Setogenesis and molting in planktonic crustaceans

The formation of new setae, termed setogenesis, is describedfor two taxa of planktonic crustaceans: euphausiids, Euphausiapacifica and Thysanoessa spinifera, and a calanoid copepod,Calanus marshallae. Characteristics of setal formation and eversionat ecdysis are described from two time-series of animals preservedat known intervals in their molt cycle. Results from these laboratoryreference series indicate that previous interpretations of setogenesisin the literature can by synthesized to describe a dynamic processof setal formation which is common to all crustacean taxa. The morphological characteristics of developing setae are usedto designate three specific phases in the molt cycle of planktoniccrustaceans (premolt, postmolt, and intermolt). This stagingtechnique may be used to study field-oriented problems relatedto molting in small planktonic crustaceans.     

Effect of Eyestalk-Ablation on Circulating Ecdysteroids in Hemolymph of Snow Crabs, Chionoecetes opilio: Physiological Evidence for a Terminal Molt

Bering Sea snow crabs (Chionoecetes opilio) are a commerciallyimportant crab harvested in the Bering Sea. Optimal managementof this species requires an understanding of the biology ofthis crab that is currently incomplete. Fisheries managers applya continuous growth model in their management of snow crab,which assumes that male crabs increase in size throughout theirlifespan. Male snow crabs undergo a morphometric molt that leadsto a disproportionate increase in chelae size and it is stilldebated whether this molt is associated with a terminal molt.This study was conducted to determine whether adult male C.opilio are anecdysic. Using current knowledge of the hormonalregulation of crustacean growth, snow crab physiology was manipulatedto induce an increase in molting hormones (ecdysteroids). Sincefemale snow crabs are known to undergo a terminal molt afterattaining reproductive maturity, we compared ecdysteroid levelsin eyestalk-ablated terminally molted females, small-clawedmales and large-clawed males. Snow crabs were collected fromthe Bering Sea and maintained in circulating seawater at approximately6°C. Animals were either eyestalk-ablated or left intact.Ecdysteroid levels in hemolymph were quantified using an enzyme-linkedimmunosorbant assay (ELISA). Circulating ecdysteroids were significantlyhigher in small-clawed male crabs when compared to large-clawedmales or terminally molted females. Eyestalk-ablation increasedcirculating ecdysteroids in small-clawed males, but had no significanteffect on circulating ecdysteroids in large-clawed males orin terminally molted females.     

Cuticle Formation in Hemicycliophora arenaria,Aphelenchus avenae and HirschmannIella gracilis

The onset of molting in all stages of Hemicycliophora arenaria was preceded by the appearance of numerous, discrete globular structures which were termed "molting bodies" because they were present in the hypodermis only during the production of the new cuticle. In all parasitic stages the molt commenced with the separation of the cuticle from the hypodermis from which the new sheath and cuticle were differentiated. Following completion of the new sheath and cuticle most of the old outer covering was apparently absorbed before ecdysis. Electronmicrographs of body wall cross sections in molting L4 male specimens revealed the final molt to be a double molt in which an additional sixth cuticle was produced. Since both a new sheath and cuticle were produced during the molt of each stage, the sheath must be considered as an integral part of the cuticle and not as a residual cuticle or the result of an incomplete additional molt. Molting in Aphelenchus avenae and Hirschmanniella gracilis was less complex and "molting bodies" were not observed. After cuticle separation the hypodermis gave rise to a new trilaminate zone, the future cortex, and (later) the matrix and striated basal layers.     

Regulation of Crustacean Molting: A Multi-Hormonal System

SYNOPSIS. In order to increase in size, arthropods must firstmolt (shed) their confining exoskeleton. This molting processis under the immediate control of the steroid molting hormone20-hydroxyecdysone (20-HE). Both the initial rise in circulatinghormone concentration and a coordinated decline are necessaryfor successful molting. Synthesis and/or release of ecdysone,the precursor to 20-HE, is regulated by the neuropeptide molt-inhibitinghormone (MIH). We have determined the primary amino acid sequenceof MIH in the lobster, Homarus americanus. This peptide hasa high degree of identity with the lobster hyperglycemic hormone.Another endocrine factor that appears to be involved in moltingis the juvenile hormone-like terpenoid methyl farnesoate (MF).We have characterized hemolymph MF binding proteins during themolt cycle. In addition, recent data indicate that MF may stimulatethe secretion of 20-HE in vivo and in vitro.     

