Importance of epiphytic diatoms and fronds of two species of red algae as diets for juvenile Japanese turban snail Turbo cornutus.
Article Type: Report
Subject: Red algae (Environmental aspects)
Snails (Food and nutrition)
Shellfish culture (Research)
Authors: Hayakawa, Jun
Kawamura, Tomohiko
Ohashi, Satoshi
Horii, Toyomitu
Watanabe, Yoshiro
Pub Date: 04/01/2010
Publication: Name: Journal of Shellfish Research Publisher: National Shellfisheries Association, Inc. Audience: Academic Format: Magazine/Journal Subject: Biological sciences; Zoology and wildlife conservation Copyright: COPYRIGHT 2010 National Shellfisheries Association, Inc. ISSN: 0730-8000
Issue: Date: April, 2010 Source Volume: 29 Source Issue: 1
Topic: Event Code: 310 Science & research
Geographic: Geographic Scope: Japan Geographic Code: 9JAPA Japan
Accession Number: 224932792
Full Text: ABSTRACT

Survival and growth rates of juvenile turban snail (Turbo cornutus) reared on 2 red algae, an articulated coralline alga (ACA) Marginisporum crassissima and a gelidicean alga (Agar) Gelidium elegans, with natural and reduced diatom densities were compared to examine the relative importance of algal fronds and epiphytic diatoms as diets for the juvenile turban snail. Laboratory experiments were conducted for 8 or 9 wk with turban snail of 3 size groups, early juveniles with 1.26-mm and 2.89-mm shell diameters and juveniles with an 8. 1-mm shell height. The 2 diameter size groups of early juveniles grew well (22.3 [+ or -] 5.5 [micro]m/day and 16.4 [+ or -] 4.2 [micro]m/day, respectively; mean [+ or -] SD) on ACA with natural diatom densities, whereas the growth rates were significantly lower on ACA with reduced diatom densities in both size groups (6.7 [+ or -] 0.8 [micro]m/day and 4.0 [+ or -] 2.2 [micro]m/day, respectively). A significant difference in growth rate was also found between early juveniles of 1.26 mm on Agar with natural and reduced diatom densities. In contrast, no significant difference was detected in juveniles of 2.89 mm on Agar with natural and reduced diatom densities. Although turban snail juveniles of 8.1 mm reared on Agar with natural diatom densities had very high growth rates (106.1 [+ or -] 18.3 [micro]m/day), the mean growth rate of juveniles on ACA with natural diatom densities was relatively low (10.9 [+ or -] 5.8 [micro]m/day). The results of the 3 feeding experiments indicate that diatoms are an important food source for juvenile T. cornutus from 1.25 mm to approximately 4.0 mm, and that the importance of fronds of G. elegans as a food source increases as juveniles increase in size. In addition, the species composition and density of diatoms on ACA fronds collected from a nursery habitat of the turban snail were observed continuously for approximately 2 y. From the information gained on seasonal changes of diatom communities and experimental results, the importance of ACA turf in providing suitable diets for turban snail juveniles is discussed.

KEY WORDS: articulated coralline algae, benthic diatoms, coralline algal turf, feeding ecology, Gelidium elegans, Turbo cornutus, Marginisporum crassissima

INTRODUCTION

Many marine invertebrates have a pelagobenthic life cycle, and planktonic larvae can potentially disperse widely in the water. Because juvenile survivorship differs greatly among habitats, depending on their physical and biological environment (Naylor & McShane 1997, Herbert & Hawkins 2006, Walker 2007), habitat selection by larvae is crucial for survival and growth in early juvenile stages. The larval settlement preference and its ecological significance have been discussed for some marine invertebrates in laboratory experiments (Stoner et al. 1996, Roberts et al. 2004). However, the favorability for juveniles of the substratum where larvae selectively settle has not been studied sufficiently in many species.

Turbinid gastropods are widespread and abundant in coastal waters around much of the globe. They are often conspicuous herbivores and can reach relatively large sizes and locally high densities. The Japanese spiny turban snail, Turbo cornutus, is one of the largest herbivores, with a maximum size of ~140 mm in shell height (SH), and is one of the most important fishery target species on the coastal rocky shores of Japan. Our previous studies (Hayakawa et al. 2007, Hayakawa et al. 2008) showed that planktonic larvae of turban snail selectively settle on articulated (genieulate) coralline algae (ACA). In addition, juveniles preferentially aggregate on fronds of ACA rather than on other algal species (Hayakawa et al. 2008). Following settlement, turban snail juveniles stay within coralline algal turf for approximately 1 y, growing to 10-15 mm in SH (Yamazaki & Ishiwata 1987, Sasaki 2003). This strong preference for ACA species by both larvae and juveniles may be related to the favorable environment for survival and growth of juveniles provided by ACA turf, although the roles of ACA for turban snail juveniles has not yet been studied.

