Terminal Molt Research Articles

Growth patterns of the invasive crab Cronius ruber (Brachyura, Portunidae) in the Canary Islands (Central-East Atlantic)

The growth pattern, including the moult process, relative growth, and estimation of von Bertalanffy growth parameters of the invasive non-indigenus species (NIS) Cronius ruber were studied in the Canary Islands. Despite being found in both, native and invaded areas, the life cycle and ecology of this species remain unknown. The study revealed a greater abundance of females than males, with males exhibiting larger carapace sizes and weights than females. Throughout ontogeny, relative growth was observed, with males reaching carapace widths (CW) between 54.5 and 57.8 mm and females reaching 49.9 mm CW. Allometry in the growth of the males’ chelae and females’ abdomen was consistently positive. Cronius ruber was found to be a fast-growing species, reaching the postpubertal growth phase (maturity) between 0.5 and 1 year of age in males and between 0.4 and 0.8 years in females. The intermoult period increased by more than 638 % in males and 443 % in females with age. Individuals undergo 13 moults during the first phase of life (until 30 mm CW) and then reduced to 6 moults until the end of life, concluding with a final terminal moult. The lifespan until the terminal moult is approximately 2.5 years, followed by a survival time over 1.5 years. von Bertalanffy growth parameters estimated using monthly CW-frequency data for males (L∞=97.4 mm CW, k=1.89 years−1, to=0.31 years), and for females (L∞=86.72 mm CW, k=1.91 years−1, to=0.29 years). Similar results were obtained using growth increment data.

Morphometric changes at terminal molt and reproductive capacity in the kelp crab Pugettia ferox Ohtsuchi & Kawamura, 2019 (Decapoda: Brachyura: Majoidea)

Abstract It has been assumed that functional maturity and terminal molt occur simultaneously in majoid crabs. Although this assumption seems to be accurate in females, males of some majoids can mate with females even before their terminal molt. Here we describe the morphometric changes at terminal molt and test the reproductive capacity of pre-terminal molt males in the kelp crab Pugettia feroxOhtsuchi & Kawamura, 2019 (Majoidea, Epialtidae, Epialtinae). We measured the post-pseudorostral carapace length (PCL), claw length (CL), claw height (CH), and pleon (abdomen) width (AW) of crabs to estimate the morphological features distinguishing pre- and post-terminal molt individuals, namely relative claw length (CL/PCL) ≥ 0.660 in males and relative abdominal width (AW/PCL) ≥ 0.487 in females. Males with pre-terminal molt morphological features were observed for behavioral maturity through encounters with mature virgin females under laboratory conditions. More than half of the males copulated with a female and then underwent terminal molt. These females bred clutches, and larvae hatched. These results indicate that pre-terminal molt males of P. ferox can be functionally mature.

Tanner crab at southwestern Kamchatka: fishing, stock condition and specifics of the assessment

Results of the current stage of research on tanner crab at the southwestern coast of Kamchatka, harvested in the Kamchatka-Kuril fishing subzone are presented. The stock dynamics since 2001 is analyzed, with special attention to its assessing and forecasting. Features of the crab biology associated with terminal molting are described. The algorithm for separation of the tanner crab recruits into morphometrically mature and immature males is developed that allows to calculate probability for a particular size group of morphometrically immature males to undergo the terminal molt and join to the commercial stock. This algorithm is applied for the data collected at southwestern Kamchatka since 2010. Two peaks in total male abundance of tanner crab were observed in 2005–2006 and 2020–2022. Landing of this species was banned in the Kamchatka-Kuril fishing subzone in 2019–2020, as the result of the local population abundance drop in the previous two years, but was resumed in 2021 after its full restoration. Possible reasons for underestimation of the crab recruitment are discussed, related both with features of the species biology and methodological problems. The population in the Kamchatka-Kuril fishing subzone is currently the most abundant for tanner crab, and this stock in 2020–2022 has reached the historical peak. The fishing of tanner crab in the last three years is distinguished by high catches and high rate of the total allowable catch (TAC) utilization.

