Recruitment to adult habitats following marine planktonic development in the fiddler crabs, Uca pugilator, U. pugnax, and U. minax

Three congeneric species of fiddler crabs, Uca pugilator (Bosc, 1802), U. pugnax (Smith, 1870), and U. minax (LeConte, 1855), co-occur in estuaries along the east coast of North America, from Cape Cod to northern Florida. Although U. minax adults are generally found at lower salinities than the other two species, the distributions of all three species overlap to some degree. The distribution of megalopae and juvenile fiddler crabs (from first crab stage to those with a carapace width of 3.0 mm) was examined at four sites along a salinity gradient (from 35.0±2.0 to 3.0±3.0; ) in the North Inlet Estuary, South Carolina, USA, in August 2002. A PCR-RFLP technique was developed to identify individuals from the genus Uca to species from first zoea through the early crab stages. An examination of the distribution of early life stages showed that U. minax reinvades low-salinity adult habitats at settlement, following planktonic larval development in the coastal ocean. Also, juveniles of U. pugilator were found to occupy Spartina alterniflora stands, where adult conspecifics rarely occur. Species frequencies were different for adults compared to early life stages in low-salinity areas of the marsh, where populations overlap. Settlement and survival dynamics of early life-history stages in wet and dry years likely determine the distribution of adult Uca spp. populations along salinity gradients in estuarine ecosystems.Communicated by J.P. Grassle, New Brunswick     

Ecdysial rhythms in megalopae and first instars of the Dungeness crab Cancer magister

Diel molting cycles of megalopae and first instar Dungeness crabs Cancer magister Dana captured in the Grays Harbor estuary (46° 55N; 124° 05 W) in May 1991 were studied under laboratory conditions. Sixtyone percent of the megalopae and 76% of the first instar crabs molted during periods of ambient darkness under a normal light — dark diel regime, and molting pattern was not affected by changes in the photoperiod (24 h daylight or 24 h darkness). Time until metamorphic molt increased as conspecific density increased. Habitat type (shell or mud) did not affect time until molt of megalopae and first instar crabs, nor did it affect daily molting rhythm of first instars. We hypothesize that nightly ecdysis of megalopae and first instar Dungeness crabs and density-dependent molting may be an adaptive response to predation and cannibalism among young-of-the-year.Contribution No. 875 from the School of Fisheries, WH-10, University of Washington, Seattle, Washington     

Habitat selectivity of megalopae and juvenile mud crabs (<Emphasis Type="Italic">Scylla serrata</Emphasis>): implications for recruitment mechanism

Megalopae of several crab species exhibit active habitat selection when settling. These megalopae usually select structurally complex habitats which can provide refuge and food. The portunid mud crab, Scylla serrata, is commonly found within the muddy estuaries of the Indo-West Pacific after attaining a carapace width >40 mm. Despite substantial efforts, the recruitment mechanism of juvenile mud crabs to estuaries is not understood because their megalopae and early stage crablets (carapace width <30 mm) are rarely found. We used laboratory experiments to determine whether megalopae and early stage crablets are selective among three estuarine habitats which commonly occur in Queensland, Australia. These animals were placed in arenas where they had a choice of habitats: seagrass, mud or sand, and arenas where they had no choice. Contrary to the associations exhibited by other portunid crab megalopae, S. serrata megalopae were not selective among these estuarine habitats, suggesting that they tend not to encounter these habitats, or, gain no advantage by selecting one over the others. The crablets, however, strongly selected seagrass, suggesting that residing within seagrass is beneficial to the crablets and likely increases survival. This supports the model that for S. serrata, crablets and not megalopae tend to colonise estuaries, since a selective behaviour has evolved within crablets but not megalopae.     