Turban snail juveniles need to obtain sufficient food inside coralline turf, but their main food source from ACA is still unclear because of the lack of information about feeding habits during the early juvenile stages. ACA fronds themselves may have poor dietary values for herbivores as a result of the calcified algal bodies, although adult turban snails of 2 species--T. cornutus (Yamakawa & Hayashi 2004) and T. sarmaticus (Foster & Hodgson 1998)--seem to consume ACA species actively. Yoshiya et al. (1987) found that large turban snail juveniles (>30 mm SH) actively consume fronds of ACA species in the field when microalgae such as the red alga Herposiphonia subdisticha (Rhodomelaceae) become abundant from August to October. These authors considered that large juveniles do not prefer ACA fronds as food, but ingest them secondarily with microalgae on the surface of ACA.

Previous studies showed that some macroalgae (e.g., gelidicean algae and the green alga Ulva pertusa) are good food sources for juveniles (Fujii 1998); however, both the presence of benthic diatoms on such macroalgae and the juveniles' specific distribution on ACA turf have been almost ignored in consideration of their feeding habits. Diatoms growing on ACA surfaces are likely to be food sources for early juveniles of turban snail. However, the contribution of epiphytic diatoms as food sustaining favorable survival and growth of juveniles has still not been evaluated. In addition, almost no information exists regarding species composition and density of diatoms on algal fronds in natural habitats of turban snail.

In the current study, survival and growth rates of juvenile turban snail T. cornutus reared on the ACA Marginisporum crassissima with natural and reduced diatom densities were compared to confirm the contribution of diatoms on ACA as food sources for juveniles and the dietary value of the ACA themselves. A gelidicean alga (Agar), Gelidium elegans, with natural and reduced diatom densities was also tested, and the relative importance of this alga in the diet of juvenile turban snail was reconsidered. In addition, the species composition and density of diatom communities on ACA fronds in the natural habitat of turban snail were observed continuously for approximately 2 y.

MATERIALS AND METHODS

Collection of Algae

The ACA M. crassissima and the gelidicean alga G. elegans were collected from the subtidal zone on the east coast of Sagami Bay, Kanagawa, Japan, by scuba diving. The algae without a canopy cover of larger algae such as Eisenia bicyclis were detached from rocks from a depth of 1-2 m. ACA collection was carried out monthly for observation of seasonal changes in surface diatom communities, and additionally for feeding experiments. In the additional collection, the gelidicean alga was also gathered, The collected algae were separately collected into plastic bags containing seawater and transported to the Ocean Research Institute (ORI), the University of Tokyo, Japan, within 2 h. Only clean and visibly healthy algae without observable epibenthos and epiphytes were selected for the feeding experiments. Algal fragments were prepared immediately before the feeding experiments by cutting the thalli at 30 mm (experiment 1), 35 mm (experiment 2), or 50 mm (experiment 3) from the algal apex. Six ACA fronds were separately packed in a plastic bag with small holes and fixed in 5% formalin--seawater solution for observation of surface diatom communities.

Feeding Experiments

Three feeding experiments (experiments 1, 2, and 3) using turban snail juveniles of different sizes were carried out. The turban snail early juveniles used in experiment 1 were hatched in September 2008 and were reared on plastic plates dominated by Navicula spp. as a food source until November 2008 at the Nagasaki Prefectural Institute of Fisheries (Nagasaki, Japan), after which they were transported to the National Fisheries Research Institute (NFRI; Kanagawa, Japan) in November 2008 and reared for 2 wk on diatom plates. Finally, the early juveniles were transported to the ORI and used in the experiment after a 2-day no-feeding period to equalize nutritional conditions of individuals. The average size of the turban snail at the start of the experiment was a 1.26-ram shell diameter (the size of turban snail early juveniles, individuals less than 3.5 mm SH, are generally measured in shell diameter).