Hormonal Control of Molting in Decapod Crustacea

The involvement of the molting hormone, 20-hydroxyecdysone,in the mediation of molting in decapod crustaceans is brieflyreviewed. Aspects of the secretion and metabolism of its precursor,ecdysone, are discussed. Experiments are described that demonstratethe presence of a molt-inhibiting hormone (MIH) in the sinusglands of juvenile lobsters (Homarus americanus). Assays forMIH include measurement of the molt interval and radioimmunoassayof circulating titers of ecdysteroids in eyestalk-ablated lobsters.This latter assay indicates that sinus gland extracts significantlydecrease the concentration of circulating ecdysteroids 24 hrafter injection. Data are also presented on the circulatingtiters of ecdysteroids during multiple molt cycles of lobstersfollowing eyestalk ablation. These data indicate that theremust be another factor that ultimately regulates the circulatinglevels of the molting hormone.     

Interaction Between Ecdysteroid, Eclosion Hormone, and Bursicon Titers inManduca sexta

SYNOPSIS. The end of the molting process in the tobacco hornwormincludes the rapid digestion of the old cuticle, molting fluidresorption, ecdysis of the old cuticle, and expansion and hardeningof the new cuticle. The coordination of these processes is accomplishedby three hormones. Each ecdysis during the life of Manduca appearsto be triggered by eclosion hormone. Depending on developmentalstage, the hormone comes either from the brain-corpora cardiacacomplex or from the chain of ventral ganglia. The neural programstriggered by eclosion hormone include a neuroendocrine event,the release of the tanning hormone, bursicon, thereby ensuringthat tanning of the new cuticle must follow ecdysis. Ecdysis,itself, appears to be controlled by the ecdysteroid levels sinceecdysteroid injections delay ecdysis at physiological concentrationsand in a dose dependent fashion. This delay is due to inhibitionof eclosion hormone secretion and to the retardation of theterminal phases of the molt including the digestion of the oldcuticle and the onset of sensitivity to eclosion hormone. Thus,eclosion hormone secretion and the ecdysis it triggers are coordinatedwith the end of development because both are influenced by thesame endocrine signal—the decline in the ecdysteroid titer.     

Limb Autotomy as an Investigatory Tool: Host Molt-Stage Affects the Success Rate of Infective Larvae of a Rhizocephalan Barnacle

D. M. Skinner was the first biologist to systematically investigatethe effect of multiple limb autotomy on ecdysis in decapod crustaceans.She proposed the existence of (1) limb autotomy factor anecdysis(LAF_an) which initiates precocious molting, and (2) limb autotomyfactor proecdysis (LAF^pro) which postpones proecdysis. Meantime to ecdysis did not differ significantly among groups ofsmall juvenile Callinectes sapidus with zero, two, four, andsix limbs removed. Variance was significantly less for the groupmissing six limbs; i.e., autotomy of six limbs synchronizedthe molt-cycle. These patterns were consistent with the hypothesesthat LAF^an exerts an additive effect, i.e., more anecdysialcrabs enter proecdysis as more limbs are autotomized and thata delay of proecdysis occurs above a threshold (>4) of autotomizedlimbs. Multiple limb autotomy provides investigators with easyaccess to crabs in metecdysial molt-stages which is useful instudying interactions between rhizocephalan barnacles and theirhosts. While 71% of metecdysial C. sapidus exposed to infectivelarvae of the sacculinid rhizocephalan, Loxothylacus texanus,developed the external stage of the parasite, no similarlysizedanecdysial crab was parasitized, suggesting that these crabsare not susceptible to infection during anecdysis. Size of C.sapidus at infection was inversely proportional to the numberof ecdyses between infection and emergence of the parasite,but not correlated to final host size. These data suggest thatthere is a minimum threshold for host size; smaller hosts undergomore ecdyses before attaining the threshold. Rhizocephalansare being considered as biological control agents and thereis a need to understand how they find and infect hosts. Becauselimb autotomy is such a useful research tool, D. M. Skinner'scontributions to our understanding of how limb autotomy influencesthe crustacean molt cycle will continue to pay significant dividendsin crustacean biology.     

Photoperiod and Reproduction in Lobsters (Homarus)

SYNOPSIS. Photoperiodic control of lobster (Homarus) ovipositionranges from none in the European species (H. gammarus) to atwo-phase requirement in some populations of H. americanus.In the inshore population from Massachusetts two to three monthsof shortday photoperiod appear necessary to condition the ovaryfor final vitellogenesis following long-day onset (LDO). Thestimulus of LDO is only effective near the time of molt. Ifmolt follows LDO within 120 days, egg extrusion follows themolt in about 80 days. If LDO follows the molt, extrusion occursin about 125 days. Extrusion delayed in this way delays thefollowing molt, which is also delayed by the retention of eggson the pleopods. In lobsters from Prince Edward Island the photoperiodic responseseen at laboratory temperatures (ca. 14°C) appears to besubordinated to a response to other cues, probably temperature,when females are held on a near-natural temperature regime withwinter minimum of 2°C characteristic of inshore environments.H. americanus from the migratory outer continental shelf populationexperience winter temperatures rarely lower than 8°C andprobably require photoperiodic control to initiate migrationas well as vitellogenesis. The photoperiodic response may haveoriginated in offshore populations during the Pleistocene andspread to more northerly inshore areas as habitat opened afterthe retreat of the continental ice sheet.     

