Can hermit crabs recognize social partners by odors? And why?

Individual recognition, i.e., the ability of an animal to recognize the identity of a conspecific, is a key element in the social life of many vertebrates. This phenomenon has rarely been investigated in invertebrates, particularly in pre-social arthropods, and is generally poorly understood for several reasons, including (1) the disparity between what the experiments actually demonstrated and what we implicitly mean by individual recognition, (2) the limited knowledge of the sensory channels used to recognize the social partner (particularly in the case of pre-social arthropods), and (3) the general disregard for the nature of representations that an animal constructs concerning the recognized conspecific. This article aims to review what is currently known about individual recognition mediated by pheromones in the shallow-water hermit crab Pagurus longicarpus. Methodological biases, on one hand, and promises of future success, on the other, are pinpointed while explaining the results obtained so far. Finally, an adaptive explanation is offered that attempts to solve the observed hiatus between the “virtuosism” shown by P. longicarpus in the laboratory and the apparent simplistic way of shell recruitment used in the field. Individual recognition, even if its expression is very rare in nature, might be a matter of life or death for the hermit crab that makes recourse to it.     

Analysis of multiple aspects of a repeated signal: power and rate of rapping during shell fights in hermit crabs

Repeated activities used by animals during contests are assumedto act as signals advertising the quality of the sender. However,their exact functions are not well understood and observationsfit only a limited set of the predictions made by models ofsignaling systems. Experimental studies of contest behaviortend to focus on analysis of the rate of signaling, but individualperformances may also vary in magnitude. Both of these featurescan vary between outcomes and within contests. We examined changesin the rate and power of shell rapping during shell fights inhermit crabs. We show that both rate and power decline duringthe course of the encounter and that the duration of pausesbetween bouts of shell rapping increases with an index of thetotal effort put into each bout. This supports the idea thatthe vigor of shell rapping is regulated by fatigue and couldtherefore act as a signal of stamina. By examining differentinteracting components of this complex activity, we gain greaterinsight into its function than would be achieved by investigatinga single aspect in isolation.     

Resource assessment in hermit crabs: the worth of their own shell

Animals gather information about the quality of a resource throughits assessment and behave accordingly as a result of adaptivemotivational changes. In the hermit crab Pagurus longicarpus,we investigated whether an individual was affected in its motivationto acquire a new shell by the quality of the domicile shell(own resource value [ORV]), of the offered shell (external resourcevalue [ERV]), or of both and asked whether its motivation wasaltered by the information gathered during shell investigation.We analyzed the behavior of hermit crabs inhabiting shells ofdiffering qualities and compared their willingness to acquirean offered shell—optimal, smaller than optimal, or largerthan optimal—by measuring the latency to approach it,the number of shell investigation, and its total duration. Crabsin smaller shells (SSs) approached more quick and often theoffered shell, whereas crabs in larger shells investigated theoffered shell more thoroughly. The readiness of crabs to approachthe offered shell and the extent of its investigation were independentof the ERV but were exclusively affected by the ORV, whereasthe number and duration of shell investigation did not changewith time as investigation proceeded, except for crabs in SSs.These results suggest that P. longicarpus' motivation to acquirea new shell is exclusively influenced by the value of the shellit inhabits rather than by the quality of the shell it is offeredand that this species does not gather—or does not use—informationabout ERV during investigation.     

Multiple feeding techniques in the sessile hermit crab,Discorsopagurus schmitti,inhabiting polychaete tubes

In the sessile hermit crab Discorsopagurus schmitti, four different feeding techniques are described: antennary beating, body-trap feeding, feeding on wafting particles, and scraping. This species' potential for being a generalist in its diet and opportunistic in its foraging behavior certainly opened the way for a sedentary life, allowing occupation of the unusual microhabitat provided by attached polychaete tubes. Nonetheless, deposit-feeding and/or browsing are the most conspicuous foraging strategies, when both the percentage of behavioral acts performed and the quantity of potential food collected are estimated. The question still remains of whether species' reliance on benthos as the main food source should be regarded as an efficient strategy for a sedentary life, or simply as the heritage of its not-so-remote past as a mobile, and shell-dwelling hermit crab.     