In experiment 1, growth and survival rates of early juveniles were compared for 9 wk (60 days) among the following 5 experimental treatments:

1. ACA + diatoms treatment: an ACA fragment was given in filtered seawater (FSW; 0.45 [micro]m)

2. ACA--diatoms treatment: an ACA fragment was given in FSW with 5 mg/L germanium dioxide (Ge[O.sub.2]), which inhibits diatom growth but does not affect the growth of other algae (Chapman 1973)

3. Agar + diatoms treatment: a fragment of G. elegans was given in FSW

4. Agar--diatoms treatment: a fragment of G. elegans was given in FSW with 5 mg/L Ge[O.sub.2]

5. Negative control: no feeding

The rinsed algal fragments were placed on the bottom of each 50-mL plastic pot with 30 mL FSW or Ge[O.sub.2] seawater. Three early juveniles were introduced to each pot. Five replicates were set for each experimental treatment. The pots containing an algal fragment and early juveniles were kept at 20[degrees]C and 5,000 lux, with a 12-h light/12-h dark regimen. FSW and Ge[O.sub.2] seawater in the pots were replaced every day. The shell diameter of early juveniles was measured weekly from the start of the experiment using a micrometer eyepiece of a dissecting microscope. At the time of measurement, all early juveniles were gently removed from the algal fragments with round-ended tweezers, and the algal fragments in the pots were replaced with new ones. The pot itself was also replaced every other week. Survival rate of early juveniles was assessed by counting the number of dead individuals during the experimental period. Dead individuals were immediately removed from pots. Another 5 fragments each of ACA and G. elegans from just before the start of the experiment and fragments after use in the experiment were individually packed in a plastic bag with small holes and fixed in 5% formalin-seawater solutions. These samples were used later for the diatom composition analyses on the algal surfaces.

In experiment 2, growth and survival rates of turban snail of 2.89-mm shell diameter were compared for 9 wk (63 days) among the same 5 experimental treatments as experiment 1. In this experiment, 2 algal fragments of ACA or G. elegans were fed. The early juveniles used in experiment 2 were hatched in June 2007 and reared under the same conditions as in experiment 1. They were transported to the NFRI in September 2007 and reared for 2 wk. Some were taken to the ORI and used in the experiment after 3 days of starvation. The rinsed algal fragments were placed on the bottom of each 100-mL plastic beaker with 70 mL FSW or Ge[O.sub.2] seawater, and 2 early juveniles were introduced to each beaker. Six replicates were set for each experimental treatment. The conditions for the early juveniles and methods of measurement were the same as in experiment 1. The Agar--diatoms treatment was only observed for 4 wk (26 days). Another 6 fragments of ACA just before the beginning of the experiment and ACA fragments after use in the experiment treatments (ACA + diatoms and ACA--diatoms) were fixed for diatom counting in the same way as in experiment 1.

In experiment 3, growth and survival rates of turban snail juveniles were compared for 8 wk (55 days) among several feeding conditions. The average size of the juveniles at the start of the experiment was approximately 8.1 mm SH, and was too large for rearing under the still water conditions; therefore, this experiment was conducted in flowing water in tanks at the NFRI. This rearing method prevented use of Ge[O.sub.2] solution to reduce diatom densities on ACA. Thus, to prepare treatments with different diatom densities on ACA surfaces, diatom density on ACA fragments of one treatment was artificially increased by keeping these fragments in a small cage to exclude grazers at the bottom of a running seawater tank under sunshine for a week before use in the experiment. The experimental treatments in experiment 3 were as follows:

1. ACA ++ diatoms treatment: 2 ACA fragments, on which diatom densities were increased by the previously mentioned method, were given

2. ACA + diatoms treatment: 2 ACA fragments were given

3. Agar + diatoms treatment: 2 fragments of G. elegans were given

4. Negative control: no food provided

The juveniles used in experiment 3 were from the same batch as those used in experiment 2, which continued to be reared on diatom plates and supplied with the brown alga Undaria pinnatifida at the NFRI until June 2008. Algal fragments and two juveniles were placed inside each case of PVC pipes (inner diameter, 60 mm; height, 51 mm) closed at the top and bottom with plastic mesh (mesh size, 2 x 2 mm). Ten PVC pipe cases were contained in a plastic mesh box (34 x 23 x 10 cm; mesh size, 2 x 7 mm). Each experimental treatment had 10 replicates. Four mesh boxes were held 10 cm above the bottom of the water tank (60 x 90 x 50 cm) on dish drainer baskets to allow water circulation. The water tank was supplied with seawater (22 [+ or -] 2[degrees]C) at a flow rate of 5 L/min, fully aerated and illuminated by timed fluorescent lamps (7,000 lux: 12-h light/ 12-h dark). The SH of all juveniles was measured weekly with calipers, and at the same time, the algal fragments in the cases were replaced with new ones. To avoid location effects, the mesh boxes were randomized every week. Another 6 fragments of ACA from just before the beginning of the experiment and 6 randomly selected ACA fragments after use in the experimental treatments (ACA ++ diatoms and ACA + diatoms) were fixed for measuring diatom densities in the same way as in experiments 1 and 2.

