Antennulary sensory organs in cyprids of Octolasmis and Lepas (Crustacea: Thecostraca: Cirripedia: Thoracica): a scanning electron microscopic study

Cypris larvae of the pedunculate barnacles Octolasmis angulata (Poecilasmatidae), Lepas australis, L. pectinata, and Dosima fascicularis (Lepadidae) were studied with scanning electron microscopy, focusing on the sensory setae and the attachment disc on the antennules. The antennules of O. angulata did not exhibit any remarkable trait, but carry the same number of setae as seen in most other thoracicans. The third segment is bell-shaped and quite distinct from the second and its attachment disc is surrounded by a skirt. We found several potential synapomorphies in antennulary morphology between cyprids of the lepadid species but none of them were shared with the cyprids of Octolasmis; the list of unique lepadid characters includes: one additional, preaxial seta on the second segment; multiple similar (up to eight) postaxial setae (PS3) on the third segment, unlike all other thoracicans, where there is only a single PS3; the third segment consists almost entirely of the attachment disc, which is distended and surrounded by two parallel rows of radial setae; on the fourth segment the terminal seta E is diminutive. We found no traits in cyprids of Octolasmis that seem to be adaptations to their attachment site within the branchial chamber of swimming crabs and, in particular, no similarities with cyprids of rhizocephalan barnacles, many of which also attach in the gill chamber. The synapomorphies between cyprids of the lepadid species may be adaptations to their life in the neuston.     

A new species of Tigriopus (Copepoda, Harpacticoida, Harpacticidae) from Thailand with the description of its naupliar development

Both genders of Tigriopus thailandensis sp. nov. are described from a laboratory stock raised from individuals collected from the seaweed Enteromorpha clathrata in Thailand (Bangsaen Beach, Chonburi Province). Tigriopus thailandensis sp. nov. shares with its closest relative T. japonicus Mori, 1932 two setae on the third exopodal segment of leg 4 while other congeners bear 3 inner setae. However, allobasis and exopod of antenna in both genders are much more slender and elongate than in T. japonicus. All six naupliar stages of T. thailandensis are described from the offspring of isolated females. In comparison with nauplii of T. japonicus, T. thailandensis nauplii are characterized by the following: a smaller body size throughout the naupliar phase; first antennular segment without seta, second antennular segment with only one small seta plus two longer setae; third antennular segment with additional spinules from naupliar stage II onwards; antenna bears three small spinules on the terminal exopodal segment; one additional seta on the anterior surface of the antennary basis, tubular endopod of antenna with one tiny seta midlength at naupliar stage III that increases in size; mandibular basis with several spinules on anterior surface; mandibular coxa with one spinulose seta that is smooth in T. japonicus.     

The design of the fly adhesive pad: distal tenent setae are adapted to the delivery of an adhesive secretion

Flies (Brachycera) have adhesive pads called pulvilli at the terminal tarsomere. The pulvilli are covered by tenent setae, sometimes termed tenent hairs, which serve to increase the actual area of attachment to the surface. By using transmission and scanning electron microscopy it is shown that proximal and distal tenent setae have different ultrastructures. The design of distal adhesive setae is adapted for the release of adhesive substances close to the area of contact. It is concluded that secretion injection is precisely targeted under the distal tip of a single seta.     

On the morphology of antennular sensory and attachment organs in cypris larvae of the deep‐sea vent/seep barnacles,Ashinkailepas and Neoverruca

Barnacle cypris larvae show high morphological variation in the organs used in search of and attaching to a substratum. This variation may represent adaptation to the habitat of the species. Here, we studied SEM level morphologies of cypris antennular sensory and attachment organs in a deep‐sea vent endemic species (Neoverruca sp.) and a vent/seep inhabiting species (Ashinkailepas seepiophila). We compare them with three species from other environments. The antennular morphologies of Neoverruca sp. and A. seepiophila were similar, which is consistent with recent molecular studies showing a close relationship of the two species. The setation pattern of the antennules was very conservative among species from various environments. In contrast, striking differences were observed in the structure of the attachment organ (the third antennular segment). Neoverruca sp. and A. seepiophila had no velum or a skirt surrounding the attachment disc on the third segment, while other cirripede cyprids almost always have either of these structures. In addition, both cyprids of A. seepiophila and Neoverruca sp. had the attachment disc angled toward the substratum, whereas it faces distally in cyprids from hard bottom inhabiting barnacles. We suggest that both velum/skirt and the angle of the attachment disc play an important role, when the antennules are contacting the substratum during surface exploration. Differences in attachment organ structures may be highly adaptive, enabling cirripede species to enter new habitats during evolution. J. Morphol. 277:594–602, 2016. © 2016 Wiley Periodicals, Inc.     