Shell occupancy of the intertidal hermit crab Clibanarius erythropus (Decapoda,Diogenidae) on São Miguel (Azores)

The intertidal hermit crab Clibanarius erythropus was collected at three sites on São Miguel (Azores) during low spring tides. Shells occupied were identified and measured. Crab sizes ranged from 1.78 to 13.67 mm (cephalothoracic shield length), with an average size of 4.40 ± 1.44 mm. Of the 19 different shells utilised, the most frequent were Littorina striata (23.8%), Nassarius incrassatus (22.5%) and Mitra sp. (22.0%). At Fenais da Luz, L. striata was most frequently occupied, while at Água de Alto it was N. incrassatus and, at Caloura, Mitra sp. shells were most frequently used. Shell selection appears to be determined by respective sizes of hermit crab and shell species. Small size-class crabs occupy more shell species than larger crabs. The smallest crab was found at Fenais da Luz occupying a small Bittium sp., whereas the largest crab was found at Caloura inhabiting Stramonita haemastoma.     

Fractionation of the gene-size macronuclear chromatin fragments of the binucleated eukaryote Oxytricha

Summary The macronuclear chromatin of Oxytrichia nova consists of chromatin fragments which are fully soluble in 0.2 mM EDTA and whose DNA length varies from 500–25 000 bp. The DNA migrates electrophoretically as a series of discrete bands, with specific genes present in only one or a few bands. The chromatin fragments are composed of nucleosomes and migrate electrophoretically in proportion to their DNA length. These results suggest schemes for the fractionation of undigested chromatin in order to enrich for specific genes, facilitating analysis of changes in chromatin structure associated with changes in gene expression.     

Invertebrate and protozoan epibionts on the velvet swimming crab Liocarcinus puber (Linnaeus, 1767) from Scotland

Invertebrate and ciliate protozoan epibionts of velvet swimming crabs collected near Millport (Scotland) were analysed. The ecdysis peak for male crabs was at the time of collection while that for female crabs was 2 months later. The epibionts were: the polychaetes Pomatoceros triqueter and Hydroides norvegica, the cirriped Balanus crenatus, the entoproct Barentsia matsushimana, the hydroids Leuckartiara sp. and Clytia sp., and the ciliate protozoans Ephelota plana, Ephelota gemmipara, Chilodochona quennerstedti and Cothurnia longipes. The polychaetes, cirripeds, entoprocts and hydroids, all of them with comparatively larger size, were distributed on the carapace, ventral surface of the cephalothorax and the pereiopods; meanwhile the protozoans, with smaller size, were attached also on pleopods, antennae, eyes and buccal appendages. Chilodochona quennerstedti was the epibiont most abundant on the crab, followed by Ephelota plana. Cothurnia longipes was, in contrast, the least abundant epibiont, followed by Barentsia matsushimana. The anatomical unit most colonized was the left third maxilliped, followed by the left first maxilliped and the ventral surface. The less colonized anatomical units were the left antenna and the ocular orbits. Epibionts on this crab have not been described before. Statistical analyses of the epibiont distribution on the crab were carried out. There were significant correlations between right and left appendages in 66.67% of the cases. Males and females differed significantly with respect to the distribution of epibionts on their anatomical units. The comparison analysis indicated a significant differential distribution of each epibiont species on the anatomical units of the crab. Principal component analysis grouped the epibiont species according to their colonization pattern in three clusters: (1) Ephelota plana, Ephelota gemmipara and Chilodochona quennerstedti; (2) Leuckartiara sp., Clytia sp., Barentsia matsushimana and Cothurnia longipes; and (3) Pomatoceros triqueter, Balanus crenatus and Hydroides norvegica. The hierarchical cluster analysis grouped the anatomical units of the crab in relation to their colonization in five clusters.     