Osmoregulation in fiddler crabs (<Emphasis Type="Italic">Uca</Emphasis>) from temperate Atlantic and Gulf of Mexico coasts of North America

Fiddler crabs (Crustacea: Decapoda; genus Uca) were collected from 16 locations along the temperate Atlantic and Gulf of Mexico coasts of eastern North America for osmoregulation studies. Three species, U. pugilator, U. pugnax and U. minax, were taken from habitats between Cape Cod, MA, and St. Mary's River, Nassau Co., GA Seven species, U. panacea, U. speciosa, U. spinicarpa, U. longisignalis, U. rapax, U. pugilator and U. minax, were collected in habitats between the Ochlochonee River, Liberty Co., and Thompson's Bayou, Escambia Co., along the Gulf coast of northwest Florida. To examine differences in osmoregulatory capabilities among the species, specimens were placed in various concentrations of artificial seawater (0-3450 mOsm) for 5 days. The oligohaline or "freshwater" species, U. minax, U. spinicarpa and U. longisignalis, possess the lowest average hemolymph osmolality. They are unable to control hemolymph osmolality above 2000 mOsm. On the other hand, the euryhaline species, U. speciosa, U. panacea, U. pugilator and U. pugnax, have much higher average hemolymph osmolality. They are able to withstand an osmotic challenge of 2200 mOsm or greater. Among the eight species, U. panacea and U. pugilator are able to osmoregulate across the broadest range of seawater concentrations (0-2800 mOsm). After an examination of the osmoregulation in several populations of U. minax, U. pugilator and U. pugnax, physiological adaptation is apparent in the two former species, but not the latter. Experimental evidence for capacity adaptation was examined in the freshwater species, U. minax. If slowly adapted to hypertonic seawater (1800 mOsm) for 14 days, this species is able to withstand much higher osmotic pressures than unadapted crabs. Although hemolymph isosmotic concentration remains the same, the limits for regulation are extended to higher osmolality. Consequently, as a result of studying osmoregulation in several species of Uca captured at various locations across their geographic range in the temperate zone, a clearer view of water-balance physiology is available for the genus. The Uca spp. are not uniform in osmoregulatory abilities. There is considerable inter- and intraspecific physiological variation associated with the ecological distribution of each species, respectively.     

Responses of estuarine crab megalopae to pressure,salinity and light: Implications for flood-tide transport

The megalopal larval stage of many estuarine brachyuran crabs appears to return to adult habitats by undergoing rhythmic vertical migrations which result in saltatory up-estuary transport on flood tides. Larval ascent into the water column during rising tides may be cued by changing hydrologic variables. To test this hypothesis, we investigated the responses of field-caught megalopae of the blue crab Callinectes sapidus and the fiddler crab Uca spp. to constant rates of pressure and salinity change under laboratory conditions. For both genera, pressure changes resulted in increased movement (barokinesis) and upward migration in the test chamber, with C. sapidus megalopae having a lower response threshold (2.8×10^-2 mbar s^-1) than Uca spp. larvae (5×10^-2 mbar s^-1). Similarly, larvae ascended in response to increasing salinity, with C. sapidus larvae being more sensitive. Larvae were negatively phototactic and failed to respond to pressure increases at light levels above 1.0×10¹⁵ and 1.0×10¹³ photons m^-2 s^-1 for C. sapidus and Uca spp. megalopae, respectively. Such responses are thought to explain the low abundances of larvae in the water column during daytime flood tides. Nevertheless, threshold sensitivities to increasing pressure for both genera were above levels experienced during floodtide conditions in the field. Similarly, it is unlikely that increasing salinity is sufficient to induce ascent in Uca spp. postlarvae. However, rates of salinity increase during midflood tide typically reach levels necessary to induce an ascent in C. sapidus megalopae. These results are consistent with the hypothesis that fiddler crab megalopae utilize an endogenous activity rhythm for flood-tide transport, while blue crab megalopae rely upon external cues, especially salinity changes, to time their sojourns in the water column.     

