Paulina Gebauer

Costs of delayed metamorphosis : reduced growth and survival in early juveniles of an estuarine grapsid crab, Chasmagnathus granulata

When the megalopa stage of estuarine crab species becomes competent for settlement and metamorphosis, it responds to specific chemical and physical cues from the adult environment. Delayed metamorphosis in the absence of such cues is beneficial insofar as it increases the probability of finding a suitable habitat, and it may also enhance the genetic exchange between separate populations. However, this developmental and behavioural response may incur energetic costs reducing the fitness of later life-history stages. In a laboratory investigation, we studied postmetamorphic consequences of delayed metamorphosis for growth and survival in early juvenile instars (I through V) of an estuarine grapsid crab, Chasmagnathus granulata (Dana, 1851). In competent megalopa larvae of this species, metamorphosis is induced by chemical cues from muddy substrates and conspecific adults (control treatment). After delayed metamorphosis in the absence of these cues (experimental treatment), survival and body size were significantly reduced in the first crab stage, and the duration of development to the second juvenile instar was significantly longer. Survival, moult-cycle duration, and percentage growth increments were not significantly affected in later juvenile instars. However, as a consequence of the initial reduction in development and growth, the crabs from the experimental treatment remained consistently smaller and moulted later to successive instars than in the control group. Our results indicate that delayed metamorphosis is associated with a reduced postmetamorphic fitness in an estuarine crab species.

Delayed metamorphosis in decapod crustaceans: evidence and consequences

Most marine invertebrate species exhibit a complex life cycle including a planktonic larval phase and a benthic juvenile-adult phase. Metamorphosis and settlement are the links between these phases of development. In many species, metamorphosis is triggered by specific chemical and/or physical cues, mainly associated with the adult habitat. In the absence of such cues. competent larvae can delay their metamorphosis by a few days to several months. Most investigations on the delay of metamorphosis have been realised on sessile or sedentary species. In relation to mobile decapod crustaceans. the number of such studies is low, probably because the members of this group retain their mobility after metamorphosis. and hence, may depend less on environmental cues for the induction of settlement and metamorphosis. Nevertheless. the larvae of some decapod species have been shown to depend on metamorphosis-stimulating cues. These include special types of substrates, physical or chemical traits of particular (e.g., estuarine) water bodies, as well as odors from conspecific or congeneric adults. The capacity for delay is, in the decapod species studied so far, limited and may normally end with spontaneous metamorphosis. An extended time of larval development presents the advantage of enhancing the probability for locating a suitable habitat, but it may imply, as a disadvantage, a reduction of juvenile growth or survival and a prolonged development time preceding benthic life. This paper reviews the available evidence for delayed metamorphosis in decapod crustaceans, indentifed cues, the importance of larval age at the time of contact with a cue, and costs of delayed metamorphosis. Additionally, we propose new frontiers for future investigations on delayed metamorphosis in decapod crustaceans, including the molecular identification of chemical cues, the identification of the stage(s) of the moulting cycle that is or are sensitive to such cues, the study of hormonally mediated effects on the moulting cycle, the quantification of energetic or other costs of delayed metamorphosis, and the analysis of relationships between the effectiveness of adult odors and phylogenetic proximity of larvae and adults.

Effects of substratum and conspecific adults on the metamorphosis of Chasmagnathus granulata (Dana) (Decapoda: Grapsidae) megalopae

The ability of marine invertebrate larvae to delay their metamorphosis in the absence of adequate environmental cues has been reported for numerous sedentary and sessile species. In the present study, the effect of various substrata and the presence of conspecific adults on the metamorphosis of a mobile species, the crab Chasmagnathus granulata, was evaluated. The duration of the megalopa stage in experiments with six different substrata and in the presence or absence of conspecific adults was compared in a laboratory study. In addition, the influence of natural substrata was compared with that of artificial substrata of similar grain size or texture. In a further experiment, the two most effective cues (natural mud and conspecific adults) were tested as single vs. combined factors. Natural mud and unidentified chemical cues from conspecific adults had the strongest accelerating effects on development duration to metamorphosis. With the exception of nylon threads (artificial filamentous substratum), none of the artificial substrata had a significant effect on the duration of the megalopa stage. Simultaneous exposure to natural mud and water containing chemical cues from conspecific adults accelerated metamorphosis more than each of these factors separately. Megalopae that were reared without a substratum (control) delayed their metamorphosis by 29% (about 3 days) compared with those in simultaneous contact with natural mud and rearing water of adult conspecifics. The results indicate that the metamorphosis of the megalopa of C. granulata is influenced by the presence or absence of environmental stimuli that are associated with the preferred adult habitat.

