Augusto A. V. Flores

SEXUAL MATURITY, REPRODUCTIVE CYCLES, AND JUVENILE RECRUITMENT OF PERISESARMA GUTTATUM (BRACHYURA, SESARMIDAE) AT PONTA RASA MANGROVE SWAMP, INHACA ISLAND, MOZAMBIQUE

The sesarmid Perisarma guttatum is the most abundant crab species inhabiting the lower mangrove areas at Inhaca Island, and presumably an ecologically important species in this habitat. Among the scarce available information on its population ecology, this species is known to follow a semilunar rhythm of larval release, with breeding probably extended year-round. In this study, a population of P. guttatum was examined, from which estimates of sexual maturity were carried out for both sexes, and breeding activity was monitored to describe short-term variation of reproductive intensity. In an attempt to relate reproductive cycles with recruitment patterns of young, a systematic random sampling design was used to quantify and describe the juvenile population. Reproductive parameters in adults and density estimates for juveniles were obtained from samples taken at 4-d intervals over a 3-mo period. Reproductive activity increased over the study period. Females released larvae around the new and full moon, but lunar variation of the ovigerous ratio with peaks preceding full moon periods indicated that different breeding groups may account unevenly for the reproductive output of the population. Very young recruits made up the larger fraction of the juvenile population, and their density increased from the beginning to the end of the sampling season. Density variation pattern of first-crab stages followed fortnight cycles, but slow growth prevented recruitment pulses to be tracked in size-frequency distributions. Polymodal distributions corresponded to recent recruits and older juveniles originating from previous settlement seasons. Growth rates of identified age groups suggest that sexual maturity in females is likely to be achieved after 2 years or more.

Intertidal distribution and species composition of brachyuran crabs at two rocky shores in Central Portugal

The objectives of the present study are to describe and compare the brachyuran community of rocky shores within the Central Portuguese coast and to examine the zonation patterns of the most representative species. For this, randomly placed transects were surveyed to obtain crab counts according to microhabitat and intertidal level. Repeated sampling in two different shores during two different seasons provided spatial and temporal replication for zonation analyses. Seven species were registered: Pachygrapsus marmoratus, Eriphia verrucosa, Xantho incisus, Carcinus maenas, Necora puber, Pirimela denticulata and Pilumnus hirtellus. Species density rankings are the same at both localities, but the less exposed shore presents higher diversity. While most species are mainly confined to specific microhabitats in the lower level, P. marmoratus and E. verrucosa can exploit the whole intertidal range. Regardless of shore and season, E. verrucosa is more abundant in the lower intertidal levels, while no such zonation patterns were recorded for P. marmoratus. Initial predictions concerning the effect of wave exposure and temperature on the zonation of those species are not validated after analysing the factorial model proposed. Between-shore contrasts were found instead, with higher densities recorded in the more exposed locality for both species. Possible causes of the observed patterns are discussed.

Sexual maturity, larval release and reproductive output of two brachyuran crabs from a rocky intertidal area in central Portugal

Summary Sexual maturity, larval release and reproductive output were examined and compared in the intertidal crabs Xantho incisus (Brachyura: Xanthidae) and Pachygrapsus marmoratus (Brachyura: Grapsidae). Categorising females into predefined abdominal morphotypes proved to be the most realistic procedure to estimate size at 50% sexual maturity (i.e., the size at which 50% of the individuals are mature) in both species. In males, maturity estimates based on the proportion of individuals bearing mature gonads suggested that complementary allometric techniques using the relative growth of gonopods are adequate for both P. marmoratus and X. incisus. During the first half of the breeding season, both species breed intensively producing multiple broods within a single intermoult stage. Clutch age is positively correlated to stage of gonad development, and a new egg batch can be extruded within a few days of larval release. After July, ovigerous females with advanced broods were commonly found with spent gonads indicating a gradual decrease of breeding activity. In contrast to most brachyurans studied so far, asynchronous patterns of larval release were recorded for both species. It is suggested that the lack of an entrained lunar cycle may enable breeding females to produce successive egg batches within a shorter period, thus compensating for an increased larval mortality due to sub-optimal timing of larval release. The production of offspring can be adequately estimated by egg counts in P. marmoratus, but brood loss is significant in X. incisus, female size having no effect on this feature. Differences of brood volume, controlled for variation in crab size, suggested a slightly higher reproductive effort per batch in X. incisus. The reproductive variables examined will be used in future research to generate egg production profiles for the studied populations.