Endogenous swimming rhythms in estuarine crab megalopae: implications for flood-tide transport

Up-estuary migration of crab larvae to adult habitats is thought to be accomplished by selective tidal transport in which late-stage larvae enter the water column on flood tides and remain on or near the bottom on ebb tides. This study measured endogenous rhythms in swimming by the last larval stage (megalopa) of blue crabs Callinectes sapidus and fiddler crabs Uca spp. Previous field studies found that megalopae of both species were only abundant in the estuarine water column on nocturnal rising tides. Megalopae were collected from the Newport River Estuary, North Carolina (34°41N; 76°40W) during August–September 1992 and swimming activity was recorded for 4.5 to 7 d under constant conditions with a video system. Rhythms exhibited by both genera in the laboratory were not identical to those recorded in the field. Uca spp. displayed a circatidal rhythm, with maximum swimming occurring near the time of high tide in the field. Rhythm amplitude increased when crushed oyster shells were present, which suggested that megalopae bury or cling to the substrate during quiescent periods. In contrast, C. sapidus had a circadian rhythm in which maximum swimming coincided with the day phase in the field. In most trials, the activity of blue crab megalopae was unrelated to the expected tidal cycle. It was concluded that a tidal rhythm in swimming was the behavioral basis of flood-tide transport for fiddler crab larvae. The endogenous rhythm in blue crabs does not participate in transport, which probably results from behavioral responses to environmental cues associated with flood tide.     

Shell choice in<Emphasis Type="Italic"> Pagurus longicarpus</Emphasis> hermit crabs: does predation threat influence shell selection behavior?

Pagurus longicarpus hermit crabs depend on empty gastropod shells for protection against predation. Hermit crabs avoid gastropod shells in which holes have been drilled by naticid gastropods, and hermit crabs forced to occupy drilled shells are more vulnerable to predation by green crabs, Carcinus maenas. In this study, we examined the effect of predator cues on P. longicarpus shell investigation behavior and shell choice. In paired laboratory shell choice trials, we examined hermit crab response to green crab chemical cues. We compared hermit crabs from two sites differing in the percentage of Littorina littorea shells with drill holes. The percentage of time hermit crabs spent occupying intact shells increased significantly in the presence of predator cues. The effect of predator cues on the amount of time hermit crabs spent investigating shells differed between individuals from the two sites. Predator effluent had a marginal effect on the proportion of hermit crabs initially choosing intact shells and within 15 min most hermit crabs in both treatments occupied intact shells due to shell switching. These results indicate that predation cues alter P. longicarpus shell choice behavior favoring intact shells, which provide greater protection. In summary, predation appears to be a key factor influencing hermit crab shell selection behavior.Communicated by T. Czeschlik     

Effects of chemical cues on orientation of blue crab, <Emphasis Type="Italic">Callinectes</Emphasis><Emphasis Type="Italic">sapidus</Emphasis>, megalopae in flow: implications for location of nursery areas

Megalopae (postlarvae) of the blue crab Callinectes sapidus Rathbun use flood-tide transport (FTT) for movement into and up estuaries. Since they settle around the time of slack water at the end of flood tide during FTT, it was predicted that orientation toward primary nursery areas of aquatic vegetation occurs at this time. This study tested the hypotheses that megalopae locate nursery areas by swimming upstream in the presence of chemical odors from potential nursery areas and avoid adverse microhabitats by swimming downstream when predator or adverse environmental odors are present. Megalopae were tested in a flume where they were exposed to the sequence of cues mediating FTT (i.e. 2 psu increase in salinity followed by an increase and a decrease in current speed and turbulence). The flume contained odor water either from the developmental area (offshore water), nursery area vegetation (seagrass, Zostera marina; salt marsh cord grass, Spartina alterniflora), predators (fiddler crab, Uca pugilator; mud crab, Panopeus herbstii; grass shrimp, Palaemonetes pugio), or chemicals associated with adverse environments (ammonium). Vertical positions of premolt and intermolt megalopae were similar in water devoid of estuarine chemical cues (offshore water) and water containing seagrass odor. Upstream swimming behavior (orientation) of intermolt megalopae was also similar in these waters. However, there was an ontogenetic behavioral change, as the proportion of premolt megalopae oriented upstream generally increased as the concentration of seagrass and salt marsh cord grass odor increased and as current speed decreased. Upstream orientation of premolt megalopae in response to seagrass odor decreased significantly (i.e. downstream swimming increased) in the presence of odor from U. pugilator, P. pugio, and ammonium, but not from P. herbstii. Thus, the hypothesis was supported. These results suggest premolt megalopae orient toward nursery areas by swimming upstream in response to odors from aquatic vegetation as current speeds decrease at the end of nocturnal flood tides. Moreover, these results also indicate that megalopae may discriminate among microhabitats and avoid adverse settlement habitat, as orientation toward nursery areas is reversed by predator odors and ammonium.     