Metamorphosis in a semiterrestrial crab, Sesarma curacaoense: intra- and interspecific settlement cues from adult odors

The larvae of many marine invertebrate species are able to delay their settlement and metamorphosis in the absence of characteristic cues from the adult habitat. This phenomenon was experimentally studied in the megalopa stage of Sesarma curacaoense de Man, 1892, a semiterrestrial grapsid crab that lives in the shallow coastal mangrove habitats in the Caribbean region. Duration of the development and survival to metamorphosis to the first juvenile crab stage were compared between experimental treatments, where the water was conditioned with adult crabs (“adult-conditioned water,” ACW) and control groups reared in filtered seawater. In the experiments with larvae from two different females, development duration was significantly shorter and mortality lower in water conditioned with conspecific adults. In the two control groups, the effects of supply with an artificial substrate (nylon gauze) were tested. This comparison showed that the presence of substrate did not significantly influence the time to metamorphosis, but did reduce the mortality rate. In all later experiments, the megalopae were thus routinely provided with nylon gauze as a substrate. In each of the three subsequent replicate experiments conducted with larvae from different females, survival rate and development time to metamorphosis were compared between one control group and four treatments with ACW. The effectiveness of conspecific (S. curacaoense) adult odors as metamorphosis-stimulating cue was, in these experiments, compared with that of ACW from one congener (S. rectum) and two species belonging to different genera within the Grapsidae (Armases miersii, Chasmagnathus granulata). While the rate of survival showed inconsistent patterns among repeated experiments, the development was consistently fastest with conspecific ACW, followed by ACW from S. rectum, A. miersii and C. granulata. Only the conspecific and congeneric cues had statistically significant effects (i.e. shorter development than in the controls). These response patterns suggest that chemically similar factors (presumably pheromones) are produced by closely related species and, thus, their chemical structure may reflect phylogenetical relationships within a clade.

Experimental validation of the growth rings of Loxechinus albus (Molina, 1782) in southern Chile (Echinodermata: Echinoidea)

Abstract To validate the periodic formation of growth rings in the genital plates of the Chilean edible sea urchin Loxechinus albus (Molina, 1782), an experimental sea urchin population grouped in five size classes (less than 15.0 mm, 20.0–29.9 mm, 50.0–59.9 mm, 70.0–79.9 mm and more than 100.0 mm) was confined to intertidal pools of the Marine Reserve of Mehuin. Size and weight were controlled monthly, from March 1991 to March 1992. At the same time, 50 sea urchins were collected each month from the non-manipulated population, involving the entire range of size distribution present in the study area. From these specimens, the genital plates were taken for ring reading, edge analysis and marginal increase analysis. The data were fitted to the individual growth model of Von Bertalanffy by means of a non-linear method (Marquardts Algorithm). Asymptotic test diameter (TD∞) ranged between 141.2 and 131.7 mm, K (intercept on the TD)-axis) between 0.164 and 0.104 and t0 (time at age 0; settlement on botttom) between −0.23 and −0.77. In the case of the growth equation for weight, asymptotic weight (W∞) ranged between 1207.3 and 740.1 g, and the relationship between test diameter and wet weight was W = 0.00054(TD)2.94. The regression analysis used for estimating the growth parameters showed no statistically significant differences (95%) between the manipulated and the non-manipulated populations (rings at present and past ages). Moreover, all the growth parameters overlap at the 95% confidence interval obtained from the non-linear fit, showing no statistical differences. On the other hand, a significant relationship was found between the number of plates of a single column of the coronal system and age, which could be useful for the identification of this important demographic parameter in non-destructive field studies.

Physiological responses of the southern king crab, Lithodes santolla (Decapoda: Lithodidae), to aerial exposure

The recent demand for live seafood has made Lithodes santolla a potential candidate for the live crab market. Since live transport implies long aerial exposure times, the present study determined the tolerance of L. santolla to aerial exposure and also explored the physiological status of L. santolla after six different aerial exposure times: 0, 10, 20, 30, 40 and 50h. No mortalities were recorded during emersion periods shorter than 40 h, however, all hemolymph parameters were modulated by aerial exposure. Hemolymph dissolved oxygen and pH were inversely related with the aerial exposure time (-0.016 mg L(-1) h(-1) and -0.018 h(-1), respectively), while oxyhemocyanin and hemolymph protein were positively correlated (0.006 mmol L(-1) h(-1) and 0.487 mg mL(-1) h(-1), respectively). Oxygen consumption at re-immersion was affected by aerial exposure time, with low values in animals emersed for longer than 30 h. We postulate that the unexpected high tolerance of L. santolla to aerial exposure is mainly related to adaptations to hypoxia, developed to overcome hypoxic events in their natural environment in deep water. The present findings are not only important for understanding L. santollas physiology, but also provide the first evaluation of the potential marketing of southern king crab as live seafood.