The effect of Ge[O.sub.2] on the growth of 2 sizes of early juveniles (shell diameter of 1.26 mm and 2.37 mm) was evaluated. These early juveniles were siblings of those in experiment 1. The early juveniles were reared in FSW or Ge[O.sub.2] solution on plastic plates covered with diatoms, which were used for rearing individuals at the NFRI. FSW or Ge[O.sub.2] solution in the plastic pots was changed every day, and the plastic plate was replaced every 5 days. After 13 days from initiation of rearing, shell diameter of the early juveniles was measured.

Scanning Electron Microscope Analysis of Diatoms

To observe seasonal changes in surface diatom communities, ACA fronds collected every month were observed under a scanning electron microscope (SEM; Hitachi Tabletop Microscope TM-1000; Hitachi, Tokyo, Japan). In addition, algal samples from the feeding experiments were also observed under an SEM to examine differences in food abundance among experimental treatments. For each sample, the number of diatoms on the upper surface of algal fronds was counted for 18 randomly selected fields at 2,000 x magnification. In observations of ACA fragments, however, algal surfaces around articulations of fronds were excluded, because those parts probably restrict access for grazers, and larger colonial diatom species dominated throughout the year. Because numerous larger diatom species, which were difficult to count adequately at 2,000x, were found on the fronds of G. elegans from experiment 1, additional counting was conducted. Three pieces 5 mm from the algal apex were cut from each fragment of G. elegans, and all visible diatoms were counted at 100x magnification.

Data Analyses

All statistical analyses were performed using the R-2.6.2 (R Development Core Team, 2008, Vienna, Austria) computer package. We used 1-way analysis of variance (ANOVA) with the Tukey-Kramer HSD test to examine differences in the growth rates among experimental treatments when data in the experiments were homogeneous in variance (Levene test, P > 0.05). The densities of diatoms on the ACA surface were compared between the 2 experimental treatments, and significant differences were tested with the Mann-Whitney U-test.

RESULTS

Feeding Experiments

In experiment 1, early juveniles fed ACA + diatoms and Agar + diatoms grew equally well and reached about 2.5 mm in shell diameter after 9 wk (22.3 [+ or -] 5.5 [micro]m/day, 19.1 [+ or -] 4.0 [micro]m/day, respectively; mean [+ or -] SD; Fig. 1). In contrast, growth rates of early juveniles fed ACA--diatoms and Agar--diatoms were relatively low, reaching only 1.6 mm in shell diameter at the end of the experiment (6.7 [+ or -] 0.8 [micro]m/day and 4.5 [+ or -] 2.2 [micro]m/day, respectively; Fig. 1). No significant differences were observed in the shell diameter of early juveniles at the start among the 5 experimental treatments, but the mean growth rates of the early juveniles reared on ACA--diatoms and Agar--diatoms were significantly lower than those of individuals on ACA + diatoms and Agar + diatoms (ANOVA with Tukey-Kramer HSD test, P < 0.01). No significant difference was found in juvenile growth rate either between ACA--diatoms and Agar--diatoms or between ACA + diatoms and Agar + diatoms (ANOVA with Tukey-Kramer HSD test, P > 0.05). The survival during the experiment was highest in ACA + diatoms (80.0%), followed by ACA--diatoms (66.7%), Agar + diatoms (60.0%) and Agar--diatoms (60.0%). All individuals in the negative control died within 8 wk (Fig. 2).

[FIGURE 1 OMITTED]

The early juveniles fed ACA + diatoms and Agar + diatoms grew equally well in experiment 2, and reached about 4.0 mm in shell diameter after 9 wk (16.4 [+ or -] 4.2 [micro]m/day and 19.1 [+ or -] 4.3 [micro]m/ day, respectively; Fig. 3). The individuals fed Agar--diatoms showed equivalently high growth rates for 4 wk (20.4 [+ or -] 5.6 [micro]m/ day; Fig. 3) and no significant difference was found in the growth rate from ACA + diatoms or Agar + diatoms (ANOVA with Tukey Kramer HSD test P > 0.05). The growth of early juveniles fed ACA--diatoms was very slow and reached only 3.0 mm in shell diameter at the end of the experiment (4.0 [+ or -] 2.2 [micro]m/ day; Fig. 3), and the mean growth rate for 9 wk was significantly lower than that of individuals on ACA + diatoms and Agar + diatoms (ANOVA with Tukey-Kramer HSD test, P < 0.01). No significant difference in mean growth rate was observed between ACA + diatoms and Agar + diatoms (ANOVA with Tukey-Kramer HSD test, P > 0.05). Figure 4 shows the survival of early juveniles during the experimental period in each treatment. The survival for 9 wk was highest in Agar + diatoms (75.0%), followed by ACA + diatoms (66.7%) and ACA--diatoms (50.0%). No dead individuals were observed during the first 4 wk of the experiment in Agar- diatoms, and all early juveniles in the negative control died within 4 wk (Fig. 4).