Response of mud crab (Panopeus herbstii) megalopae to cues from adult habitat

During the summer of 1995, we investigated the response of mud crab megalopae (Panopeus herbstii Milne-Edwards) to cues associated with adult habitat. Natural rock/shell substratum and the rock-associated seaweed, Fucus vesiculosus L., both induced metamorphosis of competent megalopae; natural sand substratum did not induce metamorphosis. Structural mimics of preferred substrata induced metamorphosis only when covered with natural biofilm. Clean mimics did not induce metamorphosis. Water-soluble exudates from preferred substrata showed weak induction of metamorphosis. Exudates from adult P. herbstii elicited a similar weak response. Exudates from another species of crab and from two fish predators did not induce metamorphosis. We conclude that water-soluble cues associated with major components of preferred adult habitat induce the metamorphic molt of P. herbstii megalopae. The three-dimensional structure of the substratum is not important in induction of metamorphosis, but the biofilm associated with preferred substratum plays a critical role. It is not clear whether the biofilm produces the water-soluble cue or simply provides a tactile stimulus that works in conjunction with the soluble cue.     

Hydrocarbon metabolism in the intertidal fiddler crab Uca pugnax

The fiddler crab Uca pugnax was examined for its ability to metabolize foreign hydrocarbons. The microsomal mixed function oxidase system was identified in U. pugnax tissues using Aldrin epoxidation rates as the assay. Rates were slow: 96 pM Dieldrin per mg microsomal protein per hour in the hepatopancreas, 438 pM mg^-1 h^-1 in the gill, and 228 pM mg^-1 h^-1 in claw-muscle microsomes. Using standard methods, no difference in rates could be detected between crabs living in clean areas and those living in environments highly contaminated with foreign hydrocarbons. In vivo rates of naphthalene oxidation were measured and used to calculate a clearance time for U. pugnax body tissues based on the aromatic hydrocarbon content of crabs collected from an oil-polluted salt marsh. Calculated clearance time was beyond the life span of the crab. It is concluded that this minimal ability of U. pugnax to metabolize foreign hydrocarbons partially accounts for its sensitivity to oil pollution in the environment.     

Past predation risk induces an intertidal whelk (Nucella lamellosa) to respond to more dilute concentrations of its predator’s scent

Unless sensory control is coupled with developmental flexibility, organisms are unable to produce adaptive phenotypes in response to environmental risk cues. However, empirical examples of adaptive phenotypic plasticity focus largely on development and rarely on sensory control. Here, I ask whether past exposure to predation cues affects an intertidal whelk’s (Nucella lamellosa) sensitivity to the scent of a known predatory crab (Cancer productus). I pre-conditioned whelks for 30 days with the smell of crabs eating other whelks and then, after a period of rest (14 days in fresh seawater free of predator effluent), re-introduced them to different concentrations of waterborne crab scent. Relative to controls, pre-conditioned whelks expressed crab-defences at significantly lower concentrations of crab effluent, suggesting the pre-conditioning reduced their risk–response threshold. Results remind us that the full scope of factors controlling adaptive phenotypic plasticity in natural populations remains underappreciated.     