Eyestalk transcriptome and methyl farnesoate titers provide insight into the physiological changes in the male snow crab, Chionoecetes opilio, after its terminal molt

The snow crab, Chionoecetes opilio, is a giant deep-sea brachyuran. While several decapod crustaceans generally continue to molt and grow throughout their lifetime, the snow crab has a fixed number of molts. Adolescent males continue to molt proportionately to their previous size until the terminal molt at which time an allometric increase in chela size occurs and an alteration of behavioral activities occurs, ensuring breeding success. In this study, we investigated the circulating concentrations of methyl farnesoate (an innate juvenile hormone in decapods) (MF) before or after the terminal molt in males. We then conducted eyestalk RNAseq to obtain molecular insight into the regulation of physiological changes after the terminal molt. Our analyses revealed an increase in MF titers after the terminal molt. This MF surge may be caused by suppression of the genes that encode MF-degrading enzymes and mandibular organ-inhibiting hormone that negatively regulates MF biosynthesis. Moreover, our data suggests that behavioral changes after the terminal molt may be driven by the activation of biogenic amine-related pathways. These results are important not only for elucidating the physiological functions of MFs in decapod crustaceans, which are still largely unknown, but also for understanding the reproductive biology of the snow crab.

Reproductive potential of the blue crab spawning stock in Chesapeake Bay across eras and exploitation rates using nemertean worms as biomarkers

Assessments of reproductive potential and spawning history of marine and estuarine crabs are limited due to issues with determination of age and spawning history. The spawning stock of the blue crab Callinectes sapidus in Chesapeake Bay was heavily fished and declined in the 1990s. Management actions were implemented between 2001 and 2008 to reduce spawning stock exploitation and trigger recovery. Whether these actions impacted the demography of female spawners is unknown. We assessed demographics of overwintering mature female crabs in 1992-1996 and 2020-2022 by classifying them as first-year (imminent primiparous) or second-year (primiparous and multiparous) spawners based on presence of mature nemertean worms (Carcinonemertes carcinophila) in their gill chambers. We also investigated organismal and environmental predictors of second-year spawners. We provide the first annual estimates of the proportion of multi-year spawners at the population level. Management actions reduced exploitation rates by 41% after 2008, and the proportion of second-year spawners was greater than in the 1990s. Nonetheless, exploitation rate in a given year did not predict proportion of second-year spawners in the following year. Second-year spawners tended to be smaller females, or females with a high gonadosomatic index or longevity indicators (i.e. fouling by barnacles). High proportions of large, highly fecund, overwintering first-year spawners in the 2020s support the need for enhanced protection of females in spring to allow these females to spawn. Finally, nemertean worms are a useful, easily implemented tool to determine spawning history and age-specific reproductive potential of crabs that undergo a terminal molt prior to spawning.

Large males matter: Low sperm reserves in female snow crab (Chionoecetes opilio) off Newfoundland, Canada

Large, male snow crab (Chionoecetes opilio) off Newfoundland and Labrador were heavily exploited throughout their range from 2015 to 2018 leading to biological concerns about the possibility of sperm limitation within portions of the stock. To address this, primiparous females were sampled from six sites off the island of Newfoundland to gather data on their overall reproductive health (e.g., seminal receptacle load, SL, and clutch weight) at the end of this heavily exploited period. Snow crab females had low SLs in comparison to long-term monitoring at nearby sites in the Northern Gulf of St. Lawrence but were similar to historic observations from the Eastern Bering Sea. Seminal receptacle loads were significantly influenced by the female’s carapace width, time since mating (represented by sampling month), as well as site-specific characteristics of the male population (ratio of most competitive males in the population and male time since terminal molt). Clutch weight was significantly influenced by the female’s carapace width, as well as the average size of sexually mature males and the average time since terminal molt of adult males. Our findings show the potential for sperm limitation to occur in populations depleted of large-male snow crab and by extension for excessive fishing of males to diminish reproductive capacity of the stock. These findings highlight the importance of maintaining conservative exploitation rates for snow crab that protect a substantial proportion of large adult males in order to preserve reproductive capacity.