Figure 5 shows the growth of juveniles in each experimental treatment for 8 wk in experiment 3. The mean growth rate of juveniles fed G. elegans (Agar + diatoms) was very high (106.1 [+ or -] 18.3 [micro]m/day; Fig. 5) and was significantly higher than juveniles in ACA ++ diatoms and ACA + diatoms (ANOVA with Tukey-Kramer HSD test, P < 0.001). The mean growth rates of individuals reared on ACA ++ diatoms and ACA + diatoms were very low (9.5 [+ or -] 3.7 [micro]m/day and 10.9 [+ or -] 5.8 [micro]m/day, respectively) and were not significantly different from that of juveniles in the negative control (4.1 [+ or -] 5.0 [micro]m/day; Fig. 5). The survival for 8 wk in each experimental treatment was 95.0% in Agar + diatoms and ACA + diatoms, 85.0% in ACA ++ diatoms, and 30.0% in the negative control.

The diatom densities on ACA in the 3 feeding experiments are shown in Tables 1-3. In experiments 1 and 2, diatom densities on both ACA + diatoms and ACA--diatoms decreased 1 wk after rearing of early juveniles, but diatom densities after the experiments were significantly higher on the surface of ACA + diatoms than on ACA--diatoms (Mann-Whitney U-test, P < 0.01; Tables 1 and 2). The diatom densities in ACA ++ diatoms in experiment 3 were significantly higher than in ACA + diatoms, both at the start and end of the experiment (Mann-Whitney U-test, P < 0.01; Table 3). In experiment 1, the density of diatoms on G. elegans increased in Agar + diatoms, but decreased in Agar--diatoms after rearing for 1 wk (Table 4). The mean densities of diatoms after the experiments were significantly higher on the surface of Agar + diatoms than on Agar--diatoms (Mann-Whitney U-test, P < 0.01). The number of larger diatom species (Arachnoidiscus sp. and Gephyria sp.) per 5-mm thalli was not significantly different between Agar + diatoms and Agar--diatoms (Mann-Whitney U-test, P > 0.05; Table 5).

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

No significant differences were observed in the growth rates of the 2 size groups of early juveniles between individuals reared in FSW and Ge[O.sub.2] solution (Mann-Whitney U-test, P > 0.05; Table 6). No dead individuals were observed in either size group. These results indicate that Ge[O.sub.2] had no effect on shell growth of early juveniles.

Seasonal Changes of Diatom Communities on ACA in the Field

The density and species composition of diatoms on ACA in the field from June 2007 to April 2009 are shown in Figure 6. The density of diatoms on ACA showed large seasonal fluctuations, from 100-1,500 cells/[mm.sup.2]. The density tended to be lower in summer (June to August) and higher in winter (December to February). Cocconeis spp. and Navicula spp. appeared throughout the year and accounted for more than 60% of the total diatoms. Amphora spp. also appeared almost year-round, but at lower densities. Larger colonial species, such as Licmophora spp. and Tabularia spp., occurred mainly from late autumn to early spring.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

DISCUSSION

Previous studies on the feeding habit of grazing gastropods have focused mainly on the dietary values of macroalgal species; however, the importance of epiphytic microalgae as diets has recently been suggested especially for small herbivores, including juveniles of large herbivores (Moncreiff & Sullivan 2001). The results of this study clearly indicate that diatoms on the surface of ACA are important food sources for growth of juvenile T. cornutus from 1.25 mm to approximately 4.0 mm shell diameter in the natural habitat of coralline turf. The differences in the growth rate of early juveniles between ACA + diatoms and ACA diatoms in experiments 1 and 2 are considered to be the result of significant differences in densities of diatoms. Thus, most of the nutrition for early juvenile turban snails inside ACA turf must be derived from benthic diatoms on ACA. Cocconeis spp. are highly adhesive diatoms and are considered to be a good food source for postlarval abalone, because their strong adhesion makes it possible for postlarvae to rupture diatom cell walls using the radula (Kawamura et al. 1995, Kawamura et al. 1998a). For early juveniles of turban snail species, little information is available on the differences in dietary value among diatom species, and we do not know if Cocconeis spp. are favorable food for turban snail early juveniles. However, the current results suggest that Cocconeis spp. are an important food source for turban snail early juveniles because they accounted for more than 50% of total diatoms on ACA, and broken cell walls of diatoms were found in the feces of early juveniles.