Induced anti-predator responses of the green mussel, Perna viridis (L.), on exposure to the predatory gastropod, Thais clavigera Küster, and the swimming crab, Thalamita danae Stimpson

A laboratory experiment was conducted to test whether anti-predator responses could be induced in the green mussel, Perna viridis, on exposure to waterborne cues from two predators, the muricid gastropod, Thais clavigera, and the portunid crab, Thalamita danae, and from damaged conspecifics. There were five treatments in this study. Aquaria of two treatments housed either Thalamita danae or Thais clavigera. Another three treatments housed only intact mussels, broken and intact mussels, or no mussels. No significant differences were found among these five treatments in final shell weight and tissue dry weight. Induced defensive responses were predator-specific. Experimental mussels exposed to waterborne cues of T. clavigera had a higher percentage increase in shell length, height and width, whereas those exposed to waterborne cues of T. danae had a higher percentage increase in shell width and height. Mussels raised in the presence of crabs developed thicker shell at the umbo and lip margin, whereas mussels raised in the presence of gastropods had thicker shell lip. Predator preference for mussels from the control group and for those pre-exposed to waterborne cues of either of the predators was studied. T. clavigera preferentially selected mussels from the control group, whereas no prey preference was found for T. danae, although shell breaking time for mussels exposed to a crab was longer than that for the control. While the present results should be viewed with caution in the absence of a dietary control, this was the first time that prey handling was shown to be affected by defensive changes in the morphology of mussels.Communicated by T. Ikeda, Hakodate     

Interactive effects of predation pressure and insecticide (temefos) toxicity on populations of the marsh fiddler crab Uca pugnax

The interaction of predation pressure with lethal and sublethal effects of temefos (Abate^®) an organophosphorous insecticide, were studied in field populations of Uca pugnax (S.I. Smith). Changes in fiddler crab population densities were followed in open-marsh temefos-treated and untreated test plots and in treated and untreated plots which were caged over to reduce predation by marsh birds. Temefos significantly reduced the population density of U. pugnax in the open test plots but not in the caged plots. These results indicate that temefos has a primarily sublethal effect on the crabs, the effect becoming lethal only after interaction with avian predation. Evidence from the presence of a time-lag effect in the population decrease, from a calculated predation index, and from laboratory studies reported elsewhere of behavioral alteration by temefos also supports the conclusion that temefos primarily impairs the escape response of U. pugnax: this leads to increased predation and subsequently to a decreased fiddler crab population. Such studies of sublethal effects of toxicants and field studies of interactions of lethal and sublethal effects of such compounds with natural population dynamics of affected species are necessary to evaluate possible effects of toxicants on populations.     

Predator-induced alarm responses in the common periwinkle, Littorina littorea: dependence on season, light conditions, and chemical labelling of predators

Chemically mediated alarm reactions of the common periwinkle, Littorina littorea (L.), were studied in laboratory experiments during two consecutive summers, and one intermediate autumn season. Responses to chemical stimuli were detected as crawl-out responses, i.e. movements of snails out of the water. Snails were exposed to extracts of injured conspecifics, extracts of the mussel Modiolus modiolus (L.), and water conditioned by the predatory crab Carcinus maenas (L.), which had been maintained on different diets. In experiments carried out during the summer, a significantly larger number of snails moved out of the water when exposed to chemical stimuli from injured conspecifics, compared to chemical stimuli from injured mussels or filtered seawater. These results suggest that chemical alarm substances are present in L. littorea. Water conditioned by crabs that had been fed L. littorea released significantly more crawl-out responses compared to water conditioned by crabs that had been kept on a fish diet. When tested in autumn, no significant differences were found in responses to the above-mentioned water samples. Crawl-out responses under different light regimes were also investigated. All series of experiments carried out in the dark evoked a higher number of responses compared to series that took place in light. These findings may indicate an adaptation of snails to night-active predators. In total, the current results suggest that a L. littorea diet may chemically “label” the predator crab with snail alarm substances, and that predator-induced responses of L. littorea are actually responses to conspecific alarm substances released from crabs that have been maintained on a L. littorea diet. The response to the alarm signal, however, appears to be dependent on season and light conditions; some ecological implications of these findings are also discussed. Received: 8 January 1999 / Accepted: 29 March 1999     

Factors affecting the interval between clutches in the hermit crab <Emphasis Type="Italic">Pagurus nigrivittatus</Emphasis>

