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Lithodoid and paguroid crabs are morphologically assigned to the superfamilies Lithodoidea and Paguroidea, respectively. Molecular analyses, however, have revealed closer genetic proximity of the lithodoid crabs to the family Paguridae than to other families of Paguroidea, provoking a long debate. We investigated the length and nucleotide sequence variation of the nuclear ribosomal DNA internal transcribed spacer 1 (ITS1) in lithodoid and paguroid species. Uniquely short ITS1s (215–253 bp) were observed in seven lithodoid species belonging to the families Lithodidae and Hapalogastridae. In contrast, ITS1 length varied considerably in 13 paguroid species belonging to the families Coenobitidae, Diogenidae, and Paguridae. Short-to-long ITS1s (238–1090 bp) were observed in five species of the family Paguridae, and medium to long ITS1s (573–1166 bp) were observed in eight species of the families Coenobitidae and Diogenidae. Interestingly, ITS1s of considerably different sizes coexist in individual paguroid species. Nucleotide sequence analysis indicated that the short ITS1s observed in the family Paguridae were descendants of longer ITS1s and were homologous to the short ITS1 of lithodoid species. ITS1 sequences of the families Coenobitidae and Diogenidae shared no nucleotide elements similar to those of lithodoid and pagurid species. These molecular signals indicate that the short ITS1 in pagurid lineage was passed on to lithodoid lineage, strongly supporting the “hermit-to-king” crab hypothesis.

Cheliped dimorphism is common in decapod crustaceans, and many decapods have a pair of different-sized chelipeds, i.e., one side being larger (major) than the other (minor), a phenomenon known as heterochely. Anomuran crabs from the family Porcellanidae exhibit a true crab-like body shape, with relatively large chelipeds for their body size, and they show a sexual size dimorphism of chelipeds (i.e., males with larger chelipeds). However, very little is known about heterochely in porcellanid crabs. Therefore, we aim to elucidate heterochely and cheliped dimorphism in the porcellanid crab Petrolisthes japonicus (De Haan, 1849). We examined the laterality of the chelipeds and performed allometric growth analyses of the cheliped dimensions. Heterochely was observed in most of the specimens, and major chelipeds occurred randomly on either the right or left sides of the body, regardless of the crab size. Major and minor cheliped size of males surpassed female cheliped size, and males increased the growth of both major and minor chelipeds to the same degree during the breeding season, thereby inducing the distinct intersexual size dimorphism of the cheliped. This is probably attributable to sexual selection (i.e., male-male competition). Intersexual dimorphism as an energy allocation strategy was also evident with females allocating their energy evenly to both major and minor cheliped growth without changing the chela shape; regardless of the body size, whereas males were investing more energy in major cheliped growth, resulting in wider chelae with increasing body size. Consequently, the major cheliped may have a greater role in male-male competition in P. japonicus.

In the Sea of Japan, the mature red snow crab Chionoecetes japonicus is distributed at a depth of 500���2,700 m. However, its planktonic larvae occur at depths shallower than 300 m. To better understand the factors influencing larval dispersal and settlement of the red snow crab in its natural habitat, we tested the effects of temperatures ranging from 1��C to 18��C on the survival, developmental period, and growth of red snow crab zoeal and megalopal larvae. The mean number of days from hatching to second zoeae and megalopae and from megalopae to the first crab instar was significantly shorter at warmer temperatures. Moreover, the relationships between mean temperatures and larval periods were well described by heat summation theory equations and a nonlinear thermodynamic Sharpe-Schoolfield-Ikemoto (SSI) model. The higher survival rate, intrinsic optimum temperatures in the SSI model, and larger carapace width suggested that the optimum temperatures for the development of zoeal and megalopal stages of the red snow crab are 9���10��C and 7���8��C, respectively. A lower optimum temperature in the megalopal stage than in the zoeal stages was considered an adaptation to facilitate deeper vertical distributions in the megalopal stage.