The Megalopa and Juvenile Development of Pachygrapsus Transversus (Gibbes, 1850) (Decapoda, Brachyura) Compared With Other Grapsid Crabs

Megalopae were reared in the laboratory to the 7th crab stage. The megalopa and 1st crab stage are described and juvenile development was studied with emphasis on pleopodal differentiation. The megalopal phase, is easily identified, and shares with those of other Grapsinae and Plagusiinae big size, the presence of many natatory setae, and a series of conspicuous teeth on the inner margin of the dactyli from the 2nd to 4th walking leg. These features are regarded as adaptive for settlement in a wave-swept environment, such as the rocky marine intertidal where most of those species live. Fast development of juvenile pleopods is another characteristic of these subfamilies. In Pachygrapsus transversus, the sexes can be distinguished from the 2nd crab stage. Gonopod differentiation in males and the basic segmentation of all four pleopod pairs in females are already concluded at the 5th instar. A review of the available information indicated that settlement of large megalopae and fast juvenile development, preceding a precocious sexual maturity, are trends in Grapsinae and Plagusiinae. On the other hand, the Sesarminae pass through a more extensive juvenile instar sequence and presumably a delayed maturity.

Population dynamics of the shore crab Pachygrapsus marmoratus (Brachyura: Grapsidae) in the central Portuguese coast

The population dynamics of the shore crab Pachygrapsus marmoratus was studied at two different rocky shores, Cabo Raso and Avencas, within the central Portuguese coast. The population structure at these shores, obtained by pooling monthly samples, revealed marked discrepancies. Unimodal, right-skewed size–frequency distributions at Avencas contrasts with the bimodal distributions obtained at Cabo Raso, corresponding to early recruits and larger adults. Recruitment timing and intensity together with habitat complexity are probable causes of the differences observed. At Cabo Raso an intense recruitment season was recorded from September to November, but very small juveniles were always present in samples. Monthly polymodal distributions are apparent but the size structure too complex to identify age groups through the sampling period. At Avencas, the influx of new juveniles is temporally restricted and year groups recognizable from main recruitment events taking place around October. Young-of-the-year crabs grow quickly to mature size and join the reproductive stock in the next breeding season from late March to early September in the study region. The growth pattern in both sexes follows a seasonal trend with minimum growth during the breeding months and maximum growth rates from late winter to early spring. Life span may reach four years, probably more in males. At Avencas, population maintenance seems to be highly dependent on favourable conditions promoting adequate settlement intensity and timing. Early recruitment is presumed to enhance the reproductive output of young-of-the-year individuals which comprise the bulk of breeding crabs. At Cabo Raso settlement rates are much higher and recruitment timing may be less critical since most breeding crabs belong to the 1-y+ class.

The use of artificial benthic collectors for assessment of spatial patterns of settlement of megalopae of Carcinus maenas (L.) (Brachyura: Portunidae) in the lower Mira Estuary, Portugal

Artificial benthic collectors have been widely used for the assessment of settlement rates of decapod crustaceans. However, to date no consistent works have addressed spatial patterns of settlement in different estuarine habitats, and no specific studies targeted the interaction of artificial surfaces with the surrounding natural substrate. It may be expected that the artificial surface may produce a different thigmotactic response when compared to the natural substrate, which may limit the use of this technique for assessment of natural settlement rates. In this study the settlement rates of megalopae of the estuarine crab Carcinus maenas were addressed, specifically deploying artificial benthic collectors in different habitats both intertidal and subtidal in the lower Mira estuary. A number of experiments were performed concerning stratification and temporal fluctuations of settlement. Further, the interaction of collector surface with the surrounding substrate was investigated, by comparing settlement rates in natural and artificial substrates in different habitats. Results have shown significant differences in settlement between different estuarine habitats, both in spatially replicated experiments and in a high-resolution temporal experiment. However, comparison between settlement rates in artificial and natural substrates has shown that there is a strong interference between collectors and surrounding substrate, limiting interpretation of results concerning settlement rates in artificial substrate alone.

External factors determining seasonal breeding in a subtropical population of the shore crab Pachygrapsus transversus (Gibbes, 1850) (Brachyura, Grapsidae)

Summary Monthly samples of the shore crab Pachygrapsus transversus from two distinct annual periods showed that ovigerous females are present throughout the year in the population. However, the relative ovigerous abundance of specimens among sexually mature changes from less than 10% during the winter months to almost 80% in summer. Linear correlations for each year revealed that both temperature and photoperiod were positively associated with relative abundance of ovigerous females. Multiple regression analyses suggested that photoperiod was the main factor affecting breeding in this species. Timing of observed reproductive pattern may enhance larval survival because of particular oceanographic conditions in the study region and favor early juvenile development due to certain species-specific growth features.