Interval between reproductive events is an important factor for iteroparous animals because it determines the number of clutches throughout life. This study examined whether female size, clutch size, shell size and prenuptial molting affected the clutch interval in the hermit crab Pagurus nigrivittatus. Precopulatory guarding pairs of P. nigrivittatus were sampled in the field and kept in the laboratory until the female extruded eggs. The clutch interval of each female was assessed as one of two types of relatively “short” and “long” intervals by checking whether the guarded female had eggs and/or egg cases from the preceding brood or not when the guarding pair was collected. The clutch interval was longer in females with prenuptial molting than those without molting and these females usually grew larger at the prenuptial molt. This suggests that female P. nigrivittatus with a long interval might allocate energy into growth at the expense of the number of clutches during the current reproductive season. The allocation to growth is theoretically predicted to decrease with female size. Gastropod shell size is also known to affect the reproductive activity in hermit crabs. However, female size did not significantly affect the clutch interval in P. nigrivittatus, and the effect of gastropod shell size on clutch interval was not consistent with previous empirical studies. These results may be caused by differences in the gastropod species of shell occupied by the females of P. nigrivittatus.     

Flood tide transport of blue crab, Callinectes sapidus, postlarvae: behavioral responses to salinity and turbulence

Blue crab (Callinectes sapidus) postlarvae (megalopae) use flood tide transport to move upstream in estuaries during nocturnal flood tides. The megalopae have a endogenous diel rhythm in activity that is inconsistent with this tidally timed behavior. Thus, it is hypothesized that this behavior is regulated by behavioral changes in response to exogenous cues associated with tidal currents. In a laboratory flow tank, blue crab megalopae were exposed to simultaneous changes in salinity and turbulence to simulate tides in an estuary. On simulated flood tides, megalopae ascended upon exposure to a salinity increase, remained swimming during times of high turbulence, and descended at times of low turbulence. Turbulence stimulated swimming for several hours, approximating the duration of tidal currents in estuaries. Swimming was inhibited by decreasing salinity on simulated ebb tides. These results support a model for regulation of flood tide transport by blue crab megalopae as follows: (1) blue crab megalopae are stimulated to swim into the water column by increasing salinity associated with flood tide; (2) megalopae remain swimming during flood tide in response to high levels of turbulence; (3) megalopae descend at the end of flood tide, when current speed and turbulence decline to low levels; and (4) megalopae are inhibited from swimming on ebb tides by the associated salinity decrease. This is the first model for regulation of flood tide transport in a species lacking a tidal rhythm in activity.     

Energy balance of <Emphasis Type="Italic">Octopus maya</Emphasis> fed crab or an artificial diet

The present study was designed to evaluate the effect of a natural prey (the crab Callinectes sp.) and an artificial diet (pellet with squid paste and offered as a paste) on the survival and assimilation efficiency of subadult octopuses with 486 g of initial live weight. In order to reach this goal, the effects of the type of diet on energetic balance were assessed by recording ingestion rate (C), respiratory rate (R = R routine, R _rout + R apparent heat increment, R _AHI), ammonia production rate (U = U routine, U _rout + U post-prandial, U _PP) and biomass production (P) of Octopus maya during its growing process. Energy lost from faeces (H) was calculated as H=C−(U+R+P) and assimilated energy (As) as R + P. Octopuses fed an artificial diet had almost five times higher ingestion rate compared to that observed in octopuses fed crab. However, growth rate and production (P) were high in octopuses fed crab in comparison to octopuses fed artificial diet. An inverse relation between faeces (H) and type of food was observed, indicating that animals lost 77% of the ingested energy when fed artificial diet and only 5% when fed crab. A higher assimilation and production efficiency were obtained in octopuses fed crab (P/As: 61%) than in animals fed the artificial diet (P/As: −5%). The routine O : N ratio for animals in fasting was 9.1 and 2.3 for octopuses being fed crabs and the artificial diet, respectively. The post-alimentary O : N ratio was 3.6 and 2.2 for animals fed crabs and the artificial diet, respectively. This indicates that animals fed on both diets rely almost exclusively on protein. Based on energy balance data, a value of 472 kJ week⁻¹ kg⁻¹ of live octopus was estimated as the energy needed to obtain a growth rate near 9 g day⁻¹ (2.8% BW day⁻¹) for O. maya subadults. The total crab biomass needed to obtain 1 kg of fed O. maya biomass was calculated. A comparison with other different energy balance measurements made in other octopus species indicates that O. maya and Enteroctopus megalocyathus (Pérez et al. 2006) tend to be more efficient by channelling more ingested energy to biomass production (P = 69.5% of C) than O. vulgaris (P = 23% of C; Petza et al. 2006) or Paraledone charcoti (P = 4% of C; Daly and Peck 2000).     