Turbinid gastropods are capable of feeding on many species of macroalgae from the juvenile to adult stages (Foster & Hodgson 1998, Fujii 1998). Because little is known about development of the digestive apparatus, such as the radula of turbinid gastropods, when the shift in feeding habit from diatoms to macroalgae occurs is still unclear. In experiment 1, growth rates of early juveniles on Agar + diatoms were significantly higher than those on Agar--diatoms. This means that early juveniles of 1.25-mm shell diameter cannot consume the algal body (epithelium) of G. elegans, which is reported to be a favored food of larger juveniles, and diatoms on the algal surface are a more important food for early juveniles of this size. In contrast, growth rates of early juveniles were nearly the same in Agar + diatoms and Agar--diatoms in experiment 2. This indicates that early juveniles of 2.90-mm shell diameter can ingest the algal body of G. elegans, and that this alga is a favored food source for rapid growth of early juveniles rather than epiphytic diatoms on G. elegans. Thus, the shift to macroalgal feeding from diatom feeding is thought to have started between a 1.25-mm shell diameter and a 2.90-mm shell diameter.

However, early juveniles with a 2.90-mm shell diameter did not grow well on ACA--diatoms in experiment 2. Coralline algal species are characterized by extremely high ash content from the calcium carbonate and thus are considered to be poor-quality food sources for many herbivores, including turban snail species belonging to Turbinidae (Uchiba et al. 1982, Foster & Hodgson 1998). Takami et al. (1997a) showed that the crustose coralline algae (CCA) Lithophyllum yessoense was not an important food source for postlarval H. discus hannai (shell length, 0.5 mm and 1.5 mm) compared with diatoms on the CCA. Similarly, Alfaro et al. (2007) reported that a turbinid gastropod, T. smaragdus, seems to prefer epiphytes and diatoms on macroalgae to the seaweeds themselves. The results in the current study suggest that possible food materials derived from the ACA M. crassissima, such as polysaccharides, are not sufficient for growth of early juveniles of turban snail.

In experiment 3, growth rates of juveniles with an 8.0-mm SH on ACA with increased diatom densities were not significantly different from those on fresh ACA, although the diatom density was 5 times the original density. In this SH size (8.0 mm), juveniles seem to have fully shifted from diatom feeding to macroalgal feeding, and the energetic contribution of diatoms on algal surfaces is thought to be much less than that of macroalgae with high dietary value like G. elegans.

Although various environmental factors such as light, water temperature, and nutrient concentration affect diatom density and species composition on artificial substrata, grazing pressure by herbivores seems to be a major factor influencing seasonal transitions of the benthic diatom community (Kawamura & Hirano 1992, Kawamura et al. 1992). Species of the adhesive prostrate type, including Cocconeis spp., seem to have slow growth rates but very strong resistance to grazing, and can remain dominant under relatively high grazing pressure by herbivores, such as in the area dominated by CCA (Kawamura et al. 1992, 2004). Coralline algal turf provides habitats for diverse macrofaunal assemblages of very high densities (Kelaher et al. 2001). Especially in summer, densities of invertebrates inside coralline algal turf increase with new recruits of many herbivore species, including T. cornutus (Hayakawa unpublished data). Moreover, the strong grazing pressure of herbivores inside ACA turf appears to lead to the low-density dominance of Cocconeis spp. in diatom communities on ACA during summer. The grazing pressure appear to decrease with decreasing water temperature from autumn to winter, which may lead to an increase in diatom density and the appearance of larger colonial species with lower tolerance to grazing by herbivores (Kawamura & Hirano 1992).

In the field, turban snail larvae settle on coralline algal turf from July to early October (Sasaki 2003, Hayakawa unpublished data). Because this is when diatom densities on ACA are low, food abundance may be inadequate for the newly settled individuals. However, the nutrient demand of early juveniles is relatively small, and other possible food sources on ACA in the natural environment, such as the polysaccharide layer of ACA themselves, bacterial assemblages, and trail mucus of gastropods, which have been reported as important food sources for postlarval abalone on CCA (Garland et al. 1985, Takami et al. 1997b), may sustain survival and growth of turban snail early juveniles in addition to benthic diatoms. From autumn to winter, an increasing nutrient demand along with growth of turban snail juveniles can be fulfilled by the increase in the density of epiphytic diatoms on ACA, which was thought to be a main food source for juveniles. Turban snail juveniles of T. undulatum (Worthington & Fairweather 1989) and T. cornutus (Yamazaki & Ishiwata 1987) appear to change their habitats from inside to outside coralline algal turf as they grow. Yamazaki and Ishiwata (1987) considered that the habitat transition of turban snail juveniles at an SH of around 10 mm in early summer might be caused by increased nutrient demands. This timing of habitat transition corresponds with the season when the density of benthic diatoms starts to decrease. Because coralline algal turf is important for turban snail early juveniles also as shelters from various disturbances, the temporal changes in diatom density on ACA turf and the shift in feeding habit may not be the only reason for the habitat change of turban snail juveniles; however, it appears to influence strongly the habitat selection of juveniles.

