness and stained with Mallory-Heidenhain. All histological sections were examined with a light microscope at 40x, 100x and 400x magnification. Each section (individual) was put into one of four categories (early active stage, late active stage, mature stage, partially spawned stage) as modified from Ropes and Stickney (1965) and Ropes (1968). A description of these categories is given in the account of pipi reproductive biology (Hooker and Creese, 1995). Spawning and Larval Development.—Twenty three days after collection of the adult pipi, they were removed from the conditioning tank, and 34 were preserved as above for histological exami- nation. The remaining individuals were spawned using a combination of temperature shock (27°C) and a dilute sperm solution. Spawning male and female pipi were separated so the eggs and sperm were collected separately and the eggs fertilized with a dilute sperm solution. The fertilized eggs were placed in 300-liter tanks of filtered (1 µm) seawater at a density of approximately 20 eggs ml- 1. Every 2 d the resultant larvae were sieved out, and the tank cleaned and refilled with fresh (10 µm) seawater at 20°C ± 1°C. Veligers were fed the same two microalgal species used to condition the adults at a concentration of approximately 105 cells per ml of culture tank. Pediveligers were settled in a downweller arrangement with a small amount of sand from the Whangateau Harbor. In HOOKER: LARVAL DEVELOPMENT 227

Figure 1. Morphometric measurements used in larval pipi (after Chanley and Chanley, 1980). early September 1991, 3 mos after the adults had spawned, the juveniles were moved to an upweller. In January 1992 they were individually tagged, using glue-on plastic mollusc tags (Hallprint, Aus- tralia). Tagged pipi were replaced in the upwellers and fed on raw seawater. In July 1992, 80 were put out in the Whangateau Harbor in hanging nets. The nets were suspended from a stationary raft, approximately 2 m from the bottom, in 8 m of water near the entrance of the Whangateau Harbor. Batch 2.—Once the techniques for larval rearing had been perfected, a second batch of pipi was reared to thoroughly describe the larvae as an aid to identification in plankton samples. Previous descriptions of mesodesmid larval development had ceased at time of settlement (Redfearn, 1982; 1987). It is useful however, to also describe the development through to the post metamorphic shape of the juveniles commonly found in field samples. Therefore, the second batch describes the larval and early post-larval development of pipi. In late October 1993, pipi were observed spawning at the mouth of the Whangateau Harbor. Adult specimens were collected from this site and taken to the laboratory. To obtain information on egg size, a haphazard selection of 10 females was “stripped” and their eggs measured. Thirty animals were placed in a 300-liter tank of 1-µm filtered seawater, at 28°C. These pipi took much longer to spawn than those in batch 1 and actual spawning was not observed, occurring during the night. Trochophore larvae were present in the tanks the following morning, and these were col- lected, a sample measured and the remainder re-suspended in the larval tanks. Veliger larvae were 228 BULLETIN OF MARINE SCIENCE, 61(2): 225–240, 1997

Figure 2. Histograms of the proportion of conditioned adult pipis in each gonad stage. A, wild pipis at the start of conditioning (7 May 1991). B, pipis conditioned in tanks for 23 d. C, wild pipis at the end of conditioning period (4 June 1991). cultured as previously described. Larvae were fed as in batch 1 but with the addition of two further microalgal species: Pavlova lutheri, CS-182 (at all developmental stages), and for late-stage ve- ligers, Chaetoceros gracilis was also fed. Measurements.—Every 2 d a sample of larvae was removed, photographed and video taped. A total of 599 larvae was measured covering the full range of sizes. The dimensions measured (Fig. 1) followed those used by Chanley and Andrews (1971) and Chanley and Chanley (1980). Total length, total height, hinge length (on straight-hinge veligers only), anterior end length, anterior shoulder, posterior shoulder, shoulder height, umbo length, umbo height and depth were measured. The depth was measured on an additional 299 individuals from first veliger stage until settlement. Once settled, total length, total height, anterior end length, anterior shoulder, posterior shoulder, shoulder height, umbo length, and umbo height were measured on juvenile pipi up to 36 d old. All microscopic measurements were made using a computer image analysis system (Jandel Video Analysis Software, 1990). The accuracy and precision of this system was ascertained by repeatedly measuring a Gallenkamp, 1 mm stage graticule, and comparing a range of lengths, microscope objectives and videoed and microscope samples. The 10x microscope objective was used for ani- mals from 50 to 200 µm and the 4x objective for animals above 200 µm. The JAVA video analysis system was extremely precise, (range; mean = 0.050560 µm, SE = 0.00027 to mean = 0.40269 µm, SE = 0.0009). This high precision is consistent over both the size range measured, and the microscope and video systems. The system was also accurate when com- pared to the stage micrometer (95% confidence interval range; mean = 0.09078 ± 0.00061 to mean 0.40269 ± 0.00204). The range in percentage difference between the stage graticule and the mean HOOKER: CLAM LARVAL DEVELOPMENT 229 is from 2.96 to 0.01 with most below 1%. The video did not influence the accuracy compared to using the image directly from the microscope. Scanning Electron Microscope.—Larval samples for scanning electron microscope analysis were rinsed in distilled water, then placed in a 5% sodium hypochlorite solution until the shells had separated (Lutz et al., 1982). The resultant valves were washed in distilled water and placed in absolute ethanol. They were later dropped onto a glass plate, splutter coated with 15 nm of gold and examined using a scanning electron microscope.