Evidence for Sperm Limitation in the Louisiana Blue Crab Spawning Stock

AbstractMale‐targeted fishing pressure has the potential to skew population sex ratios and to result in sperm limitation in females, where the quantity or quality of sperm received during mating is insufficient for the female to achieve her full lifetime reproductive potential. Blue crabs Callinectes sapidus are heavily targeted by commercial and recreational fisheries throughout much of their range, with fishing pressure biased toward males. Additionally, blue crabs are particularly susceptible to sperm limitation because mating occurs during a short window following the female’s terminal molt and stored sperm is used to fertilize all future broods. We investigated sperm quantities in recently molted and mated female blue crabs in southeastern Louisiana to assess the potential for sperm limitation in this area. Sperm quantities were an order of magnitude lower than those previously observed in other areas. Brood production modeling indicated that most of the female blue crabs in this region are heavily sperm limited and unable to achieve their lifetime spawning potential. On a population level, this sperm limitation may be partially responsible for recent declines in landings and estimates of abundance. Although insufficient data exist to assess the precise cause of this sperm limitation, these results should be incorporated into future stock assessments and management decisions for the Louisiana blue crab spawning stock.

Factors influencing size-structured models’ ability to estimate natural mortality

Natural mortality (M) is crucially important for stock assessment since it strongly influences estimates of spawning stock biomass, MSY and fishing mortality. Variation in M can occur between sexes, maturity stages and temporally. Estimates of M are confounded with those of catchability, recruitment, and growth. Previous studies, which focused on age-structured population dynamics models, suggest that it would be better to estimate M rather than pre-specify it. Size-structured population dynamics models, used for species that are hard to age such as crabs, have unique characteristics such as a size-transition matrix and the possibility of a terminal molt, which could impact the ability to estimate M. Therefore, this study explores the ability of size-structured stock assessment models to estimate M for several scenarios. These scenarios include whether the operating model and estimation method have a terminal molt, whether M is constant, time-varying, or sex- and maturity stage-specific, the quality/sample size of the biomass index and the size-composition data and whether growth is estimated simultaneously with M. The results show that size-structured models can estimate time-varying, sex- and maturity stage-specific M when the estimation method mirrors the operating model. Surprisingly, terminal molt does not affect the ability to estimate M. However, estimating growth simultaneously with M has a negative impact on the ability to estimate M but a positive effect on the quality of the estimates of spawning stock biomass.

Setal descriptions, locations, and abundances: Their impacts on decorating behaviors in the spider crab Libinia (Crustacea: Epialtidae).

The setal abundances and distributions of the spider crab Libinia are examined, using two species. These spider crabs decorate their bodies with environmental materials and camouflage from predators. These decorations attach to hooked setae, but other types setae are prevalent in body regions where decorations occur. Adult crabs undergo a terminal molt and lose the ability to regrow setae. Little is known about setal changes over the lifetime in these crabs and if hooked setae abundance decreases with age, resulting in a loss of camouflage strategy in larger individuals. The objective of this study is to determine the prevalence and abundance of hooked and non-hooked setae in the decorated body regions and if these setal patterns vary by species and body size. Scanning electron microscopy is used along with computer software to describe, count, and measure the setal coverage of hooked and non-hooked setae in body regions. Small individuals have more hooked setae than larger individuals and the two species have different setal abundances of hooked and non-hooked setae. Non-hooked setae cover much surface areas of crabs, attach decorations, and remain on the crabs even when hooked setae are damaged and broken. Setal morphologies and fouling are described among different sized crabs and the two species, with setal types and morphologies being similar but fouling being different. Large individuals likely employ a different camouflage strategy than small individuals due to losing hooked setae but retaining non-hooked setae.

The occurrence rates of adolescent with hard shell and terminal molt of male snow crab <i>Chionoecetes opilio</i> in the Sea of Japan off Kyoto Prefecture

The occurrence rates of adolescent with hard shell and terminal molt of male snow crab <i>Chionoecetes opilio</i> in the Sea of Japan off Kyoto Prefecture

Ocean acidification alters properties of the exoskeleton in adult Tanner crabs, Chionoecetes bairdi.