Osmoregulation in Larvae of the Land-Crab, Cardisoma guanhumi Latreille

Daily studies were made of the osmoregulatory abilities of Cardisomaguanhumi from hatching to the end of larval development. Theseresults were compared to those of similar research on osmoregulationby larvae and megalopa of four species of estuarine, littoral,and sub-littoral crabs. The comparison shows that larvae ofC. guanhumi possess the same kinds of adaptations for waterintake at the time of molt that were found in Rhithropanopeusharrisii. Land-crab larvae hyperregulated in 10 p.p.t. sea waterand hyporegulated in water of 40 p.p.t. for experimental periodsfor 2 hr during the first one-third of their development. Duringthe remainder of larval life, they hyporegulated against 15p.p.t. in intermolt periods and became isosmotic with, or hyporegulatedagainst, 10 p.p.t. at the time of molting. From the time ofhatching, the osmoregulatory pattern of developing C. guanhumifits them for deep penetration of estuaries and for crossingsteep saline gradients. This pattern is evidence for a strongerand more enduring control of water balance, especially at thetime of molting, than we have found in non-terrestrial species.     

Effects of food quality on individual growth and development in the freshwater copepod Boeckella triarticulata

To quantify the demographic effects of food quality, and specificallyof the ‘poorquality’ cyanobecterium Anabaena flos-aquae,we reared individual Boeckella triarticulata (Copepoda, Calanoida)on two diets (monospecific Cryptomom sp. versus mixed Cryptomonas-Anabanmadiets) and quantified individual growth and developmental trajectoriesby examining exuviae produced at each molt, from hatching tomaturity. Size at molting was less variable among individuals,within and between diets, than age. Food quality had significanteffects on male sizes at molting and on stage-specific dailygrowth rates of both sexes; these effects were strongest duringlate naupliar and all copepodite stages Tke med Cryptomonas-Anabaenadiet significantly slowed development, particularly of copepoditestages. As a consequence of these effects, individuals raisedon the mixed diet were smaller and older at maturity. Withina given diet, individual differences explained much, if notmost, of the variation exhibited in growth and development.By following growth and development of a large number of individualsthroughout their Life cycles, we show that individual femalesproduce variable offspring, indicative of a bet-hedging life-historystrategy, and that B.triarticulata (like other calanoids) cangrow, develop and survive on diets that include ‘poorquality’ cyanobacteria. ¹Present address: Department of Biological Sciences, Universityof Rhode Island, Kingston, RI 02881, USA     

Variation in foraging investment during the intermolt interval and before egg-laying in the spiderNephila clavipes (Araneae: Araneidae)

Repeated observations were made of marked, freeliving females in three disjunct populations of the orb-weaving spider Nephila clavipes,recording orb size and structure and barrier web complexity. These longitudinal studies revealed regular patterns of variation in orb size associated with the molting cycle and cycle of egg maturation. Juveniles maintained in an insectary under a controlled feeding regime exhibited similar variation in orb size. All spiders ceased orb renewal prior to molting and the majority replaced the viscid orb with a nonviscid platform prior to molting. Differences among the populations in molting web structure are discussed in light of differences in the physical and biological environments. Cessation of orb renewal corresponded temporally with reported physiological changes associated with the molt cycle and possibly reflect changes in resource partitioning between growth and foraging during the molt cycle.     

Demographic variation in timing and intensity of feather molt in migratory Fork‐tailed Flycatchers (Tyrannus s. savana)

Understanding the annual cycle of migratory birds is imperative for evaluating the evolution of life‐history strategies and developing effective conservation strategies. Yet, we still know little about the annual cycle of migratory birds that breed at south‐temperate latitudes of South America. We aged, sexed, and determined the progression and intensity of body, remige, and rectrix molt of migratory Fork‐tailed Flycatchers (Tyrannus s. savana) at breeding sites in southern South America and at wintering sites in northern South America. Molt of both body and flight feathers occurred primarily during the winter. In early winter, a similar proportion of young and adult flycatchers molted remiges and rectrices, but remige molt intensity (number of remiges molting) was greater and primary molt progression (mean primary feather molting) more advanced in adults. In late winter, remige molt intensity and primary molt progression did not differ between age groups. We found no difference between males and females either in the proportion of individuals molting in winter or in the intensity or progress of remige molt. Our results suggest that the nominate subspecies of Fork‐tailed Flycatcher undergoes one complete, annual molt on the wintering grounds, and represents the first comprehensive evaluation of molt timing of a migratory New World flycatcher that overwinters in the tropics. Given that breeding, molt, and migration represent three key events in the annual cycle of migratory birds, knowledge of the timing of these events is the first step toward understanding the possible tradeoffs migratory birds face throughout the year.     

