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Biological rhythms with lunar components are common in nature. In the sea, the moon's gravitational pull on earth is the principal cause of the tides, which normally reach maximum amplitudes every new and full moon. Many populations synchronize spawning to this time. Some choose either the new or the full moon, implying that moonlight is important; but one lunar phase usually has higher tides than the other, and many species select the phase with the higher tide to improve the offshore transport of their progeny. However, tidal dominance by one lunar phase is not constant; it switches between new and full moon every seven months. We tested the influence of this 14-month “syzygy inequality cycle” (SIC) on lunar synchrony by sampling 11 populations of intertidal crabs at two locations in East Africa for 21 months. Eight populations synchronized larval release with the SIC. Tidal cues were more important than moonlight in entraining the reproductive rhythm, although two populations synchronized spawning to t...

A model was developed for the growth of intertidal algae with photosynthesis simulated both in air and in a tidal water column. Morphological data on dry mass per unit area and length-area relationships were used to separate the growth of different fucoid species. The relative growth rate of fronds at any height on the shore depended on a trade-off between net photosynthetic performance and tolerance to desiccation. 2. The simulated zonation patterns and growth rates were consistent with those observed previously for Fucus spp. and Pelvetia canaliculata. 3. The simulated growth of Ascophyllum nodosum was always slower than for the other species. This species did not form its characteristic distribution zones in simulations without including further processes in the model. However, Ascophyllum collected from the field could be separated into upper and lower shore morphologies which formed separate zones when they were simulated in competition with each other. 4. Several hypotheses were proposed concerning the relative locations and sharpness of interspecies boundaries on the shore. Zonation patterns were relatively insensitive to changes in most model parameters except the desiccation rate.

The megalopae and first crab instars are described for three of the four Indo-west Pacific species of the grapsid crab Percnon—P. abbreviatum, P. guinotae and P. planissimum. Keys are provided to these three species and for the Atlantic/East Pacific P. gibbesi. The validity of separating P. planissimum and P. gibbesi is discussed, as are the implications of the unusually large megalopa in Percnon.

Twenty-nine species of the Grapsidae and 15 species of the Ocypodidae were collected from the littoral zone in the area around Dar es Salaam. Nine species of Grapsidae and three of Ocypodidae were new records for the East African area, bringing the total of recorded species for the region to 35 and 26 respectively. The distribution of these crabs indicates a very limited endemism in the western Indian Ocean, with the majority of species forming an attenuated extension of the abundant Indo/West Pacific brachyuran fauna. The habitat preference of each species is described. Within each subfamily there is a relative uniformity in general habitat preference, mode of life and manner of feeding. The species of each subfamily are then separated by substrate choice and preferred level on the shore. The greatest numbers of species occur in the most diverse habitats—the creek mangrove, the coastal mangrove and the reef flat.

The true crabs or Brachyura can be divided into a primitive section with integumental spermathecae unconnected with the oviducts, and an advanced section with spermathecae formed as dilations of the oviducts: no intermediate forms are known. Previously the Raninidae, one of the primitive families, has been considered to have a single median sper-matheca, as in the presumed ancestral Astacura. This conflicted with their generally more evolved condition compared to the other primitive families. This investigation shows that the basic raninid structure is as in other primitive crabs, with paired spermathecae lying within endosternite 7/8. Due to the acute dorsal flexion of the posterior sternites the spermathecae have come to open anteriorly on sternite 7 rather than on sternal suture 7/8, and due to narrowing of the sternum the paired openings may appear as a single median one. This revised structure confirms the Raninidae as the most advanced of the primitive Brachyura, but unfortunately does not contribute to an understanding of the course of evolution of the spermathecal condition in the advanced Brachyura.

The larval and post-larval development of Percnon gibbesi, from a combination of laboratory-reared and plankton-caught material, is described, and larval characters within the Plagusiinae are discussed. The synonymy of Percnon and Pluteocaris is established.

The biology of the fourteen species of spider crabs occurring in the Isle of Man area was investigated, particular attention being given to their distribution, food, moulting cycle and breeding. The distribution of a species is related to the type of bottom deposit, and members of a genus have complementary distributions. There is a variable degree of diet selection and food availability is an important factor in determining distribution. The moult of puberty, which in all cases is the terminal eedysis, is well defined in all species. There are two patterns of breeding, one continuous throughout the year, the other with a single annual hatching. With few exceptions the relative growth of the chelae is allometric, exhibiting marked sexual dimorphism.