Post-autotomy appendage movement has evolved independently in several vertebrate and invertebrate taxa. Moving appendage may divert a predator’s attention away from the body, allowing the prey to e...

Temperature affects ectotherm's biological processes and plays a fundamental role in determining their geographical distribution. We elucidated the temperature adaptation of juveniles of two mud crab species in the genus Scylla De Haan, 1833 (in De Haan, 1833-1850): Scylla paramamosain Estampador, 1949 and Scylla serrata (Forskål, 1775), which occur mainly in temperate and subtropical/tropical areas in Japan, respectively. The first instar crabs (C1) were raised from three broods for each species and were individually cultured to molt to C2-C5 at different temperatures (15.2-30.5 °C℃). In S. paramamosain, C1 juveniles could molt to C2 at 15.4 °C and the survival rate was reduced at 29.8 °C through C4-C5. In S. serrata, C1 juveniles could not molt to C2 at 15.2 °C and the survival rate was not affected by temperature thereafter. The intermolt period was shortened at higher temperatures in both species. The carapace width of juveniles increased with increasing temperature but became smaller at 27.6-30.2 °C in S. paramamosain, whereas it appeared to reach a plateau at > 24 °C in S. serrata. The lower threshold temperature (95 % confidence interval) for the development of C1 juveniles was estimated as 13.65 °C (13.51-13.77 °C) for S. paramamosain, and 15.44 °C (15.12-15.72 °C) for S. serrata. Thus, juveniles of S. paramamosain and S. serrata adapted to lower and higher temperature conditions, respectively, reflecting their biogeographical distribution.

We examined the effects of temperature and salinity on larval survival, duration, and growth of three amphidromous atyid shrimps, Caridina leucosticta, C. multidentata, and C. typus to infer larval dispersal strategy in the sea. Larvae were reared under 25 combinations of five different temperature (20, 23, 26, 29, and 32°C) and salinity (4.25, 8.5, 17, 25.5, and 34 ppt) levels. Interspecific variability was detected in larval performance: C. leucosticta larvae were able to survive to moult into the juvenile stage at the lower salinity condition (8.5 ppt), C. multidentata larvae exhibited an ability to adapt to the wide range of salinity condition (17–34 ppt), and C. typus larvae adapted to the higher salinity condition (34 ppt) better than C. leucosticta larvae. Larval duration was less variable between species. Thus, salinity adaptation of larvae may play an important role in mediating the larval dispersal of the three Caridina species in the sea. Larval dispersal range may be most limited near the river mouth for C. leucosticta, and be moderate for C. typus, and C. multidentata larvae may be able to disperse broadly under the high salinity condition of the open sea.

The East Asian common octopus Octopus sinensis is a bottom-dwelling species, but goes through a planktonic phase as ‘paralarvae’ during the first several weeks of life. As the swimming performance of the paralarvae is relatively poor, their dispersion relies primarily on ocean currents; however, vertical swimming may play an important role in their horizontal dispersion by controlling their buoyancy in the current. Within this context, ontogenic changes in vertical swimming in relation to the ambient water flow are key to understanding the dispersion properties of the paralarvae and the subsequent onset of settlement, yet this information remains largely unknown. Here, we investigated the effects of different water flows (downward × 4 velocities, stagnant and upward × 5 velocities) on the vertical swimming position in test columns using laboratory-raised O. sinensis paralarvae aged 0, 5, 10 and 15 days during the day and at night. Paralarvae younger than 10 days ( 5 mg) commenced short clinging behaviour, and their swimming positions dispersed downwards during daytime regardless of the water flow, indicating the onset of the transition to settlement. In contrast, their position shifted to near the upper limit at night by swimming upwards, even under downward water flows. Our results suggest that O. sinensis exhibits characteristic behaviour that alternates between diurnal descent dispersion with clinging (hyperbenthic trait) and nocturnal upward swimming (planktonic trait) during the transition to permanent benthic life.