Radioimmunoassay and the Characterization of Crustacean Ecdysteroids

The development of the radioimmunoassay (RIA) for ecdysteroidshas permitted the sequential measurement of circulating ecdysteroidson individual organisms. In addition, used in conjunction withvarious chromatographic techniques, it has assisted in the identificationof ecdysteroids secreted by molting glands. The sensitivityof the assay has also permitted the monitoring of the rate ofsecretion of ecdysteroids by molting glands exposed to variousin vitro conditions. This latter approach will be of great utilityin determining the nature and effect of molt regulators (inhibitors)present in other tissues.     

Physiological evaluation of temperature effect on the growth processes of the mysid, Neomysis intermedia Czerniawsky

Ingestion, respiration, and molting loss rates were measuredover the 3 – 29°C range in Neomysis intermedia. Weightspecific rates of these physiological processes ranged from2 to 140% body C day^–1 for ingestion, from 2 to 15% bodyC day^–1 for respiration, and from 0.1 to 5% body C day^–1for molting loss. All weight-specific rates showed a logarithmicdecrease with a logarithmic increase in body weight, and a logarithmicincrease with a linear increase in temperature below 20 or 25°C.The effect of temperature, however, was different between thephysiological rates, with a large temperature dependency foringestion (Q₁₀ = 2.6 –3.9) and molting loss (Q¹⁰ = 2.9– 3.6) and a moderate temperature dependency for respiration(Q₁₀ = 1.9 – 2.1). Calculated assimilation efficiencychanged with body size, but was constant over the temperaturerange examined. Allocation of assimilated materials varied witha change in temperature, reflecting the different temperaturedependence between physiological processes. It was deduced thatthe strong temperature dependency of the growth rate in N. intermediaobserved in the previous studies resulted from the large temperatureeffect on ingestion and assimilation rates, superimposed bythe different allocation of assimilated materials. ¹Present address: Department of Botany, University of Tokyo,Hongo, Tokyo 113, Japan     

A Quantitative Analysis of Flight Feather Replacement in the Moustached Tree Swift Hemiprocne mystacea,a Tropical Aerial Forager

The functional life span of feathers is always much less than the potential life span of birds, so feathers must be renewed regularly. But feather renewal entails important energetic, time and performance costs that must be integrated into the annual cycle. Across species the time required to replace flight feather increases disproportionately with body size, resulting in complex, multiple waves of feather replacement in the primaries of many large birds. We describe the rules of flight feather replacement for Hemiprocne mystacea, a small, 60g tree swift from the New Guinea region. This species breeds and molts in all months of the year, and flight feather molt occurs during breeding in some individuals. H. mystacea is one to be the smallest species for which stepwise replacement of the primaries and secondaries has been documented; yet, primary replacement is extremely slow in this aerial forager, requiring more than 300 days if molt is not interrupted. We used growth bands to show that primaries grow at an average rate of 2.86 mm/d. The 10 primaries are a single molt series, while the 11 secondaries and five rectrices are each broken into two molt series. In large birds stepwise replacement of the primaries serves to increase the rate of primary replacement while minimizing gaps in the wing. But stepwise replacement of the wing quills in H. mystacea proceeds so slowly that it may be a consequence of the ontogeny of stepwise molting, rather than an adaptation, because the average number of growing primaries is probably lower than 1.14 feathers per wing.     

The molt cycle and its hormonal control in Rhithropanopeus harrisii larvae

Stages of the zoeal and megalopal molt cycles of Rhithropanopeus harrisii were characterized by the appearance of epidermal cells in the spines and antennae. Eyestalk removal of the beginning of the last zoeal instar slightly accelerated the molt cycle. Fourth-instar larvae which had undergone eyestalk ablation during the third instar progressed through the molt cycle significantly faster than did control larvae. Eyestalkless megalopae also demonstrated an enhanced molting rate. The results suggested that the larval eyestalks contain a factor (molt-inhibiting hormone) which plays a role in controlling the molting rate.