Hermit crabs, humans and Mozambique mangroves

There is a complex interrelationship between upper shore hermit crabs (such as Coenobita sp. and Clibanarius sp.), coastal human populations and mangrove forests in Mozambique. The abundance, activity, shell selection and behaviour of three species of hermit crab are related to the level of mangrove cover. With increased density of mangrove trees, the study species of hermit crab changed in abundance, tended to become diurnal, spent more time feeding and were clustered in larger groups when doing so, and selected longer spired shells. All five of the same variables are also linked to the proximity and activity of humans through both direct and indirect actions. Direct effects included a tendency to nocturnal activity with proximity to human activity; indirect effects included increased and more clumped food supplies, and shell middens from intertidal harvesting and deforestation. Mangroves are important to local human populations as well as to hermit crabs, for a wide variety of (similar) reasons. Mangroves provide storm shelter, fisheries and fishery nursery grounds for adjacent human settlements, but they also harbour mosquito populations and their removal provides valuable building materials and fuel. Hermit crabs may be useful (indirectly) to coastal human populations by being a source of food to certain commercial species, and by quickly consuming rotting/discarded food and faeces (thereby reducing disease and pests). They can also cause minor problems to coastal human populations because they use shells of (fisheries) target mollusc species and can be more abundant than the living molluscs, thereby slowing down effective hand collection through confusion over identification. The mixture of positive and negative attributes that the three groups impart to each other in the Quirimba Archipelago, northern Mozambique, is discussed.     

Balanion masanensis n. sp. (ciliophora: prostomatea) from the coastal waters of Korea: morphology and small subunit ribosomal RNA gene sequence

The planktonic ciliate Balanion masanensis n. sp. is described from living cells, from cells prepared by quantitative protargol staining (QPS), scanning electron microscopy (SEM), and transmitted electron microscopy (TEM) preparations, and the sequence of its nuclear small subunit rDNA (SSU rDNA) is reported. This species is almost ovoid with a flattened anterior oral region when the cells are alive and stained. The flattened anterior region of a living cell often forms a dome with the perimeter receded in a groove, and this region is easily inflated or depressed. In SEM photos, a brosse of six to nine monokinetids (or possibly three to five dikinetids) was observed inside the circumoral dikinetids. In TEM photos, circumoral microtubular ribbons were observed below the oral cilia, which along with the oral flaps were 8-16 microm in length. The cytostome is a slight funnel-like central depression on the flattened anterior end. The morphological characteristics of this ciliate are identical to those of the genus Balanion (Order Prorodontida). The ranges (and mean+/-standard deviation) of cell length, cell width, and oral diameter of living cells (n=23-26) were 27-43 microm (35.2+/-4.6), 25-32 microm (28.6+/-2.3), and 25-30 microm (27.6+/-1.3), respectively, while those of the QPS-stained specimens (n=70) were 23-37 microm (30.6+/-3.5), 26-35 microm (30.7+/-2.2), and 26-33 microm (29.5+/-1.5), respectively. Forty-six to 55 somatic kineties (SKs) were equally spaced around the cell body and extended from the oral to near the posterior regions with 24-50 monokinetids per kinety. Each kinetid bore a cilium 2.8-7.2 microm long. A caudal cilium (ca 14 microm long) arose on the posterior end. The single ellipsoid macronucleus is 6.8-13.4 x 6.8-10.5 microm, accompanied by a single micronucleus (2.0-2.8 x 1.5-2.5 microm) visible only in QPS specimens. Because, the cell size, the number of SKs, and the number of kinetosomes per SK of this ciliate were much greater than those of Balanion comatum and Balanion planctonicum, the only two Balanion species so far reported, we have established B. masanensis n. sp. When properly aligned, the sequence of the SSU rDNA of B. masanensis n. sp. (GenBank Accession No. AM412525) was approximately 9% different from that of Coleps hirtus (Colepidae, Prorodontida) and 12% different from that of Prorodon teres (Prorodontidae, Prorodontida).     

Interference and exploitation components in interespecific competition between sympatric intertidal hermit crabs

This study was designed to evaluate the effect of interference and exploitation competition in shell partitioning between two hermit crab species (Pagurus criniticornis and Clibanarius antillensis). Field samples revealed that shells of the gastropod Cerithium atratum were the main resource used by both hermit crab species and that Pagurus used eroded or damaged shells in higher frequency than Clibanarius. The exploitative ability of each species was compared between species in the laboratory using dead gastropod (Cerithium) baits to simulate predation events and signalize newly available shells to hermit crabs. Pagurus reached the baits more rapidly than Clibanarius, but this higher exploitative ability did not explain shell utilization patterns in nature. Another experiment evaluated the dominance hierarchy between these two hermit crab species and revealed that Clibanarius was able to outcompete Pagurus for higher quality shells in agonistic encounters. This higher interference competitive ability of Clibanarius in relation to Pagurus may explain field observations. Nevertheless, Pagurus may be responsible to enhance shell availability to other hermit crab species that have lower ability to find and use newly available shells. Differently, the poorer condition of shells used by Pagurus, the higher ability of this species to attend gastropod predation events and its higher consumption rate by shell-breaking crabs (Menippe nodifrons) may increase its predation risks, thus revealing the disadvantages of such an exploitative competitive strategy for hermit crabs.     