The importance of diatoms on ACA as diets for turban snail early juveniles was clearly shown; however, food availability was not thought to be stable in the natural habitat of ACA turf. In some abalone species, it has been suggested that the dietary value for the juveniles differs among benthic diatom species, which have different morphologies, sizes, frustule strengths (Kawamura et al. 1998a, 1998b, Daume et al. 2000, Takami & Kawamura 2003). Thus, further studies on seasonal changes in species composition and density of epiphytic diatoms should be combined with the additional information of the difference of dietary values for turban snail early juveniles among diatom species. Other factors influencing survival and growth of turban snails, such as competition for benthic diatoms with small herbivores inside ACA turf, are needed to examine the favorability of coralline algal turf as a nursery for the turban snail.

[FIGURE 6 OMITTED]

ACKNOWLEDGMENTS

We thank the staff of the Nagasaki Prefectural Institute of Fisheries for their cooperation in rearing turban snail larvae and juveniles. We are also grateful to the staff of the National Research Institute of Fisheries Science for their kind help in rearing turban snail juveniles and carrying out the experiments. This research was supported in part by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS) for Young Scientists, Japan. J. H. was financially supported by a research fellowship from JSPS.

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JUN HAYAKAWA, (1) * TOMOHIKO KAWAMURA, (1) SATOSHI OHASHI, (2) TOYOMITU HORII (3) AND YOSHIRO WATANABE (1)

(1) Ocean Research Institute, The University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164-8639, Japan; (2) Nagasaki Prefectural Institute of Fisheries, Taira, Nagasaki, Nagasaki 1551-4, Japan; (3) National Research Institute of Fisheries Science, Fisheries Research Agency, Nagai, Yokosuka, Kanagawa 238-0316, Japan

* Corresponding author: E-mail: jhayakawa@ori.u-tokyo.ac.jp
TABLE 1.
Mean density of benthic diatoms on ACA Marginisporum
crassissima (n = 5) before and after experiment 1.

                   Density (cells/[mm.sup.2], mean [+ or -] SD)

                                                 After

                                               Filtered
                                               Seawater
Species                  Before             (ACA + Diatoms)

Cocconeis ssp.    925.1 [+ or -] 285.7   255.1 [+ or -] 142.9
Amphora ssp.      160.8 [+ or -] 132.5    35.5 [+ or -] 27.6
Navicula ssp.     432.6 [+ or -] 148.0   170.8 [+ or -] 101.8
Licmophora ssp.     5.5 [+ or -] 9.93              ND
Tabularia ssp.     68.4 [+ or -] 82.2     48.8 [+ or -] 66.8

                    Density (cells/
                   [mm.sup.2], mean
                      [+ or -] SD)

                         After

                   Filtered Seawater
                       Dissolved
                    with Ge[O.sub.2]
Species              (ACA--Diatoms)

Cocconeis ssp.     86.5 [+ or -] 22.7
Amphora ssp.               ND
Navicula ssp.      55.5 [+ or -] 34.2
Licmophora ssp.            ND
Tabularia ssp.     11.1 [+ or -] 13.6

ND, not detected.

TABLE 2.
Mean density of benthic diatoms on ACA Marginisporum
crassissima (n = 6) before and after experiment 2.

                  Density (cells/[mm.sup.2], mean [+ or -] SD)

                                              After

                                             Filtered
                                             Seawater
    Species             Before           (ACA + Diatoms)

Cocconeis ssp.    173.7 [+ or -] 79.6   127.6 [+ or -] 81.4
Amphora ssp.              ND             14.8 [+ or -] 13.4
Navicula ssp.      31.4 [+ or -] 25.7    16.6 [+ or -] 15.3
Licmophora ssp.           ND                    ND
Tabularia ssp.            ND                    ND

                    Density (cells/
                      [mm.sup.2],
                   mean [+ or -] SD)

                         After

                   Filtered Seawater
                       Dissolved
                   with Ge[O.sub.2]
    Species         (ACA--Diatoms)

Cocconeis ssp.    25.9 [+ or -] 21.8
Amphora ssp.              ND
Navicula ssp.      7.4 [+ or -] 13.4
Licmophora ssp.           ND
Tabularia ssp.            ND

ND, not detected.

TABLE 3. Mean density of benthic diatoms on ACA Marginisporum
crassissima (n = 6) before and after experiment 3.