RESULTS

Histological Analysis.—Histological examination of gonads showed that pipi had been successfully conditioned at the completion of the first experiment (batch 1). The sub- sample of wild pipi collected before conditioning, on 7 May 1991, were composed of nearly equal proportions of early active stage and late active stage gonads (Fig. 2, A). After 23 d of conditioning, the proportions had changed dramatically to 3% (one indi- vidual) in the late active stage and 3% spawned. The remaining 94% were mature (Fig. 2, B). This very high proportion of mature individuals was seldom observed in natural pipi populations despite extensive sampling (Hooker and Creese, 1995). Gonads of wild pipi at this time (beginning of June 1991) had changed very little from those collected in early May, with only one individual in the sample being mature (Fig. 2, C). Spawning.—Attempts at inducing spawning in pipi collected directly from the wild failed during May and June 1991. All attempts at manually stripping eggs and sperm from individuals throughout the year always resulted in very low numbers of healthy veligers. Although fertilization was successful at these times, very few embryos devel- oped to the veliger stage. Almost all died shortly after. Conditioned pipi from May 1991 spawned within 1.5 h of raising the temperature and within minutes of adding the sperm solution. Male pipi spawned first, followed soon after by the females. Spawning adults extended their siphons and pinched the end slightly. Eggs and sperm were repeatedly released in bursts lasting a few seconds or less. Spawn- ing continued for up to 1 h. Once disturbed (e.g., picked up) a spawning adult would briefly close its valves, then resume spawning again in a few seconds. Larval Development (batch 2).—Average sizes of eggs and trochophore larvae ob- tained from the second spawning (Fig. 3a, b) are presented in (Table 1). Straight-hinge veligers developed within 24 to 36 h after spawning (Fig. 3d). Most larvae had lost their “D” shape by day 6 (Fig. 3e) with the rudimentary umbo appearing as a slight saddle shape in the hinge region. Growth slowed down at this stage presumably as a result of the formation of prodissoconch II. By day 10, the larvae had become more rounded (Fig. 3f). By day 14 the umbo became more prominent (Fig. 3g). Pediveligers were first seen after 16 d. Most larvae had a well developed foot by day 18 and were alternating between actively crawling and swimming (Fig. 3h). At this stage they responded to the presence of sand by rapidly digging and burying themselves in it. By day 22 most larvae had settled. At no stage of their development was an eyespot observed. Metamorphosed animals had discernible siphons and gill bars. The change to the adult form was gradual with most pipi relatively unchanged from the larval shape 1 wk after settlement (Fig. 3i). Two weeks after settlement some juveniles had assumed the adult shape (Fig. 3j). At this stage there was wide variability in the stage of development, with most individuals retaining their larval shape while others were still 230 BULLETIN OF MARINE SCIENCE, 61(2): 225–240, 1997

Figure 3. Photomicrographs of developmental stages of pipi (note, all photographs are not to scale; scale bar refers to veliger larvae only). (A) fertilized egg with polar body (56µm), (B) dividing egg; four cell stage (58µm), (C) trochophore larvae (57 µm). Veliger larvae: (D) straight hinge stage, day 2-4, (E) early umbo stage, day 6-8, (F) mid umbo stage, day 10-12, (G) late umbo stage, day 14-16, (H) settlement stage, day 18. Post-settlement: (I) 28 d old (310µm), (J) 36 d old (670µm), (K) adult HOOKER: CLAM LARVAL DEVELOPMENT 231