Cypris larvae of acrothoracican barnacles (Thecostraca: Cirripedia: Acrothoracica)

We used SEM to investigate the morphology of the cypris larvae from a range of species of the Cirripedia Acrothoracica, representing all three families and including the first detailed account of cyprids in the highly specialized Cryptophialidae. Special attention was given to the head shield (carapace), the lattice organs, the antennules, the thoracopods, the telson and the furcal rami. The cypris larvae of the Acrothoracica fall into two morphological groups; those of the Trypetesidae and Lithoglyptidae have a well-developed carapace (head shield) that can completely enclose the body and sports fronto-lateral pores, numerous short setae and lattice organs perforated by numerous small, rounded pores and a single, conspicuous terminal pore. The fourth antennular segment has the setae arranged in subterminal and terminal groups. There is a developed thorax with natatory thoracopods and a distinct abdomen and telson. In comparison, the cyprids of the Cryptophialidae exhibit apomorphies in the morphology of the carapace, the antennules and the thorax, mostly in the form of simplifications and reductions. They have a much smaller head shield, leaving parts of the body directly exposed. The shield is conspicuously ornamented by deep pits and hexagonally arranged ridges and bears a few, very long setae but lacks fronto-lateral pores. The lattice organs have numerous elongated pores, but no large, terminal pore. The fourth antennular segment has all the setae clustered in one terminal group. The thorax and thoracopods are rudimentary and not suitable for swimming. These reductions and simplifications in morphology correlate with cryptophialid cyprids being unable to swim. They can only disperse by antennular walking resulting in small, but highly gregarious populations of adults. The variations in antennular morphology and telson structure were traced for the genera of the families Lithoglyptidae and Trypetesidae. The traditional non-cladistic taxonomy in the suborders Pygophora (Cryptophialidae+Lithoglyptidae) and Apygophora (Trypetesidae) was based largely on symplesiomorphies in adult morphology and cannot be upheld. The Lithoglyptidae and Trypetesidae may form a monophylum, but evidence remains scarce. We expect that the use of larval (cyprid) characters will in the future play an important part in more detailed phylogenetic analyses of the Acrothoracica and also shed new light on their reproductive ecology.     

Description of Eucyclops tziscao sp. n., E. angeli sp. n., and a new record of E. festivus Lindberg, 1955 (Cyclopoida,Cyclopidae, Eucyclopinae) in Chiapas,Mexico

Two new species of the freshwater cyclopoid genera Eucyclops are described, Eucyclops tziscao sp. n. and E. angeli sp. n. Both species belong to the serrulatus-group defined by morphological features such as: the presence of distal spinules or hair-like setae (groups N1 and N2) on frontal surface of antennal basis; the fourth leg coxa with a strong inner spine that bears dense setules on inner side, yet proximally naked (large gap) on outer side; and a 12-segmented antennule with smooth hyaline membrane on the three distalmost segments. Eucyclops tziscao sp. n. is morphologically similar to E. bondi and E. conrowae but differs from these species in having a unique combination of characters, including a caudal ramus 4.05±0.25 times as long as wide, lateral seta of Enp3P4 modified as a strong, sclerotized blunt seta, coxal spine of fourth leg with inner spinule-like setules distally, and sixth leg of males bearing a strong and long inner spine 2.3 times longer than median seta. Eucyclops angeli sp. n. can be distinguished by an unique combination of morphological features: the short caudal ramus; the long spine on the sixth antennular segment of A1; the presence of one additional group of spinules (N12’) on the caudal surface of A2; the presence of long setae in females, or short spinules in males on the lateral margin of fourth prosomite; the strong ornamentation of the intercoxal sclerite of P4, specially group I modified as long denticles; the distal modified setae of Exp3P3 and Exp3P4 in females and males; and the short lateral seta of P5. Finally, we report on a new record of E. festivus in México, and add data on morphology of the species.     