Ocean acidification can affect the ability of calcifying organisms to build and maintain mineralized tissue. In decapod crustaceans, the exoskeleton is a multilayered structure composed of chitin, protein and mineral, predominately magnesian calcite or amorphous calcium carbonate (ACC). We investigated the effects of acidification on the exoskeleton of mature (post-terminal-molt) female southern Tanner crabs, Chionoecetes bairdi Crabs were exposed to one of three pH levels - 8.1, 7.8 or 7.5 - for 2 years. Reduced pH led to a suite of body region-specific effects on the exoskeleton. Microhardness of the claw was 38% lower in crabs at pH 7.5 compared with those at pH 8.1, but carapace microhardness was unaffected by pH. In contrast, reduced pH altered elemental content in the carapace (reduced calcium, increased magnesium), but not the claw. Diminished structural integrity and thinning of the exoskeleton were observed at reduced pH in both body regions; internal erosion of the carapace was present in most crabs at pH 7.5, and the claws of these crabs showed substantial external erosion, with tooth-like denticles nearly or completely worn away. Using infrared spectroscopy, we observed a shift in the phase of calcium carbonate present in the carapace of pH7.5 crabs: a mix of ACC and calcite was found in the carapace of crabs at pH 8.1, whereas the bulk of calcium carbonate had transformed to calcite in pH7.5 crabs. With limited capacity for repair, the exoskeleton of long-lived crabs that undergo a terminal molt, such as C. bairdi, may be especially susceptible to ocean acidification.

Direction, ontogeny, and mechanism of the temperature-size rule operating in a large marine crab, Chionoecetes opilio

Abstract The classic temperature-size rule (TSR) states that ectotherms mature smaller in warmer than in colder conditions; the reverse TSR is the opposite response. We combined field observations with laboratory experiments and published information to synthesise the response of snow crab (Chionoecetes opilio), a marine brachyuran with determinate growth, to temperature. Size at onset of physiological maturation/maturity and after terminal moult (TM) were positively related to temperature, thus indicating the reverse TSR. Moult increment varied little with temperature, but crabs were larger at instar in colder than in warmer water due to an initial difference in settlement size that propagated to higher instars, suggesting classic TSR prior to settlement. The pattern of increasing TM size with temperature was caused by crabs moulting more times before TM in warmer than in colder water. Intermoult period (IP) declined exponentially with temperature, and lower instars were more temperature sensitive than higher instars. Temperature effects on IP were strong enough to explain changes in size and instar number at TM under a possible time-invariant maturation schedule. Skip moulting was observed in the smallest crabs reared in the laboratory and resulted in high mortality. The reverse TSR in snow crab seems to be adaptive to coping with ectotherm predation.

Temporal and spatial variability in size-at-maturity for the eastern Bering Sea snow and Tanner crab (Chionoecetes opilio and C. bairdi)

Growth in size stops at the terminal molt (molt-to-maturity) for the eastern Bering Sea snow and Tanner crab (Chionoecetes opilio and C. bairdi) and understanding size-at-maturity (SAM) dynamics is critical to manage their fisheries, which target and retain large mature males. Bottom trawl survey data from 1982 to 2017 were analyzed with correlations and spatial regressions to identify temporal and spatial patterns of SAM and size-specific proportions mature (PM) for newshell (recently molted) crab of each sex of each species. While substantial annual variability in SAM was estimated, no long-term directional trends were estimated for either species. Recruitment variability was likely the major driver of temporal trends in SAM for both species, rather than variable ocean temperatures or other possible factors. Recruitment abundances and SAM were negatively correlated for female snow crab but positively correlated for male snow crab and both sexes of Tanner crab. Male and female snow crab SAM had similar periodicities as snow crab recruitment. SAM spatial patterns were relatively stable at decadal time-scales for both species. Annual temporal variability in PM was moderate to negligible for both species and decadal-sale variability was modest, indicating temporally stable size-specific maturation probabilities. Comparison of SAM and PM regression results show strong effects of ontogenetic movement on spatial patterns of SAM for male snow crab. Intra- and inter-species comparisons allowed for broader understanding of SAM dynamics. Strong periodicity of male snow crab SAM may have utility for fisheries management. Elucidating the mechanisms responsible for periodic recruitment for these species will help provide a more complete understanding of both SAM dynamics and general population dynamics. With increasing ocean temperatures in the EBS, the mean size of mature crab for these species may increase but would certainly be accompanied by a decrease in snow crab abundance, the more stenothermic of the two species.