Mass mortality due to necrosis signs occurred in hatchery-reared zoea stage larvae of the mud crab Scylla serrata in Okinawa, Japan, and a causative bacterium was isolated. In this study, we identified and characterized the bacterium by genome analysis, biochemical properties and pathogenicity. The bacterium was a Gram-negative, non-motile, long rod, forming yellow colonies on a marine agar plate. It grew at 20-33°C (not at 37°C) and degraded chitin and gelatin. Phylogenetic analysis of the 16S rRNA gene sequence identified the bacterium as Aquimarina hainanensis. Genome sequence data obtained from Illumina MiSeq generated 29 contigs with 3.56 Mbp in total length and a G + C content of 32.5%. The predicted 16 chitinase genes, as putative virulence factors, had certain homologies with those of genus Aquimarina. Experimental infection with the bacterium conducted on larvae of four crustacean species, brine shrimp Artemia franciscana, freshwater shrimp Caridina multidentata, swimming crab Portunus trituberculatus and mud crab S. serrata, revealed that this bacterium was highly virulent to these species. The present study suggests that the bacterium caused mass mortality in mud crab seed production was A. hainanensis and can be widely pathogenic to crustaceans.

Because temperature strongly influences biological processes of ectotherms, they must adapt to the thermal conditions of their habitats. We evaluated the upper thermal tolerance of early juveniles of six terrestrial hermit crab species in the family Coenobitidae (genera Birgus and Coenobita), B. latro, C. brevimanus, C. cavipes, C. purpureus, C. rugosus, and C. violascens, that occur in the northwestern Pacific region of Japan. A total of 30 laboratory-raised juveniles (approximately 1 mm in shield length) carrying gastropod shells were individually stocked in small plastic cups with sandy bottoms in temperature-controlled incubation chambers at ∼27°C. The temperature was increased by 1°C every 48 h, and the juveniles were observed until all the crabs had died. The median upper lethal temperature was estimated as the temperature at which 50% of the test juveniles had died. The median upper lethal temperature estimates significantly varied among the species. Coenobita violascens, which mainly occurs in mangrove estuaries with lower thermal conditions, had the lowest median upper lethal temperature values. The median upper lethal temperature values estimated for B. latro were slightly lower than those for the other Coenobita species, probably reflecting its cryptic nature in natural habitats during the juvenile stage. The most northerly distributed species, C. purpureus, had the highest median upper lethal temperature values, suggesting the existence of common physiological mechanisms that regulate both upper and lower thermal tolerance abilities.

Pathogenic marine fungi, Lagenidium thermophilum is known causative agent in the crustacean industry. Current disinfection practice in hatchery has risks and negative impacts which prompts suitable substitute to synthetic antifungal agents. Thus, this study was conducted to evaluate the antifungal potential of postbiotic from four potential probiotics towards marine oomycetes, L. thermophilum IPMB 1401. The screening test showed that the Lactobacillus plantarum GS12 and Bacillus cereus GS15 postbiotics were positive for antifungal activity on L. thermophilum IPMB 1401. These two bacterial extracts have minimum inhibitory concentration （MIC） at 50%. The toxicity assay on MIC level of the postbiotic revealed that the cumulative mortality of brine shrimp nauplii exposed to B. cereus postbiotic was significantly lower compared to L. plantarum GS12 postbiotic and formalin. This indicates a high potential of B. cereus GS15 as a prospect for alternative control method for fungal infections in the crustacean culture industry.