Role of maxilla 2 and its setae during feeding in the shrimp Palaemon adspersus (Crustacea: Decapoda)

The movements of the basis of maxilla 2 in Palaemon adspersus were examined using macro-video recordings, and the morphology of its setae was examined using both scanning and transmission electron microscopy. The basis of maxilla 2 performs stereotypical movements in the latero-medial plane and gently touches the food with a frequency of 3-5 Hz. The medial rim of the basis of maxilla 2 carries three types of seta. Type 1 is serrate, type 2 and 3 are serrulate, and type 2 has a prominent terminal pore. Type 2 is innervated by 18-25 sensory cells whose cilia protrude through the terminal pore and are in direct contact with the external environment. The structure of type 2 setae indicates that they are mainly gustatory, although still bimodal due to their innervation by presumed chemosensory and mechanosensory neurons. Distally, the three types of setae have a complex arrangement of the cuticle involving water-filled canals, which may serve to improve flexibility. Type 1 and 3 setae have fewer sensory cells (4-9) but probably also have a bimodal sensory function. The function of type 1 setae is probably to protect type 2 setae, while type 3 setae might serve to groom the ventral side of the basis of maxilla 1.     

First report of antennular attachment organs in a barnacle nauplius larva

Larvae released from Newmaniverruca albatrossiana were cultured in the laboratory until the cypris stage. The brood size of individuals was low, about 60 larvae per brood. The exact number of instars was not determined. Early instars had the morphology normally seen in lecithotrophic nauplii of thoracican cirripedes. They had uniramous antennules with a few apical setae and biramous antennae and mandibles equipped with natatory setae only. Neither antennae nor mandibles carried any enditic spines or setae and the mouth cone was diminutive. The last nauplius stage obtained in our cultures was typical except in the structure of antennules. The head shield was enlarged but not flexed down, the antennae and mandibles were virtually unchanged from earlier instars, and the ventral thoracic process was well developed but without any external appendages. In contrast, the antennules had the complex shape and segmentation otherwise seen only in cypris larvae, where they are used for bipedal walking on the substratum in search of a settlement site. The similarity included the specialized shape of the first two antennular segments and the specialization of the third as an attachment organ. Nauplii just prior to this last instar had simple, straight antennules but completely lacked setae and instead terminated bluntly in what appears to be an incipient attachment organ. The presence of cypris-like antennules in late nauplii has not previously been recorded in cirripedes. We suggest that this will allow the larvae to attach on the substratum temporarily before they reach the cypris instar and this will increase the chance of settling successfully on their rare substratum (sea urchin spines). The specialization in late N. albatrossiana nauplii will therefore decrease mortality during the larval phase and thus counterbalance the very low breeding potential in this deep-sea species.     

The peripheral and central antennular pathway of the Caribbean stomatopod crustacean Neogonodactylus oerstedii

Although stomatopod crustaceans use their chemical senses in many facets of behavior, little is known about their chemosensory neural pathways, especially in comparison to the better-studied decapod crustaceans. We examined the stomatopod Neogonodactylus oerstedii to determine organizational aspects of peripheral and central neural pathway of antennules, which is a major chemosensory organ. We describe the three flagella of the triramous antennule as the medial, dorsolateral, and ventrolateral flagella. The primary branch point is between the medial flagellum and lateral flagella, and the secondary branch point is at the junction of the dorsolateral and ventrolateral flagella. The antennule bears at least three types of setae, based on their external morphology. Simple setae are present only on the medial flagellum and ventrolateral flagellum, organized as a tuft of 10-15 setae on each flagellar annulus. Aesthetasc setae and asymmetric setae occur only on the distal annuli of the dorsolateral flagellum, with each annulus bearing a row of three aesthetascs and one asymmetric seta. DiI fills of the antennular nerve near the junction of the flagella show that sensory neurons in the antennular flagella project to two neuropils in the ipsilateral midbrain-the olfactory lobe (OL) and lateral antennular neuropil (LAN). The OL is glomerular and has rich serotonergic innervation, a characteristic of the OL in decapods. The LAN is bi-lobed and stratified as it is in decapods. However, the LAN of stomatopods differs from that of decapods in being relatively large and containing extensive serotonergic innervation. The median antennular neuropil of stomatopods has sparse serotonergic innervation, and it is more diffusely organized compared to decapods. No accessory lobes were found in N. oerstedii. Thus, the stomatopod antennular flagella have the same two, highly organized parallel pathways common to decapods-the OL pathway and the LAN pathway.     

Antennular sensilla of the brine shrimp, Artemia salina.