Stratified settlement and moulting competency of brachyuran megalopae in Ponta Rasa mangrove swamp, Inhaca Island (Mozambique)

Information on recruitment processes of mangrove crustaceans is very limited, in spite of the great importance of these environments for the coastal zone. This study describes the settlement patterns of brachyuran crabs at Ponta Rasa mangrove swamp, Inhaca Island, in order to assess if settlement patterns reflect adult distribution. Hoghair filter collectors were deployed at different strata within the mangrove, and at the adjacent intertidal flat as control. Sampling was conducted daily for a period of 82 days. The collected megalopae were reared in the laboratory to assess their moulting competency and to enable identification. Settlement intensity was tested for association with wind stress, from different directions and averaged over time lags up to 6 days. A total of 960 megalopae settled during this study. Only 8% of the megalopae that settled at the control site belonged to mangrove dwelling species. Settlers inside Ponta Rasa swamp were exclusively mangrove species. Ilyograpsus paludicola settled mainly among Rhizophora mucronata and the mixed area of Ceriops tagal and Bruguiera gymnorrhiza, where adults occur, and Perisesarma guttatum, the most abundant sesarmid at Ponta Rasa, settled mainly at the creek. Neosarmatium meinerti settled among Avicennia marina pnematophores, very close to the adult populations. Megalopae of mangrove taxa that settled outside the mangrove took longer to moult than the ones settling inside the swamp. In the intertidal areas of the mangrove, settlers took an average of 1–2 days to moult, whereas in subtidal areas time to moult was on an average 3–5 days. Thus, both distribution of settlers and moulting competency suggest that settling follows adult distribution. Tides have a strong influence on settlement at Ponta Rasa, with the effect that, due to the high sill at the entrance, neap tides do not penetrate the mangrove. The analysis of settlement data suggested a significant effect of wind-driven transport on onshore migration. There is however no obvious interpretation for the pattern observed. Probably, the distribution of different larval patches and the complex dynamic structure of currents within Maputo Bay and adjacent areas influence the settlement pattern.

Camouflage through colour change: mechanisms, adaptive value and ecological significance

Animals from a wide range of taxonomic groups are capable of colour change, of which camouflage is one of the main functions. A considerable amount of past work on this subject has investigated species capable of extremely rapid colour change (in seconds). However, relatively slow colour change (over hours, days, weeks and months), as well as changes arising via developmental plasticity are probably more common than rapid changes, yet less studied. We discuss three key areas of colour change and camouflage. First, we review the mechanisms underpinning colour change and developmental plasticity for camouflage, including cellular processes, visual feedback, hormonal control and dietary factors. Second, we discuss the adaptive value of colour change for camouflage, including the use of different camouflage types. Third, we discuss the evolutionary–ecological implications of colour change for concealment, including what it can tell us about intraspecific colour diversity, morph-specific strategies, and matching to different environments and microhabitats. Throughout, we discuss key unresolved questions and present directions for future work, and highlight how colour change facilitates camouflage among habitats and arises when animals are faced with environmental changes occurring over a range of spatial and temporal scales. This article is part of the themed issue ‘Animal coloration: production, perception, function and application’.

POSTLARVAL STAGES AND GROWTH PATTERNS OF THE SPIDER CRAB PYROMAIA TUBERCULATA (BRACHYURA, MAJIDAE) FROM LABORATORY-REARED MATERIAL

Abstract The spider crab Pyromaia tuberculata was introduced into southeastern Brazil; ovigerous material was collected and reared in the laboratory. Morphologic changes and growth patterns of postlarval development are reported. Results show that within-stage size variation is lowest in mature stages, especially in the case of females in which there is an apparent size threshold for the last juvenile stages to undergo the puberty molt. A prepuberty molt taking place at the fourth crab stage is indicated by analyzing the allometric growth of the abdomen in females. In contrast, the same procedure using the allometric growth of chelae failed in detecting both the prepuberty and puberty molts in males. Conversely to females, which develop a complex brood chamber at the puberty molt, the enlargement of chelae was not consistent in all postpuberty males. The short instar sequence of this species, in no case exceeding nine stages, is marked by conspicuous morphologic alterations achieved at each molt. Almost all stages can be identified by examining diagnostic features of rostrum, abdomen, sternum, and pleopods.