A season-independent juvenile supply system is required for developing the aquaculture of the red swamp crayfish Procambarus clarkii. The present study aimed to test if P. clarkii females can spawn multiple times under a constant temperature (25°C) and photoperiod (14 L: 10D) – conditions that are suitable for their reproduction. When a female developed glair glands on the underside of its uropods, it was paired with a single male for mating. Females that laid the first egg batch from the middle of July to the end of October were successively cultured; large proportions of individuals then rematured (76%) and laid a second egg batch (94%) from late September to early February. Although we finished the culture experiments in early February when all the test females had laid their second egg batch, a third spawning occurred in 27% of these females. Overall, the earlier the females spawned, the more egg batches they laid. Fecundity was correlated with female body size (postorbital carapace length) and decreased in accordance with spawning time. The egg development rate was very low in some females – probably depending on male fertility. We highlight future research subjects for developing a continuous breeding system in P. clarkii.

Gastropod shells are vital resources for hermit crabs for their growth, reproduction and survival. However, the land hermit crab Coenobita spinosus, which lives in wet and dark coastal forests, she...

Metamorphosis season of megalopae to the first crab stage in snow crab Chionoecetes opilio and red snow crab C. japonicus was inferred by culturing wild-born megalopae collected from the Se...

Despite recent advances in culture technology for East Asian common octopus Octopus sinensis paralarvae using upwelling systems, securing suitable feed for the paralarvae is an unresolved issue. The zoea of the swimming crab Portunus trituberculatus is a good candidate for paralarval feed because of the high fecundity of the adult females. To investigate the effects of supplying P. trituberculatus zoeae and their feeding method on paralarvae, we cultured paralarvae with supplying different combination ratios of zoeae and Artemia (10:0, 7:3, 5:5, 3:7 and 0:10), and with or without supplementing rotifers using small‐scale (3‐L) upwelling systems. Paralarval survival rate and growth were improved when zoeae were supplied as the main feed, but reduced when the proportion of Artemia exceeded half the whole preys. Supplementing rotifers did not affect the paralarval survival and growth. Subsequently, paralarvae were cultured by supplying zoeae (partially augmented by Artemia) using three large (1‐kl) upwelling systems to assess their feeding effectiveness in juvenile octopus production. Paralarvae could be cultured at high survival rates of 77.1 ± 5.1% to reach benthic juveniles at 23 days after hatching. In conclusion, supplying P. trituberculatus zoeae augmented with Artemia under an upwelling culture system has great potential for juvenile octopus production.

Because temperature strongly influences biological processes of ectotherms, it also plays a fundamental role in determining their geographical distribution. We evaluated the low-temperature tolerance of early juveniles of six terrestrial hermit crab species in the family Coenobitidae (genera Birgus and Coenobita), B. latro, C. brevimanus, C. cavipes, C. purpureus, C. rugosus, and C. violascens that occur in the northwestern Pacific region, Japan. A total of 30 laboratory-raised juveniles (about 1 mm in shield length) carrying gastropod shells were individually stocked in small plastic cups with sandy bottoms in temperature-controlled incubatory chambers at ∼27°C. The temperature was reduced by 1°C every 48 h, and the juveniles were observed until all the crabs had died; the median lethal temperature (MLT) was estimated as the temperature at which 50% of the test juveniles had died. The MLT estimates varied significantly among the species, and the most northward distributed species, C. purpureus, had the lowest MLT values. The phylogeny, paleoceanography, paleogeography, and paleoclimatology suggest that cooler thermal regimes might have acted as an evolutionary force for the divergence of C. purpureus in the Pliocene. A negative correlation was found between the northern latitudinal limit of distribution and the MLT values, even after controlling for the phylogenetic relationships in the six coenobitids. A temperature-dependent biogeography was thus recognized in terrestrial hermit crab species in the northwestern Pacific region, and global warming is expected to affect their geographical distributions.