1. Scanning electron microscopy was used to characterize the external morphology of setae found on the antennules of adults and nauplii of the brine shrimp, Artemia salina (L.). The permeability of the antennular setae was studied by means of Slifer's crystal violet method. 2. Each antennule of an adult brine shrimp possessed a terminal cluster of sensory setae. Within a cluster there were two morphologically distinct kinds of sensilla, here designated type 1 and type 2. Three type 1 sensilla were observed on every antennule examined. The number of type 2 sensilla per antennule was usually four or five. 3. Type 1 sensilla of adults were 43 to 80 micrometer long and simple in external morphology. They were widest at the base, decreased in diameter gradually, and terminated as a finely tapered tip. No pores were resolved by scanning electron microscopy. 4. Type 2 sensilla of adults were shorter (shaft length, 12 to 23 micrometer) and displayed a single pore at the tip (average pore diameter, 0.4 micrometer). In thin section they were seen to possess a distinctive articular specialization of the cuticle at the base of the seta. 5. Dye penetration experiments indicated that type 2 sensilla were permeable to aqueous crystal violet, whereas type 1 sensilla were not. 6. The antennular setae of nauplii resembled type 1 sensilla in general shape, in being impermeable to crystal violet, and in lacking a terminal pore and basal articular specialization. Moreover, a total of three setae was normally present on each naupliar antennule, and the same number of type 1 sensilla was found on each adult antennule examined. If the three naupliar setae represent a developmental stage in the formation of three adult sensilla, available observations suggest that the larval setae are developmentally related to type 1, rather than to type 2 adult sensilla.     

Probing the settlement signals of Amphibalanus amphitrite

To achieve their reproductive potential, barnacles combine tactile exploration of surface structural properties and integration of cellular signals originating from their antennular sensory setae within a developmentally defined, temporally narrow window of settlement opportunity. Behavioural assays with cyprids coupled with biometric analysis of scanning electron microscopy-acquired images in the presence of specific chemical compounds were used to investigate how settlement on a substratum is altered in response to the presence of these compounds. Impeding tactile exploration was shown which altered cellular signalling and/or induced malformation of anatomical features of the antennular sensory setae, which disrupted the settlement behaviour of the model barnacle species Amphibalanus amphitrite. It is concluded that surface exploration by the cyprids relies on mechanical and nociception-related and calcium-mediated signals while a protein kinase C signalling cascade controls the timely metamorphosis of the cyprids to sessile juveniles.     

The ultrastructure of the so-called olfactory setae on the antennula ofDaphnia magna Straus (Crustacea,Cladocera)

A group of nine sensory setae is found on the tip of the antennula ofDaphnia magna in both sexes. Inside a seta four dendrites are situated, each with one receptor cilium. The receptor cilia extend through a liquor space into the exterior part of the seta. The exterior part of the liquor space is divided from the interior part by a knob-like thickening of the innermost layer of the epicuticle, the basal bead. The basal bead narrows the liquor space and the receptor cilia. The interior part of the liquor space is surrounded by five sheath cells, the exterior part by a thin cuticle. In the exterior part the receptor cilia branch partly and reach a terminal pellet on the tip of the seta. The terminal pellet is a thickened part of the epicuticle. It is permeable to several dissolved substances. It is the exterior part of the receptor that projects over the tip of the antennula and seems to be the entire seta. During the premoult the fifth sheath cell builds up the articulation of the seta, the fourth the basal bead, and the third the shaft of the seta. The first sheath cell forms the cuticular sheath. The organ seems to be a chemoreceptor, but the adequate stimulus is as yet unknown.     

A new species of Peltidium Philippi, 1839 (Crustacea,Copepoda, Harpacticoida) from the Pacific coast of Mexico

During the analysis of phytal meiobenthic samples collected from a rocky-sandy beach in the state of Nayarit, in the Mexican Pacific, several specimens of harpacticoid copepods were obtained and taxonomically examined. These specimens were found to represent an undescribed species of the peltidiid genus Peltidium Philippi, 1839. The new species, Peltidium nayarit sp. n. is described herein. It resembles Peltidium nichollsi Geddes and Peltidium lerneri Geddes from Bahamas but also the widespread Peltidium speciosum Thompson & Scott and Peltidium purpureum Philippi. The new species from the Mexican Pacific differs from its known congeners by its possession of a unique combination of characters, including a modified pectinate seta on the antennary exopod, three terminal setae on the second endopodal segment of leg 1, third exopodal segment of leg 1 with three elements, inner terminal claw twice as long as outer claw, female fifth leg with 5 exopodal setae, exopodal setae I-III stout, spinulose and seta IV being as long as seta V. This is the second species of the family known to be distributed in the Eastern Tropical Pacific and in Mexico. Pending additional data, the distribution of this species appears to be restricted to this area of the Mexican Pacific.     