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MEnae SnMEeaSn l)ength(µm) height(µm E4ggs546.307.9 2 T1rochophore536.800.7 3 D4ay2956.907.8719.200.84 D7ay41115.815.6966.714.44 D1ay61034.527.11516.915.94 D3ay81631.417.91412.313.75 D9ayl01143.720.41230.821.64 D1ayl21753.242.51739.852.03 D9ayl41470.152.11858.241.53 D4ay161095.441.91381.843.78 D5ayl82534.344.02719.047.18 D1ay202554.032.82034.939.64 D0ay222864.749.02648.831.63 D7ay242861.034.92645.240.03 D7ay282079.945.82354.844.42 D2ay366377.7732.55057.8498.1 swimming veligers. Linear relationships between length and height (Fig. 4a), anterior end (Fig. 4b), anterior shoulder (Fig. 4c), posterior shoulder (Fig. 4d), shoulder height (Fig 4e), adequately describe the increase in size from straight-hinge to settlement (at day 18). The slope of length versus height relationship decreases after settlement, as the length increases at a faster rate than the height to result in the adult shape in which the length is always longer than the height (Fig. 3k). Larval Hinge Structure.—Shell hinge terminology follows that from Redfearn (1982) and Redfearn (1987). Examination of scanning electron micrographs of late-stage straight- hinge larval shells revealed numerous small teeth on the provinculum; an obvious check line in prodissoconch I probably derived from the originally secreted larval shell (Fig. 5a and b). In umbo stage larvae the numerous small teeth of the provinculum became more prominent and reduced in number, and an obvious lateral groove developed (Fig. 5c). A prominent lamelliform tooth was apparent on the left valve of late-stage umbo larvae (Fig. 5d), which was fully developed by day 16 (pediveligers) (Fig. 5e and f). The larval ligament was conspicuous on late-stage larvae lying at the posterior end of the provinculum (Fig. 5f). Growth of Juvenile Pipi from Batch 1.—Settled pipi reached a mean shell length of 1.45 mm (SE = 0.08) by 30 July 1991 (2 mo-old), and small transparent byssus threads were apparent on some of the individuals. By 21 August the mean shell length had reached 2.73 mm, SE = 0.19. On 2 September the settled pipi (mean shell length 2.91 mm, SE = 0.29) were removed from the downweller and placed into an upweller. By January 1992 they had grown to a mean shell length of 13.34 mm (SE = 0.45). Growth effectively stopped when the tagged pipi were kept in raw seawater in the laboratory (Fig. 6). By July 1992, when they were put out in the harbor, their mean size was 13.40 mm, (SE = 0.44). Growth over the winter months was slow with a rapid increase in growth rate over the 232 BULLETIN OF MARINE SCIENCE, 61(2): 225–240, 1997

Figure 4. Graphs of the relationship between shell lenght and (a) height, (b) anterior end, (c) anterior shoulder, (d) posterior shoulder, (e) shoulder height. Regression relationships (i) is for larval pipis (0-18 d old) and (ii) is for settled juvenile pipis (20-36 d old). HOOKER: CLAM LARVAL DEVELOPMENT 233 following summer. Juveniles reached a mean size of 29.96 mm (SE = 0.44) by March 1993. This pattern was repeated again during 1993. The juvenile pipi had grown to a mean shell length of 37.3 mm (SE = 0.40) by December 1993. Pattern of growth of hatchery pipi was similar to wild pipi of a comparable size (Fig. 6).