The gastrointestinal (GI) microbiota is a group of complex and dynamic microorganisms present in the GI tract of an organism that live in symbiosis with the host and benefit the host with various biological functions. The communities of GI microbiota are formed by various aerobic, anaerobic, and facultatively anaerobic bacteria in aquatic species. In spiny lobsters, common GI microorganisms found in the GI tract are Vibrio, Pseudomonas, Bacillus, Micrococcus, and Flavobacterium, where the structure and abundance of these microbes are varied depending on the environment. GI microbiotas hold an important role and significantly affect the overall condition of spiny lobsters, such as secreting digestive enzymes (lipase, protease, and cellulase), helping in digesting food intake, providing nutrition and synthesising vitamins needed by the host system, and protecting the host against infection from pathogens and diseases by activating an immune mechanism in the GI tract. The microorganisms in the water column, sediment, and diet are primarily responsible for altering, manipulating, and shaping GI microbial structures and communities. This review also highlights the possibilities of isolating the indigenous GI microbiota as a potential probiotic strain and introducing it to spiny lobster juveniles and larvae for better health management.

Fungal infections have been reported as one of the main hindrances for successful crustacean seed production. In a shrimp hatchery of Universiti Malaysia Sabah, abnormal sponge development of the ornate spiny lobster Panulirus ornatus was observed and it was suspected as fungal infection due to a change in sponge colour. Fungus was isolated from egg samples of P. ornatus and transferred to Peptone-Yeast-Glucose-Seawater (PYGS) agar to identify and to reveal its morphological characteristics. Interestingly, the isolate in PYGS broth transferred into sterilized seawater did not show any characteristic feature of any asexual reproduction of Oomycete infection on crustacean. Consequently, slide culture was performed for a more detailed examination, where the fungus isolate showed septate hyphae and vesicle-like fruiting body only upon staining via Iodine-Glycerol stain. Based on the morphological characteristic features, the present isolate was determined as a non-oomycete organism and designated as IPMB LE01 strain. According to the ITS nucleotide sequence analyses of IPMB LE01, it has matched to Zasmidium musae. This species was mainly reported in plant fungal infection and rarely observed in aquatic animals. Hence, it is considered as the first report of fungus-associated on the moribund eggs of ornate spiny lobster in Sabah.

Artemia are potential food organisms for the mass culture of common octopus paralarvae but cause poor paralarval growth and mortality. To understand problems arising from Artemia use, we focused on Artemia flocculation in paralarval culture tanks; Artemia get caught up with each other, exhibit disrupted swimming, are deposited on the tank bottom and eventually die. To clarify whether paralarvae induce the flocculation of food organisms or not, we cultured newly hatched Artemia nauplii, 3-day-old metanauplii and decapod crustacean zoeae with or without paralarvae at different growth stages (weight). Flocculation occurred only when Artemia were cultured with paralarvae; metanauplii had a higher susceptibility for flocculation than nauplii. Flocculated Artemia proportion increased with increasing paralarval weight. Scanning electron microscopy revealed that flocculated metanauplii had deformed setules on their setae, with hook-shaped tips and adhesion of neighbouring tips, suggesting that flocculation may occur via a mechanism similar to the ‘hook-and-loop fastener’. As octopus paralarvae exhibit external digestion, digestive enzymes secreted by paralarvae may deform Artemia setules and result in flocculation. As flocculation did not occur when metanauplii were cultured in water in which paralarvae were cultured and then removed, causative enzymes were probably rapidly inactivated after secretion.

The present study was conducted to examine the occurrence and distribution of early juvenile land hermit crabs of the genus Coenobita on a small beach on the Boso Peninsula, Japan as a basis for understanding the important habitats that land hermit crabs use during the sea-to-land transition. The survey sites were supratidal zones at six points with heterogeneous topography, and visual surveys for early juveniles were conducted from middle July to late October in 2015 and 2016. The number of empty gastropod shells in four quadrats along the intertidal zone at each point was also counted in the 2016 survey. The occurrence of early juveniles varied among survey points and was spatially stable for two successive years. The first appearance and abundance of early juveniles varied among survey years. Early juveniles were abundant at points with a gravel bottom throughout the intertidal zone, and empty gastropod shells were also abundant at these points. The abundance of early juveniles was highly correlated with the amount of empty gastropod shells. Thus, our surveys suggested the importance of the sediment types and abundance of empty gastropod shells in the intertidal zones for settlement and landing by land hermit crabs.