Estimating the maximum sustainable yield of snow crab (Chionoecetes opilio) off Tohoku, Japan via a state‐space stock assessment model with time‐varying natural mortality

AbstractA maximum sustainable yield (MSY) obtained by maintaining or restoring fish stock levels is a tangible benefit of ecosystem services. Snow crab (Chionoecetes opilio) off Tohoku, Japan has been managed by a total allowable catch since 1996, although the abundance has not increased. Surprisingly, there was no increase after 2011, when fishing pressure was greatly reduced because of the Great East Japan earthquake. This implies that some of the crab's biological characteristics, such as recruits, natural mortality coefficient (M) and terminal molting probabilities (p), might have changed. We developed “just another state‐space stock assessment model” to estimate the MSY of the snow crab off Tohoku considering interannual variations in M and p. The multimodel inference revealed that M increased from 0.2 in 1997 to 0.59 in 2018, although it did not vary according to instars, sex or terminal molt. The parameter p also increased by 1.34–2.46 times depending on the instar growth stages from 1997 to 2018. We estimated the MSYs in three scenarios, which changed drastically if M and p were set as they were in the past or at the current values estimated from this study. This result indicated that the MSY of snow crab would also vary with time based on their time‐varying biological characteristics.

Evaluation of a direct age estimation method for terminally molted male snow crab Chionoecetes opilio (Fabricius 1788) (Decapoda: Brachyura: Oregoniidae)

Abstract Recent research suggests that calcified eyestalks and gastric mill ossicles (stomach teeth) can be used to estimate the age of some crustacean species. Along with annual growth of the endocuticle, bipartite bands in the hard tissue are believed to reflect annual growth patterns (similar to fish scales or otoliths) that are retained through repeated molt cycles. Similar bands are observed in the zygocardiac ossicles of the gastric mill from the snow crab (Chionoecetes opilioFabricius 1788). If these bands reflect annual growth, they may be used to estimate age, which could enhance understanding growth, mortality, recruitment, and age composition and improve fishery management. While some studies show that the number of bands correlates to other estimates of age for C. opilio, little evidence suggests that bands accumulate annually as growth increments independent of molting. Male C. opilio terminally molt at maturity, after which they can survive for seven years or more. Shell condition, i.e., degree of wear and epibionts on their exoskeleton, is used here and by other carcinologists as a proxy for age subsequent to the terminal molt. We estimated band counts and endocuticle thickness from thin sections of the zygocardiac ossicle of terminally molted male C. opilio across a range of shell conditions from a wild, fished stock. We found no differences in band counts (P = 0.41) or endocuticle thickness (P = 0.13) across varying shell conditions and size. These results do not support the hypothesis that band counts can be used to estimate the age of this species after the terminal molt.

Growth and longevity of the spider crab Libinia ferreirae (Majoidea, Epialtidae)

Abstract We estimated the growth parameters of the spider crab, Libinia ferreirae (age, asymptotic size and growth rate) using the von Bertalanffy growth equation model. We obtained nine cohorts for female carapace asymptotic width (CW∞) = 64.32 mm, growth coefficient (day-1) (k) = 0.0027 e t0= 0.77 days) and seven for males (CW∞ = 81.93 mm, k= 0.0021 e t0= 0.49 days). The longevity for males was higher than that for females, estimated 2,156 days (5.91 years) and 1,706 days (4.68 years), respectively. The growth curves for males and females differed (F = 34.67 e p < 0.001). Males reached gonadal maturity before morphometric maturity and occurred at 8.8 and 16.6 months of life, respectively. Females reach gonad and morphometric maturity synchronously and this was estimated to occur at about 11.42 months of life. These crabs invest a great amount of energy in growth during a brief period of their development until reaching the terminal moult. This growth strategy would bring less wear to the organism and consequently a greater longevity.