DISCUSSION

Obtaining healthy, viable embryos is one of the key factors in the culture of marine bivalve molluscs (Loosanoff and Davis, 1963; Walne, 1974; Muranaka and Lannan, 1984). To achieve this, adults may need to be conditioned prior to spawning. Wild pipi popula- tions tend to have varying, and generally low proportions of ripe individuals throughout the year, with higher proportions in spring and summer (Hooker and Creese, 1995). Tech- niques for conditioning bivalves to reproductive maturity outside their usual reproductive season are well known (Loosanoff and Davis, 1963; Hidu, 1975; Rhodes et al., 1975; Chanley, 1975; Chanley, 1981; Lipovsky, 1984; Muranaka and Lannan, 1984; Hurley et al., 1987; Castagna and Manzi, 1989; Wada, 1990; Braley, 1992). This study found that pipi could be conditioned to maturity outside their normal reproductive season, using a combination of increased feed and elevated constant water temperature. It is uncommon to have over 90% of mature individuals from one sample, as was shown by the pipi that were conditioned. This would be a major advantage in any proposed aquaculture of pipi, as future developments would rely on a hatchery source of juveniles. Preliminary at- tempts at conditioning pipi in the laboratory showed that a constant water temperature was critical, as the clams tend to spawn when the temperature fluctuates by more than 2 degrees (Hooker, unpubl. data). Pipi held in the hatchery stopped feeding and closed when given too high a concentra- tion of live algal feed, indicating that careful control of food concentrations is also neces- sary for successful conditioning in a hatchery. Identification of larvae in plankton samples can only be verified by direct techniques such as larval culture. Indirect methods such as abundance of a particular larval type in plankton samples correlated with spawning times can lead to incorrect identification. For example, Rapson (1952) illustrated several bivalve larvae, supposedly those of toheroa. These were subsequently shown to have little resemblance to the cultured larvae of toheroa (Redfearn, 1982). Other examples of mistaken larval identification in the literature are common (Loosanoff et al., 1966). To distinguish pipi larvae from the larvae of other Paphies spp, direct identification techniques were needed to confirm Booth’s (1983) pre- liminary identifications of pipi larvae, which relied heavily on extrapolation from sus- pected spawning times. As pipi and tuatua have very similar spawning times in and adjacent to the Whangateau Harbor (Hooker and Creese, 1995; Grant, 1994), it is likely that larvae from both species will co-occur in the water column increasing the potential for larval misidentifications (Parr, 1994). Pipi larvae reared in the present study were of a similar size to those of tuatua and smaller than toheroa measured at a comparable stage of development by Redfearn (1982; 1987). For example, at settlement pipi had a mean length of 234-254 µm compared to tuatua, 230-260 µm (Redfearn, 1987) and toheroa, 270-300 µm (Redfearn, 1982). Al- though length is always greater than height of a given individual pipi the difference re- mains constant throughout growth (Table 2), resulting in the late-stage umbo larvae hav- 234 BULLETIN OF MARINE SCIENCE, 61(2): 225–240, 1997

Figure 5. Scanning electron micrographs of the larval shell of pipi. Scale bar is 50µm. (A) Internal shell of a straight hinge larva and the hinge structure with many irregular teeth (x710). (B) External shell os a straight hinge larva showing the left and right valves and a distinct shell check (arrowed) (x845). (C) Hinge structure of a disarticulated shell from a 14 d old pipi showing the prominent lateral hinge groove (arrowed), the provinculum with 15 irregular peg teeth (x1000). (D) Hinge structure from a left valve of a disarticulated shell from a 16 d old pipi showing 14 irregular peg teeth and an early small lamelliform tooth (arrowed) (x710). (E) Hinge structure from a left valve of a disarticulated shell from a 16 d old pipi showing 13 irregular peg teeth and the prominent HOOKER: CLAM LARVAL DEVELOPMENT 235