Despite a growing interest in common octopus for aquaculture, massive mortalities during the planktonic paralarval stage still hamper production. Prior to die-off during rearing trials for paralarvae, it could be observed that the food-capturing paralarvae tended to be carried downward in a current induced by the aeration system, eventually depositing the animals on the tank bottom. The aim of this study was to investigate the effect of upwelling water-flow, that might support paralarval swimming and floating, on paralarval survival and growth. A new artificial upwelling rearing system using a strong downward flow in the center to the bottom of the tank was developed. The effects of the new system in combination with supplying natural zooplankton (mainly decapod crustacean zoeae) or Artemia were evaluated using three different broods hatched from three mothers (broods A, B, and C). As food items for zooplankton, n-3 highly unsaturated fatty acid (n-3 HUFA)-fortified rotifers and digestible microalgae Nannochloropsis oculata were supplemented. The hatchling dry weight was significantly different among broods, showing heavier weight in order of broods A > C > B. Use of the upwelling rearing system significantly improved the rate of paralarval survival as compared with using the standard system, and paralarval growth was significantly faster when reared in the upwelling system with zooplankton supplementation. Paralarvae in the upwelling + zooplankton group kept high survival rates over 80% until 18 days after hatching (DAH). The survival rate of the tank of brood A could achieve 80.8% at 20 DAH, however, those of broods B and C dropped to 8.2% and 54.4% at 20 DAH, respectively, presumably due to combined effect of small hatchling size and insufficient food supply. The octopuses of broods A and C reached benthic stage by 25 and 23 DAH, with survival rates of 51.0% and 45.3%, respectively. The overall results suggest that an inadequate water-flow environment in combination with poor food quality and quantity are major causes of paralarval mortality of the common octopus. For culturing paralarvae, the combined use of an upwelling rearing system and supplying decapod crustacean zoeae supplemented with n-3 HUFA fortified rotifers and digestible N. oculata was recommended.

Microalgae concentrates are important for hatchery operations but have been found to contain high concentrations of bacteria. Therefore, this study investigated the effects of four thermal disinfection and sterilisation treatments, low‐temperature long‐time (LTLT; 62–65°C, 30 min), high‐temperature short‐time (HTST; 72–75°C, 15 s), ultra‐high‐temperature (UHT; 120–150°C, 1–3 s) and high‐temperature high‐pressure (HTHP; 121°C, 0.2 MPa, 15 min), on concentrates of the microalgae, Chaetoceros gracilis, Chlorella vulgaris (fortified with eicosapentaenoic acid and docosahexaenoic acid) and Nannochloropsis oculata. The concentrates had reduced bacterial counts following LTLT, HTST and UHT and were completely sterilised by HTHP. Furthermore, cell dispersibility was retained in all treatment groups except HTHP‐treated C. gracilis. Rotifers that were cultured with HTHP‐treated C. vulgaris and N. oculata experienced significant population increases, whereas the survival rate and growth of Artemia nauplii significantly improved when supplied with HTHP‐treated N. oculata, indicating that HTHP enhances the digestibility of this microalga. HTHP did not affect the total lipid content and fatty acid composition of the microalgae. These results suggest that HTHP‐treated C. vulgaris and N. oculata could be used for the nutritional enrichment of rotifers and Artemia, while avoiding the risk of bacterial contamination, and would have an increased shelf life at room temperature.