Elemental composition (C,N, H) and energy in the development of Pagurus bernhardus (Decapoda: Pagurida) megalopa

No differences in development time and mortality were detected between starved and fed laboratory raised megalopa of Pagurus bernhardus. The average time of development in 138 megalopa was determined as 7.3±0.1 (95% CI) days. During megalopa development P. bernhardus loses about 7% in dry weight (DW), 17% in carbon (C), 6% in nitrogen (N) and 17% in hydrogen (H). During development C/N ratio and individual energy content descend about 14 and 22% respectively. Weight specific energy content decreases by 17% in the first 3 d and remains constant at 12.3±0.3 (95% CI) J·(mg DW)^-1 thereafter. About another 25% in individual energy content was lost by molting to crabs. The measured compounds do not follow a steady decrease. The possibility is discussed that a period of low energy cost (about the first half of development) alternates with times of higher energy expenditure mainly based on lipids. A fixed physiological program different from starvation capability is indicated for P. bernhardus megalopae. By comparing megalopae hatched in two different seasons and years reference is given to the variability in growth pattern.     

Y-maze experiments on the chemotactic behaviour of the tetrabranchiate cephalopod<Emphasis Type="Italic"> Nautilus pompilius</Emphasis> (Mollusca)

In contrast to the modern cephalopods, the nautiloids use primarily the chemosensory sense to explore their environment. So far there have been few studies on sexual-selection processes in solitary-living nautiloid cephalopods, but it can be posited that conspecifics are also discovered by odour. In order to determine whether a special area of the rectum, the rectal gland, plays a role in the intraspecific communication processes of Nautilus pompilius, Y-maze experiments were performed. We tested the reaction of juvenile, early-adolescent male and female, and adult male N. pompilius to homogenates of the rectum of male and female conspecifics. As negative controls, homogenates of gills or mantle, or seawater were used. To check the set-up of the experiment, carrion was presented as a positive attractant. We demonstrated that the adult (mature) males significantly preferred the rectum homogenate containing the stimulus of females, whereas the homogenates from males and/or females had no influence on the behaviour of immature animals of both sexes. Our behavioural studies provide evidence that sexually mature male N. pompilius are attracted to females by excretions of the rectum.Communicated by O. Kinne, Oldendorf/Luhe     

Effects of macroalgal structural complexity on nearshore larval and post-larval crab composition

Larval and post-larval crab distribution was surveyed in three different habitats in Kachemak Bay, Alaska to determine temporal and spatial variability. Distribution varied temporally and spatially from June 2005 to September 2006. Nine sites of varying habitat complexity were surveyed monthly using scuba surveys and light traps to measure habitat variables and quantify crab zoeae and megalopae. A total of 10,016 crabs belonging to seven families were identified. Four species comprised the majority (97%) of the total crab assemblages and included Cancer oregonensis, Fabia subquadrata, Telmessus cheiragonus, and Pugettia gracilis. Peak abundances occurred in summer but varied on small temporal scales with species. No single bay-wide variable determined the appearance of all species. Depending on species, appearance may be influenced by seasonality of environmental variables. Spatially, highest abundances occurred in habitats with less structural complexity. Spatial differences in crab abundance may have resulted from variability on large scale physical transport mechanisms and not kelp-mediated flow alterations.