Figure 6. Graph of the growth of individual tagged hatchery pipis (reared in the laboratory and put out in the harbor, numbers tagged = 80) and wild pipis ( collected from the wild at approximately the same size as the hatchery pipis, numbers tagged = 197) suspended in nets in the Whangateau Harbor. ing a relatively rounder appearance than the equivalent stage of development of the tuatua and toheroa larvae. With practice these differences are sufficient to distinguish between live samples of the species under the light microscope (Parr, 1994). The results here confirm that pipi, toheroa and tuatua larvae can be distinguished by shell shape. Detailed examination of larval hinge structures from cultured specimens can also be a useful identification procedure for planktonic bivalves (Rees 1950; Loosanoff et al., 1966; Lutz et al., 1982). The internal structures of larval pipi, tuatua and toheroa are similar. The number of provincular teeth on late-stage larvae is too variable for identification to the species level, as all three species have between 11 and 14 teeth. Toheroa larvae can be distinguished from those of both tuatua and pipi, as they have a spatulate tooth and a peg tooth that are absent in tuatua (Redfearn, 1987) and pipi. Late- stage larval pipi and tuatua are similar in their larval hinge structures but can be distin- guished by the larval ligament which is central in tuatua and toheroa and at the posterior end in pipi. It is doubtful that straight-hinge stage specimens of these three species can be reliably separated on internal shell characteristics. The present work confirms Booth’s (1983) finding on pipi identification. Pipi pediveligers were often seen swimming to the bottom of a watchglass and “test- ing” the surface, moving vigorously around with their foot, and swimming off again with their velum. In the absence of any settlement surface, pipi settled on the bottom of the 236 BULLETIN OF MARINE SCIENCE, 61(2): 225–240, 1997 htgnelobmu,htgnelegnih,redluohsroiretsop,redluohsroiretna,dneroiretna,htped,thgiehsvhtgnelllehslavralfo) n 7 7 7 7 8 8 1 3 3 3 3 2 2 . ) 2 8 9 1 2 9 9 9 9 2 6 9 ( ...... 09 09 09 09 07 06 03 n r a e f d e R * 7 0 a 2 0 1 . . m s . . . 0 0 1 4 o o 1 . 4 3 . r 1 1 c 4 4 1 3 f 0 i - - - + - + + r a h h h h h h t t h t t t t t t t n a g g g g g g e g n n n n n n D v n e e e e e e o e l l l l l l i s * l s e * 9 6 6 5 9 i 6 s 6 8 6 e 8 4 l 0 h 4 ...... r p ) 0 0 0 0 0 0 0 g a 7 e 8 rr =8 =7 =6 =4 =6 P =2 =1 9 1 ( n nn 9 9 9 9 51 4 7 r a 14 16 10 19 17 16 e f d e R 2 9 9 9 9 0 4 9 m ...... o 03 03 03 03 02 01 01 r f a t a a t D a l * u 8 0 . g 1 . 6 . . 0 7 1 9 . n . 8 . . a 4 5 1 a s 4 1 2 0 i 3 - - - + - + o - r h h h h h t h c h t t t t t t i t b r g g g g g g t g u n n n n n n s n n e e e e e e o e e l l l l l l i s l v s e 7 0 l 6 4 9 i 0 s s 8 7 9 e 5 0 0 h 6 e ...... r i p 0 0 0 0 0 0 0 g h a e p rr =8 =7 =7 =4 =1 P* =0 =3 a P , nn t 5 5 5 5 72 7 6 5 7 a l 47 41 49 42 13 28 4 1 u g n a i r t 2 9 9 9 9 2 4 5 9 5 ...... b 08 06 06 06 08 06 06 06 06 u s s e i h p a 4 . 0 2 P 8 5 . . 0 . . 5 s ,silartsuaseihpa 1 5 . 1 5 i 8 . 9 . 0 l 0 1 3 2 2 1 6 + 2 a r(spihsnoitalernoissergeR.2elbaT - - - + + - + h r t 4 h h h h t h h h . t t t t t t t s g 7 g g g g g g g u n 1 n n n n n n n n e a - l e e e e e e e o l l l l l l l h s i t 5 s e 3 9 6 9 i 2 4 2 g s 2 . 5 7 6 0 h 5 5 2 e n ...... r 0 e p l 0 0 0 - 0 0 0 0 g a e =8 =7 =6 =3 =0 P* rr =3 =4 =3 =5 Pa rofthgiehobmudna re re th dlu dlu gie th o h htg d o t hs n hs gie g h e n n roiretso re e roiretn roiretn el h l t dlu e o o h htp g b b g i o ni e m e m h H0 A9 A0 P1 H2 U2 D7 S7 U0 HOOKER: CLAM LARVAL DEVELOPMENT 237 plastic larval tank or downweller tray. When pediveligers were given sand as a settlement surface, they immediately “tested” the substratum and actively buried themselves, using their muscular feet, often moving among sand grains much larger than themselves. Metamorphosis is not a rapid event in pipi, with the attainment of the adult shape taking up to two weeks after settlement. Redfearn (1974) described a post-metamor- phosed, settled stage of toheroa found in the wild, as having an adult shape. It is unlikely that this was a settling stage toheroa, but instead, is probably a 2-3 mo-old juvenile toheroa. The growth of juvenile pipi in the hatchery given only raw seawater was very slow. This was probably because there was little food coming through the seawater system of the laboratory. Once juvenile pipi were put into the harbor, growth was similar to that of wild pipi subjected to the same treatment. This growth rate is similar to that of in-situ pipi (Hooker and Creese, in prep.) and suggests that reseeding hatchery-reared pipi into the natural environment has potential.

ACKNOWLEDGMENTS

Thanks are due to R. Creese who provided valuable support and advice, and his critical and constructive comments on the manuscript. I also thank R. Blackburn, M. Kampman, J. Evans and B. Doak who helped in the development of the Leigh Marine Laboratory, hatchery. C. Grant, M. Hansson and the many student assistants who helped in the day to day running of the hatchery. R. Eagar who prepared the SEM photographs, B. Davy for her work on the histological preparations and I. McDonald for his developing and printing of photographs. The original manu- script was greatly improved by A. Jeffs and an anonymous referee. This work was supported in part by a DSIR fellowship. Contribution no. 55 of the School of Environmental and Marine Science, University of Auckland.

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DATE ACCEPTED: May 19, 1995

ADDRESS: Leigh Marine Laboratory and School of Environmental and Marine Sciences, University of Auckland, Private Bag 92019, Auckland, New Zealand. CURRENT ADDRESS: National Institute of Water and Atmospheric Research, P.O. Box 109695, Newmarket, Auckland, New Zealand.