An “Ethnic” Arboretum

Part I of this three-part series deals generally with sound and its pollution. Part II will move on to measurement, effects, and indoor learning activities. Outdoor activities and references will make up Part III.     

Evolutionary palaeoecology of symbioses between bryozoans and hermit crabs

Modern hermit crabs form associations with many organisms which encrust, bore into, or cohabit the living chambers of gastropod shells occupied by the crabs. Among these hermit crab symbionts are bryozoan species which develop massive, commonly multilayered, colonies encrusting hermit crab shells. These colonies extend the living chamber of the crab through a characteristic process of helicospiral tubular growth originating from the shell aperture. The scant information available on the ecology of Recent bryozoan‐hermit crab symbioses is reviewed. Symbioses have been recorded from intertidal to upper slope environments, and from tropical to cold temperate zones. None of the hermit crab species are obligatory symbionts of bryozoans, and the majority of the modern bryozoan species involved are also not obligatory symbionts. Fossil examples always lack the hermit crabs, which have a poor fossilization potential; however, the distinctive tubular growth pattern and other features of the bryozoans enable recognition of ancient examples of the symbiosis. The earliest inferred associations between bryozoans and hermit crabs date from the Mid Jurassic, but associations remained uncommon until the Neogene. A remarkably wide taxonomic diversity of Recent and fossil bryozoans are known or inferred symbionts of hermit crabs. The broad evolutionary pattern of the association demonstrates multiple originations of the symbiosis by bryozoans belonging to at least 5 cyclostome and 12 cheilostome families. Only the Miocene‐Recent cheilostome family Hippoporidridae has an evolutionary history closely tied to symbiosis with hermit crabs. There is no evidence for coevolution.     

Protection of the hermit crab Pagurus pollicaris say from predators by hydroid-colonized shells

Only one study has shown that a hydroid-colonized gastropod shell was a deterrent to predation on hermit crabs. In the present study, the hydroid-colonized shell protected the hermit crab Paguruspollicaris Say from the shell-crushing stone crab Menippe mercenaria (Say) and the non-shell-crushing octopus Octopus joubini Robson. The shell-crushing calico crab Hepatus epheliticus (Johansson) was not deterred, however, by a hydroid-colonized shell.     

Resource partitioning between sexes in the "unconventional" hermit crab, Calcinus tubularis

Together with Calcinus verrilli from Bermuda, the Mediterraneanhermit crab C. tubularis is unique in that it exhibits a sharpsexual dimorphism in resource use in which one sex, the male,occupies loose shells and the other, the female, occupies attachedtubes. Field surveys and laboratory experiments addressed twoissues that help understand male-female resource partitioningin this species. First, the value of shells as shelters differsbetween sexes. Shells furnish mobility to their inhabitants,but also greater opportunities to grow because they can providelarger size than can the tubes, whereas both mobility and largedimensions are crucial selective factors for the reproductiveoutcomes of males only. In fact, egg production seemed not tobe affected by females' sedentary life and the number of eggswas not related to the female size. Second, previous shelterexperience plays a role in reducing male-female competition.I found that individual crabs, once presented simultaneouslywith a shell and a tube, more likely selected the type of shelterthat they were collected in. Then, in male-female competitionexperiments, pairs composed of one crab found in a shell andone crab found in a tube and offered a shell and a tube in conjunctionoccupied the housing in accordance to the shelter previouslyoccupied without any influence of their sex or size. The questionremains as to how, why, and when such a sexual dimorphism inthe distribution of C. tubularis between microhabitats did occurduring a hermit crab's life.     

Sequence of a Euplotes crassus Macronuclear DNA Molecule Encoding a Protein with Homology to a Rat Form-I Phosphoinositide-specific Phospholipase C

A 604-base pair macronuclear DNA molecule from the hypotrichous ciliate Euplotes crassus was cloned and its DNA sequence determined. The DNA sequence contains an open reading frame capable of encoding a protein 141 amino acids in length. The putative protein contains significant sequence similarity to other eukaryotic proteins, including the rat form-I phosphoinositide-specific phospholipase C.