The hermit crab's nose—antennal transcriptomics
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Abstract:
In the course of evolution, crustaceans adapted to a large variety of habitats. Probably the most extreme habitat shift was the transition from water to land, which occurred independently in at least five crustacean lineages. This substantial change in life style required adaptations in sensory organs, as the medium conveying stimuli changed in both chemical and physical properties. One important sensory organ in crustaceans is the first pair of antennae, housing their sense of smell. Previous studies on the crustacean transition from water to land focused on morphological, behavioral and physiological aspects but did not analyze gene expression. Our goal was to scrutinize the molecular makeup of the crustacean antennulae, comparing the terrestrial Coenobita clypeatus and the marine Pagurus bernhardus. We sequenced and analyzed the antennal transcriptomes of two hermit crab species. Comparison to previously published datasets of similar tissues revealed a comparable quality and GO annotation confirmed a highly similar set of expressed genes in both datasets. The chemosensory gene repertoire of both species displayed a similar set of ionotropic receptors (IRs), most of them belonging to the divergent IR subtype. No binding proteins, gustatory receptors (GRs) or insect-like olfactory receptors (ORs) were present. Additionally to their olfactory function, the antennules were equipped with a variety of pathogen defense mechanisms, producing relevant substances on site. The overall similarity of both transcriptomes is high and does not indicate a general shift in genetic makeup connected to the change in habitat. Ionotropic receptors seem to perform the task of olfactory detection in both hermit crab species studied.Keywords:
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Hermit to King, or Hermit to All: Multiple Transitions to Crab-like Forms from Hermit Crab Ancestors
The Anomura presents the greatest degree of morphological disparity in the decapod Crustacea, with body forms ranging from the symmetrical and asymmetrical hermit crabs to squat lobsters and king crabs. The phylogeny of the anomurans has been fraught with controversy. Recent debate has focused primarily on the phenomenon of carcinization, the evolution of crab-like form from a non–crab-like ancestor, focused chiefly on derivation of king crabs from asymmetrical hermit crabs—the "hermit to king" hypothesis. We show by phylogenetic analysis of five nuclear protein-coding gene sequences that hermit crabs have a single origin, but surprisingly, that almost all other major clades and body forms within the Anomura, are derived from within the hermit crabs. The crab-like form and squat lobster form have each evolved at least twice from separate symmetrical hermit crab ancestors. In each case, a carcinization trend can be posited via a transition series from the initial symmetrical long-tailed hermit crab form, through the intermediate squat lobster or asymmetrical hermit crab form, to the final crab-like form. Adaptation to dextral shell habitation evolved at least twice, once in an exclusively deep-water clade and once in the common ancestor of all other asymmetrical hermit crabs (from which king crabs are derived). These remarkable cases of parallelism suggest considerable phenotypic flexibility within the hermit crab ground plan, with a general tendency toward carcinization. Rather than having a separate origin from other major clades, hermit crabs have given rise to most other major anomuran body types.
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A heterosexual pair of hermit crabassociated alpheid shrimp Aretopsis amabilis De Man, 1910 was found inside the empty shell of burrowing spatangid sea urchin of the genus Platybrissus.The finding and ecology as well as possibility of social monogamy in this shrimp species are discussed. РЕЗЮМЕ. Разнополая пара креветок-альфеидAretopsis amabilis De Man, 1910 -вида, обычно ассоциированного с раками-отшельниками, найдена живущей внутри пустого панциря зарывающегося морского ежа-спатангиды рода Platybrissus.Особенности находки, экологии и возможной социальной моногамии у этого вида креветок обсуждаюся в данной статье.
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Abstract The behaviour exhibited by the hermit crab Pagurus longicarpus in response to an empty shell varied in the presence of cues from conspecific individuals according to its familiarity or not with them. This binary discrimination was independent of the conspecific's relative size and was based on chemical signatures, an ability that this species shares with a few other aquatic invertebrates. From our results, olfaction appeared to be the dominant sensory channel in P. longicarpus ' binary discrimination, but the combination of two signal components from visual and olfactory channels resulted in the enhancement of the response displayed by the receiver. Besides, crabs reacted differently when exposed to their own odour than to the odour of familiar (as well as unfamiliar) conspecifics, suggesting that recognition in this species can be more refined than a binary discrimination and that chemical ‘badges’ may be attributes of individual crabs.
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Foraging selectivity of the predator Octopus digueti on the hermit crab Pagurus albus was investigated in the laboratory. The animals were collected from the lower tidal regions of Bahia Cholla, Sonora, Mexico in the northern Gulf of California. Hermit crab shell‐preference experiments with the field‐predominant gastropod shell Cerithium stercusmuscarum established a relationship of ideal‐fit which served as a reference guide in preparing presentation trials. A tether line attached to the shells of three hermit crabs was used to achieve simultaneous encounters with an octopus. Feeding preference was determined by noting which of the hermit crabs was first extracted from it's shell. Presentations of three equal‐sized hermit crabs in different‐sized shells resulted in 70% preference for tight‐fitted hermit crabs. Presentations of three different‐sized hermit crabs each in ideal‐fitted shells resulted in 90% preference for the largest hermit crabs.
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Anomura
octopus (software)
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