                     Density (cells/[mm.sup.2], mean [+ or -] SD)

                                      Before

                           Fresh           Diatom-Cultivated
                        Algal Frond           Algal Frond
Species               (ACA + Diatoms)      (ACA + + Diatoms)

Cocconeis ssp.       81.3 [+ or -] 34.1  318.0 [+ or -] 186.7
Amphora ssp.         11.1 [+ or -]  9.9  140.5 [+ or -]  95.8
Navicula ssp.        48.1 [+ or -] 28.6  369.7 [+ or -] 250.7
Licmophora ssp.       9.2 [+ or -] 17.8            ND
Tabularia ssp.              ND             7.4 [+ or -]  11.5

                     Density (cells/[mm.sup.2], mean [+ or -] SD)

                                      After

                           Fresh           Diatom-Cultivated
                        Algal Frond           Algal Frond
Species               (ACA + Diatoms)      (ACA + + Diatoms)

Cocconeis ssp.       14.8 [+ or -] 16.7   114.6 [+ or -] 82.6
Amphora ssp.                ND             27.7 [+ or -] 26.9
Navicula ssp.         9.2 [+ or -] 14.7    16.6 [+ or -] 13.6
Licmophora ssp.             ND                    ND
Tabularia ssp.              ND                    ND

ND, not detected.

TABLE 4.
Mean density of benthic diatoms on G. elegans (n = 5) before
and after experiment 1.

                  Density (cells/[mm.sup.2], mean [+ or -] SD)

                                               After

                                              Filtered
                                              Seawater
    Species             Before            (Agar + Diatoms)

Cocconeis ssp.    133.1 [+ or -] 94.9   219.6 [+ or -] 125.1
Amphora ssp.        3.7 [+ or -]  9.1    20.0 [+ or -]   5.0
Navicula ssp.      53.6 [+ or -] 84.3    66.5 [+ or -]  22.2
Licmophora ssp.           ND             13.3 [+ or -]  14.5
Tabularia ssp.     14.8 [+ or -] 24.0    20.0 [+ or -]  27.6

                    Density (cells/
                     [mm.sup.2],
                   mean [+ or -] SD)

                         After

                   Filtered Seawater
                       Dissolved
                   with Ge[0.sub.2]
    Species         (Agar--Diatoms)

Cocconeis ssp.    55.5 [+ or -] 93.2
Amphora ssp.              ND
Navicula ssp.     33.3 [+ or -] 24.9
Licmophora ssp.           ND
Tabularia ssp.    13.3 [+ or -] 14.6

ND, not detected.

TABLE 5.
Mean density of large benthic diatoms per 5-mm thalli of
G. elegans (n = 5) before and after experiment 1.

                      No. of Diatoms (cells, mean [+ or -] SD)

                                                  After

                                                Filtered
                                                Seawater
      Species               Before          (Agar + Diatoms)

Gephyria sp.          12.2 [+ or -] 7.6     20.1 [+ or -] 3.7
Arachnoidiscus sp.     2.7 [+ or -] 1.3      3.9 [+ or -] 1.8

                       No. of Diatoms
                        (cells, mean
                         [+ or -] SD)

                      Filtered Seawater
                          Dissolved
                       with Ge[O.sub.2]
      Species          (Agar--Diatoms)

Gephyria sp.          16.3 [+ or -] 15.9
Arachnoidiscus sp.     2.1 [+ or -] 1.0

ND, not detected.

TABLE 6.
Effect of rearing seawater with a 5-mg/L concentration of
Ge[O.sub.2] on the growth of early juvenile top shell Turbo cornutus.

                      Initial Shell           Daily Growth
    Rearing             Diameter          Rate ([micro]m/day,
   Seawater       (mm, mean [+ or -] SD)   mean [+ or -] SD)

Filtered (0.45
  [micro]m)
  seawater         1.23 [+ or -] 0.11      14.1 [+ or -] 5.1
Seawater
  dissolved
  with
  Ge[O.sub.2]      1.29 [+ or -] 0.09      13.5 [+ or -] 3.3
Filtered (0.45
  [micro]m)
  seawater         2.38 [+ or -] 0.14      16.4 [+ or -] 8.4
Seawater
  dissolved
  with
  Ge[O.sub.2]      2.36 [+ or -] 0.12      17.8 [+ or -] 6.8

                         No. of
    Rearing            Individuals
   Seawater             Measured

Filtered (0.45
  [micro]m)
  seawater                 12
Seawater
  dissolved
  with
  Ge[O.sub.2]              12
Filtered (0.45
  [micro]m)
  seawater                  8
Seawater
  dissolved
  with
  Ge[O.sub.2]               8
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