Physiological energetics of the estuarine crab Hemigrapsus crenulatus (Crustacea: Decapoda: Varunidae): responses to different salinity levels

Hemigrapsus crenulatus is an abundant and frequent decapod crustacean inhabiting estuarine environments, where it must tolerate large shifts in salinity. The present study evaluates the effect of salinity (5, 13, 21 and 30 psu) on the adult physiological processes related to the energy balance. The growth potential (SFG) and the respired oxygen:excreted nitrogen ratio were used as indices of stress. Ingestion, excretion and respiration rates showed a significant dependence on salinity, being higher at low salinities. The assimilation efficiency remained constant along the studied salinity gradient. The assimilation and ingestion rates were inversely related with the salinity. Given this scenario, the growth potential remained constant within the studied salinity gradient, as did the oxygen:nitrogen ratio. The results suggest that the increased energy losses at low salinity due to respiration and excretion are compensated by an increment in the ingestion rate, contributing to the success of H. crenulatus in dynamic habitats such as estuaries.

Reproductive biology and population parameters of Petrolisthes laevigatus (Anomura: Porcellanidae) in southern Chile: consequences on recruitment

The present study describes the reproductive cycle and population structure of the intertidal crab Petrolisthes laevigatus, an abundant inhabitant of the upper rocky intertidal zone, as well as the relationship between seasonal patterns in the release of its planktonic larvae and recruiting. A total of 22 900 individuals was collected along the coast of southern Chile between May 2001 and March 2003. The structure and density of the adult population (males and females) was relatively constant throughout the study period, although seasonal variations were observed in these two parameters of the benthic population (males, females, juveniles and megalopae), principally due to the incorporation of new individuals (recruitment) into the intertidal zone. Petrolisthes laevigatus presented a prolonged reproductive period. Females with initial eggs were present from the end of summer through to the middle of the following summer (11 months) and the breeding season lasted seven months (from August to February), as re£ected in the six months of intertidal settlement (October^ March). The interannual patterns and variations observed in the settlement along the southern Chilean coastline were related to the patterns in the abundance of potential released planktonic larvae. Petrolisthes laevigatus, unlike most temperate species, has a broad reproductive and settlement period in the south of Chile, which allows a highly stable structure and population abundance in this intertidal zone.

Thermal biology of the sub-polar-temperate estuarine crab Hemigrapsus crenulatus (Crustacea: Decapoda: Varunidae)

ABSTRACT Optimum temperatures can be measured through aerobic scope, preferred temperatures or growth. A complete thermal window, including optimum, transition (Pejus) and critical temperatures (CT), can be described if preferred temperatures and CT are defined. The crustacean Hemigrapsus crenulatus was used as a model species to evaluate the effect of acclimation temperature on: (i) thermal preference and width of thermal window, (ii) respiratory metabolism, and (iii) haemolymph proteins. Dependant on acclimation temperature, preferred temperature was between 11.8°C and 25.2°C while CT was found between a minimum of 2.7°C (CTmin) and a maximum of 35.9°C (CTmax). These data and data from tropical and temperate crustaceans were compared to examine the association between environmental temperature and thermal tolerance. Temperate species have a CTmax limit around 35°C that corresponded with the low CTmax limit of tropical species (34–36°C). Tropical species showed a CTmin limit around 9°C similar to the maximum CTmin of temperate species (5–6°C). The maximum CTmin of deep sea species that occur in cold environments (2.5°C) matched the low CTmin values (3.2°C) of temperate species. Results also indicate that the energy required to activate the enzyme complex (Ei) involved in respiratory metabolism of ectotherms changes along the latitudinal gradient of temperature. Summary: The preferred temperature of the intertidal temperate crab Hemigrapsus crenulatus was determined to be between 12°C and 25°C. Oceanic warming would trap this species between tropical and polar temperatures, causing increasing population stress with unknown ecological consequences.

Physiological responses of the ghost shrimp Neotrypaea uncinata (Milne Edwards 1837) (Decapoda: Thalassinidea) to oxygen availability and recovery after severe environmental hypoxia

Hypoxia is a common and widespread phenomenon in aquatic ecosystems, imposing a significant challenge for the animals that inhabit such waters. In different habitats, however, the characteristics of these hypoxic events may differ, therefore imposing different challenges. We investigated the tolerance of adult ghost shrimp Neotrypaea uncinata (an intertidal mudflat dweller) to different partial pressures of oxygen (pO2), severe hypoxia (2 kPa) and recovery from hypoxia after different exposure times, mimicking the natural tidal cycle (6 h and 12 h). We calculated critical oxygen tension and categorize the adult ghost shrimps as oxyregulators (R value=75.27%). All physiological measurements (metabolic rate, oxyhemocyanin, hemolymph protein and lactate concentrations) were affected by exposure to low partial pressures of oxygen, but most of them recovered (with exception of metabolic rate) control values (21 kPa) after 6h under normoxic conditions. Low metabolic rate, high release of hemolymphatic proteins and anaerobic metabolism are suggested as response mechanisms to overcome hypoxic events during low tide.