Skeletal deformities are one of the serious problems in hatchery‐reared longtooth grouper (Epinephelus bruneus). In this study, seed production of longtooth grouper was carried out in four large hatchery‐grade tanks. Osteological development of the vertebral column, caudal‐fin skeleton and dorsal‐fin supports in larvae and juveniles was described. The vertebral ontogeny started at 4.8 mm total length (TL) with the formation of neural spines 2 and 3, and the vertebral centra began forming at 5.3 mm TL. By 16 mm TL, all vertebral bony elements were formed except for the dorsal ribs. The onsets of the ontogeny of the caudal‐fin skeleton and dorsal‐fin supports occurred at approximately 5.1 mm and 3.9 mm TL, respectively, and the completion of these bony elements occurred at approximately 20 mm TL. The incidence of deformity was examined; some type of deformity was observed in 57%–68% of the specimens. These deformities mainly occurred as three types: lordosis, around vertebrae 1–5; saddleback‐like syndrome, around vertebrae 7–11; and vertebrae fusion, around vertebrae 22–24. The incidence of prehaemal lordosis was significantly higher among individuals with an uninflated swim bladder than among those with an inflated swim bladder.

Larval sinking causes larval mass mortality during seed production in the horsehair crab, Erimacrus isenbeckii. Under normal light conditions, horsehair crab larvae generally show negative phototactic behaviour and sink in their rearing seawater. It has been proposed that culturing larvae in the dark may prevent larval sinking. Herein, we examined the effect of photoperiods on horsehair crab larval survival and development to facilitate the development of larval rearing techniques that prevent sinking. Newly hatched larvae were reared with Artemia to the first crab stage in 2‐L beakers under five photoperiods: 0L:24D, 6L:18D, 12L:12D, 18L:6D and 24L:0D. Larvae survived and molted to the first crab stage under all tested photoperiod conditions. The survival rate improved with increasing light period, whereas the developmental period for each larval stage decreased with increasing light period. Longer light periods increased the carapace length at the first crab stage. Our results suggest that larvae could be cultured to the first crab stage in large‐scale tanks under constant darkness. However, significantly improved larval performance under longer photoperiodic conditions indicates a need for developing alternative culture techniques to control larval behaviour in the seed production tank.

Settlement is the process whereby benthic octopuses with a planktonic paralarval phase make a behavioural change from being active swimmers to assume adult life on the seabed . This process is poorly understood, mainly because octopus juveniles are highly cryptic in the natural habitat and they lack any distinct morphological changes (i.e. metamorphosis) in concert with settlement. To investigate behaviour during the settlement process, ontogenetic changes in swimming, clinging and shelter-utilizing behaviours were examined during both day- and night-time using laboratory-raised East Asian common octopus, Octopus sinensis, aged from 10 to 28 days after hatching (ranging from 1.7 to 16.4 mg mean dry weight, DW). The accompanying change in feeding habit from planktivorous to benthivorous was assessed by comparing rates of consumption of planktonic zoea (of the swimming crab Portunus trituberculatus ) and benthic juveniles of the Manila clam , Ruditapes philippinarum . During the daytime, octopus behaviour changed rapidly from swimming to clinging onto the surface of the wall and bottom of the test tank during growth from 3 to 9 mg DW. At night, the frequency of clinging behaviour increased along with growth but the behavioural change progressed more slowly, indicating a tendency to continue swimming during darkness. Shelter utilization during the day commenced mainly after reaching 8 mg DW, after which shelter use gradually increased with growth. In contrast, shelter utilization at night was rarely observed throughout the experiment. Regarding octopus prey preference , consumption of clam juveniles increased with growth while zoeae consumption remained constant. The increase in clam consumption, however, was gradual, indicating that the shift of feeding habit from planktivorous to benthivorous is by no means an abrupt transition. These results indicate that O. sinensis requires a considerable period of time to complete the transition to benthic life. The alternating behaviours of diurnal clinging and nocturnal swimming in the water column may facilitate safe migration of the octopuses from the location to which they have been passively advected towards a more suitable benthic habitat while minimizing their exposure to visual predators. Since shelter utilization and feeding on bivalves are likely to require the use of a certain degree of force and complex coordination of the arms and suckers, the extended, gradual increase in shelter utilization frequency and clam consumption may be attributable to the degree of arm and sucker development.