José Paula

A continental scale evaluation of the role of limpet grazing on rocky shores

It is critical for our knowledge of biodiversity and ecosystem processes to understand how individual species contribute to ecosystem processes and how these contributions vary in space and time. We used a manipulative field experiment in five locations over 17° of latitude [from southern Portugal to the Isle of Man (British Isles)] to determine the relative response of rocky intertidal algal assemblages released from control by the grazing of limpets. Response ratios showed that when limpets were removed there was a trend of effects from north to south. In the north, grazing had a strong effect on algal assemblages, but removing grazers reduced spatial variability in assemblages. In the south, the effect of limpet grazing was far weaker and removal of grazers had a much reduced impact on spatial variability. Here we show a clear trophic control of an ecosystem in that grazing by limpets not only determines macroalgal abundance overall but also modifies ecosystem stability via variability in cover of algae.

A comparison of alternative methods for estimating population density of the fiddler crab Uca annulipes at Saco Mangrove, Inhaca Island (Mozambique)

Visual counts of surface-active crabs both by binocular and burrow counting methods have been used in many studies to estimate population density. However, their reliability has not yet been assessed comparatively. Three methods for estimating the abundance of fiddler crabs Uca annulipes in a mangrove forest (Inhaca Island, Mozambique) were compared from three different sub-areas: two sub-areas inundated only during spring tides and one sub-area inundated in both spring and neap tides. Burrow, binocular and direct (excavation) counting methods were performed by plotting ten 0.25 m2 quadrats in each sub-area over the four moon phases. Overall densities (per 0.25 m2) differed according to method, sub-area and lunar phase. Burrow count overestimated crab density by up to 20%, while binocular count underestimated density by up to 41%. Correlation coefficient estimated for both counting methods showed that burrow count gives better density estimates than binocular count (0.91 and 0.56, respectively). Sex ratios were also investigated within the three sub-areas and at the moon phases. Males are dominant throughout the studied period except during new moon and first quarter, indicating that when the number of gravid females is low, sex ratio bias for binocular count is minimal.

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.

Are fiddler crabs potentially useful ecosystem engineers in mangrove wastewater wetlands

The effect of different organic-rich sewage concentration (0%, 20% and 60% diluted in seawater) and absence or presence of mangrove trees on the survival, bioturbation activities and burrow morphology of fiddler crabs species was assessed. After 6 months, males of both species always showed higher survival ( approximately 80%) when compared to females ( approximately 20%). Crabs inhabiting pristine conditions achieved higher survival (67-87%) than those living in sewage-exposed mesocosms (40-71%). At 60% sewage loading, fiddler crabs processed less sediment (34-46%) during feeding and excavated slightly more sediment (45-80%) than at pristine conditions. While percent volume of the burrow chambers increased (13-66%) at contaminated mesocosms for both vegetation conditions, burrows were shallower (approximately 33%) in bare cells loaded with sewage. The results show that fiddler crabs presented moderate mortality levels in these artificial mangrove wetlands, but mainly in sewage impacted cells. However, they still function as ecosystem engineers through bioturbation activities and burrow construction.

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.

Rhythms of larval release of decapod crustaceans in the Mira Estuary, Portugal

Rhythms of larval release of the most abundant decapod crustaceans in the Mira Estuary, Portugal, were observed based on plankton samples made over consecutive ebbing tides during one lunar cycle in June 1986. Most species showed a semi-lunar rhythm of larval release, centered on crepuscular high tides around the quarter moons. The study suggests that the larval-releasing activity is connected to the hour of the high tide rather than to tidal amplitude.

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.

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.

Early-life exposure to climate change impairs tropical shark survival

Sharks are one of the most threatened groups of marine animals worldwide, mostly owing to overfishing and habitat degradation/loss. Although these cartilaginous fish have evolved to fill many ecological niches across a wide range of habitats, they have limited capability to rapidly adapt to human-induced changes in their environments. Contrary to global warming, ocean acidification was not considered as a direct climate-related threat to sharks. Here we show, for the first time, that an early ontogenetic acclimation process of a tropical shark (Chiloscyllium punctatum) to the projected scenarios of ocean acidification (ΔpH = 0.5) and warming (+4°C; 30°C) for 2100 elicited significant impairments on juvenile shark condition and survival. The mortality of shark embryos at the present-day thermal scenarios was 0% both at normocapnic and hypercapnic conditions. Yet routine metabolic rates (RMRs) were significantly affected by temperature, pH and embryonic stage. Immediately after hatching, the Fulton condition of juvenile bamboo sharks was significantly different in individuals that experienced future warming and hypercapnia; 30 days after hatching, survival rapidly declined in individuals experiencing both ocean warming and acidification (up to 44%). The RMR of juvenile sharks was also significantly affected by temperature and pH. The impact of low pH on ventilation rates was significant only under the higher thermal scenario. This study highlights the need of experimental-based risk assessments of sharks to climate change. In other words, it is critical to directly assess risk and vulnerability of sharks to ocean acidification and warming, and such effort can ultimately help managers and policy-makers to take proactive measures targeting most endangered species.

Monitoring anthropogenic sewage pollution on mangrove creeks in southern Mozambique: A test of Palaemon concinnus Dana, 1852 (Palaemonidae) as a biological indicator

Tropical coastal ecosystems, such as mangroves, have a great ecological and socioeconomic importance for adjacent systems and local populations, but intensive environmental impact monitoring is still lacking, mainly in East Africa. This study evaluated the potential anthropogenic disturbance on Palaemon concinnus population structure and fitness. Palaemon concinnus populations from one peri-urban (domestic sewage impacted) and two pristine mangrove creeks were studied by sampling nearly 100 shrimps per location every 15 days for 12 months. The shrimps at the peri-urban location were larger, experienced longer reproductive periods, presented higher proportion of ovigerous females and better embryo quality when compared with shrimps inhabiting pristine locations. Physiological indices (RNA/DNA ratio) were similar between shrimps at pristine and peri-urban mangroves. However, a higher level of parasitation by a Bopyridae isopod, Pseudione elongata indicated some degree of stress on the host at the peri-urban mangrove, with potential effects on the host population dynamics.