Large‐Scale Movements of Postcopulatory Female Blue Crabs Callinectes sapidus in Tidal and Nontidal Estuaries of North Carolina

AbstractFor migratory species, understanding the timing, direction, and mechanism of migration is critical for successful fisheries management. We investigated migratory movements of postcopulatory female blue crabs Callinectes sapidus in three estuaries with different tidal regimes in North Carolina using a mark–recapture study. All crabs tagged were within about 2 weeks of the terminal molt to maturity (mating generally takes place immediately after the molt). In general, distances traveled (mean ± SE = 6.8 ± 0.6 km) and net movement rates (0.5 ± 0.05 km/d) were quite low compared to those in previous studies of mature (but not necessarily recently molted) female crabs, suggesting that rapid seaward migration does not take place until several weeks after mating, presumably upon production of a clutch of eggs. Although most crabs moved short distances (85% moved &lt; 15 km), movement was generally oriented seaward, indicating that some down‐estuary movement occurs prior to oviposition, especially in crabs mating in low‐salinity estuaries that would be unsuitable for embryonic and larval development. Crabs moving seaward in the weeks between molting/mating and first oviposition were likely walking rather than swimming and taking advantage of seaward currents, as indicated by the low movement rates and similarities among the three areas.

Определение широкопалости у крабов-стригунов рода Chionoecetes в полевых условиях

Определение широкопалости у крабов-стригунов рода Chionoecetes в полевых условиях

Moulting and growth in earlier and later moulters of adolescent male snow crabs (Chionoecetes opilio) (Brachyura: Majoidea) under laboratory conditions

Growth and moulting of male adolescent snow crabs were investigated in a laboratory culture experiment. The moulting season of adolescents was separated into two groups of earlier and later, and they were regarded as normal and skip moulters, respectively. The existence of male snow crab skip moulters was first demonstrated in the Sea of Japan. Although the precise moulting season of the earlier moulter could not be revealed owing to the captive method, the later moulter is suspected to moult approximately 1 year after the earlier moulter. Earlier and later moulters were not different in growth of carapace width or chela height during the pubertal and terminal moult. Our moult and growth results of earlier and later moulters will be useful for predicting recruitment to legally fishable (>90 mm carapace width) or valuable (hard-shelled adult) populations of male snow crab in the Sea of Japan.

Integrated analysis of sexual maturation through successive growth instars in the spider crab Leurocyclus tuberculosus (Decapoda: Majoidea)

An integrative analysis of sexual maturity associated with growth was developed for the spider crab Leurocyclus tuberculosus (H. Milne Edwards and Lucas, 1842). Sexual maturity was characterized based on gonadal, morphological, morphometric, and functional sexual maturity. Progress in sexual maturation was described through 13 growth stages (instars) detected by the examination of size (carapace width) frequency distributions. Mature females displayed mature ovaries, developed vaginae, open gonopores, allometric changes in the abdomen, and ovigerous stage in the transition from instar IX to instar X. Sexually mature males presented spermatophores in the distal vasa deferentia and allometric changes in several measurements of the right chela in the transition from instar X to instar XI. However, two prepubertal phases were recognized in both sexes separated from each other by a prepubertal critical molt. Preceding the second critical molt, gonopores were sealed and vasa deferentia showed no spermatophores, and therefore neither sex was able to mate. The integrated analysis of size at maturity and size frequency distributions showed that in both sexes molt to gonadal, morphological, morphometric, and functional sexual maturity occurred in advance of the terminal molt, in contrast with patterns observed in other Majoidea.