More than one hundred well-preserved specimens of a cancrid crab, Cancer gibbosulus (De Haan), were found on about 2m² of a bedding plane in the Pleistocene Higashiyatsu Formation (0.4Ma). The crabs were found embedded in living position on the mud bed and in some cases in shallow hollows. No crab with a disarticulated carapace or appendages, nor any overturned crab, was found. This population of crabs is interpreted to be a fossil association. The sex of 81 females and 6 males could be determined among a total of 113 individuals. The mode of abdominal opening indicate that the females were ovigerous.

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Microlaimus sp., a free-living marine benthic nematode, was cultured in the laboratory and its life cycle was investigated. The culture was established using a liquid medium to which a diatom (Licmophora sp.) was added as food. Mature females of Microlaimus sp. continuously deposited more than 41 eggs over 60 days at 20°C. Eggs hatched generally 4 days after deposition as juveniles of about 120μm in length. They grew to about 230μm in 5 days, and 350μm in 10 days after hatching. The sexual characters such as spicules in males and vulvae in females were observed at 12-14 days after hatching. Body length at maturation was about 400μm for males and 450μm for females. Both males and females continued to grow up to a length of 600μm. The generation time of this species was found to be as short as about 20 days. All these results suggest a high productivity potential for Microlaimus sp.

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A small ocypodid crab, Ilyoplax pingi, was found to cohabit with another, larger ocypodid crab, Macrophthalmus banzai, by two means: lodging in the burrows of M. banzai or occupying their own burrows connected with those of M. banzai. In the former case, the number of I. pingi observed emerging from a burrow of M. banzai was 4.0 on average and at most 16. In the latter case, the number of I. pingi burrows connected with one burrow of M. banzai was 2.8 on average and at most 10. The main activity of I. pingi utilizing M. banzai burrows was feeding (or feeding and waving). The activity spaces of several I. pingi either from the same M. banzai burrow or from their own burrows connected with the same M. banzai burrow overlapped considerably. Fifteen to 28% of I. pingi utilizing M. banzai burrows began to wander within 5 min of exitting the burrow. It appears that M. banzai burrows are utilized by wandering I. pingi as temporary shelters.

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The influence of the tidal cycle decreases with the distance from the sea, and this may affect the behavioral timing systems in estuarine animals. In addition, the circatidal rhythm of these animals may be controlled by light-sensitive systems. To investigate the timing systems in estuarine animals, the larval release activity of a semi-terrestrial crab, Sesarma erythrodactylum, was monitored in the laboratory without any tidal influence. The larval release rhythm free-ran under constant dim light conditions, which suggests that the timing of release is under the control of an endogenous clock. The free-running period was somewhat different for each individual. Under an artificial 24-h light-dark (LD) cycle in phase with that in the field, the timing of release coincided with high tides at night. In contrast, the rhythm changed to match a phase-shifted 24-h cyclic light regimen. These results demonstrate that a light-sensitive mechanism is certainly involved in the circatidal timing systems of S. erythrodactylum. The role of the 24-h LD cycle is not only to shift the synchrony of the timing of release onto the other high tide when necessary for maintaining a nocturnal schedule, but also to drive the phase of the circatidal rhythm. While the tidal behavioral rhythms of intertidal animals reflect two parameters of the tidal cycle, i.e., the 12.4-h period and the tidal amplitude, those of estuarine crabs lose synchrony with the tidal amplitude and show a nocturnal pattern instead. These properties of the circatidal rhythm of larval release in S. erythrodactylum and other intertidal and estuarine crabs can be explained by a coupled oscillator hypothesis.

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The micro-habitat, size composition, and movement pattern of the muricid snail Muricodrupa fusca were investigated. In contrast to two other muricids, Morula musiva and Thais clavigera, this species exclusively inhabits tidepools and its shell size is small. The adaptive implications of its small size are discussed in relation to the environmental conditions of its main habitat. Small size presents some advantages for living in tidepools with limited food resources. The major disadvantages of a small size, namely low tolerance to desiccation and increased vulnerability to predators, appear to be avoided by inhabiting tidepools. Mu. fusca moved landward in September and seaward in February, and reproduction is proposed as the reason for this migration.

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The vertical distribution and life cycle of two free-ranging polychaetes, Arabella iricolor (Montagu) and Nereis nichollsi Kott, were studied on a rocky intertidal shore at Shirahama, Wakayama Prefecture, where two mussels, Septifer virgatus (Wiegmann) and Hormomya mutabilis (Gould), formed vertically contiguous beds. The abundances of both polychaetes increased downshore within the mussel beds. Multiple linear regression analyses against polychaete density revealed that only shore height tended to be negatively correlated for both species. Adult polychaetes with eggs or sperm in their coelom were collected from May to August for A, iricolor and from April to August for N. nichollsi. Newly settled juveniles (<0.5mm width) were found abundantly within algal turf formed in the gaps of the H. mutabilis bed in August and September for A. iricolor and in June to September for N. nichollsi. No large polychaetes (>2mm width) of both species were found within the algal turf throughout the year. Many small polychaetes of both species were collected in the H. mutabilis bed in August to December. The cohort increased in size but decreased in density towards the next summer in the mussel bed, and disappeared by October. It appears that both polychaetes have a longevity of one year and reproductive periods chiefly from May to August, followed by recruitment from June to September. Juveniles seem to settle exclusively into the algal turf and migrate with growth into the H. mutabilis bed and even the S. virgatus bed.

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The incidence of limb loss and bald chelipeds in the Japanese mitten crab Eriocheir japonica in the sea was investigated at Tsuyazaki, Fukuoka Prefecture, Japan, in 1990-1991. Frequency of crabs with missing limbs increased remarkably in males from the beginning (September-December) to the end (May-July) of the reproductive season (26.8% to 83.3%), but only slightly in females (26.5% to 53.1%). Cheliped loss increased in males, from 8.9% to 33.3%, but showed no apparent change in females (5.8% to 4.1%) over the same period. Similarly, the average number of missing limbs per crab among all the sampled specimens increased markedly in males (0.36 to 1.71), but not as much in females (0.45 to 0.86). The site of limb loss was significantly different between sexes, with a lower frequency of cheliped loss in females compared to males, but there was no significant site specificity of loss in either sex. Increases in frequencies of limb loss reflect the deterioration in physiological condition after migration and reproduction and no regeneration of limbs by moulting in the sea. Higher frequencies of limb loss in males, especially of chelipeds, reflect their more active use of their larger chelipeds in the sea. Occurrence of bald chelipeds increased in both sexes towards summer (3.7% to 66.7% in males and 2.9% to 53.1% in females), with no significant difference between sexes in each season. Thus, the causes of baldness may differ from those of limb loss which more clearly reflect the behavioural differences between sexes.

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