The modality of the dominance signal in snapping shrimp (Alpheus heterochaelis) and the corresponding setal types on the antennules

Previous behavioural experiments showed that snapping shrimp lacking lateral antennular filaments, i.e. without chemosensory aesthetascs, lose the ability to distinguish between conspecifics that are inexperienced in fighting and former winners. A chemosensory dominance signal was assumed to be present, although other receptors unique to the lateral filaments may have been responsible for the behavioural changes. In the present study, the antennules of snapping shrimp were examined for differences between the lateral and medial antennule filaments to identify the modality of the dominance signal. We found six different types of setae and two types of pores. A new probably bimodal setal type is described, the broad long simple seta. Only the chemosensory aesthetascs and their associated hydrosensory companion setae are unique to the lateral filament. Thus we conclude that the dominance signal is chemical, because a hydrodynamic signal would be also received by the simple setae distributed on both filaments.     

The structure of anoline (Reptilia: Dactyloidae: Anolis) toe pads in relation to substratum conformity

Adhesive toe pads of geckos house modified components of vascular and/or connective tissues that promote conformity of the setal fields with the locomotor substratum. Similar modifications have been claimed for the digits of Anolis, but evidence for them is not compelling. Angiographic and histological investigations of Anolis failed to identify any evidence of either an intralamellar vascular reticular network or a central sinus. Instead, their vascularity more closely resembles that of lizards in general than that of pad‐bearing geckos. The loose connective tissue of the toe pads likely contributes to their general pliability and flexibility, promoting localized compliance with the substratum. Through the shedding cycle, the lamellae change shape as the replacing setae elongate. The outer epidermal generation lacunar cells on the inner lamellar faces simultaneously hypertrophy, providing for compatibility between overlapping lamellae, enabling reciprocity between them. This contributes to continuing compliance of the setal fields with the substratum. Overall, digital structure and attachment and release kinematics of the toe pads of Anolis are very similar to those of geckos exhibiting an incipient adhesive mechanism. Both lack major anatomical specializations for promoting conformity of the setae with the locomotor substratum beyond those of the seta‐bearing portions of the epidermis.     

Fine structural analysis of the fibrillar adhesion apparatus in ladybird beetle

The attachment system on the ladybird beetle Harmonia axyridis is composed of a pair of pretarsal claws and adhesive pads at the tarsal segments. The claws, which are connected to the pretarsal segment, are mainly used to hold the rough substrates by their apical diverged hooks. In contrast, the adhesive pads have an adhesive function when landing on smooth surfaces. They are interspersed at the ventral adhesive pad of each tarsomere, and are composed of two kinds of hairy setae. The discoid tip seta (DtS) is located at the central region of each adhesive pad. The DtS has a spoon‐shaped endplate with a long and narrow shaft. In contrast, the pointed tip seta (PtS) is interspersed along the marginal regions of each adhesive pad, and has a hook‐shaped spine near the tip. In the present study, we found numerous fine cuticular pores beneath the setae, which seem to be related to the secretion of some adhesive fluids. It may be deduced that ladybird beetles can attach to smooth surfaces more effectively by employing adhesive fluids filling in surface crevices to overcome problems cause by their larger size endplates.     

The modality of the dominance signal in snapping shrimp (Alpheus heterochaelis) and the corresponding setal types on the antennules

Previous behavioural experiments showed that snapping shrimp lacking lateral antennular filaments, i.e. without chemosensory aesthetascs, lose the ability to distinguish between conspecifics that are inexperienced in fighting and former winners. A chemosensory dominance signal was assumed to be present, although other receptors unique to the lateral filaments may have been responsible for the behavioural changes. In the present study, the antennules of snapping shrimp were examined for differences between the lateral and medial antennule filaments to identify the modality of the dominance signal. We found six different types of setae and two types of pores. A new probably bimodal setal type is described, the broad long simple seta. Only the chemosensory aesthetascs and their associated hydrosensory companion setae are unique to the lateral filament. Thus we conclude that the dominance signal is chemical, because a hydrodynamic signal would be also received by the simple setae distributed on both filaments.     

Morphology and histology of secretory setae in terrestrial larvae of biting midges of the genus Forcipomyia (Diptera: Ceratopogonidae)

Apneustic larvae of the genus Forcipomyia possess unique secretory setae located on the dorsal surface along the body in two rows, one pair on each thoracic and abdominal segment and two pairs on the head. Morphological and histological studies of secretory setae in fourth instar larvae of Forcipomyia nigra (Winnertz) and Forcipomyia nigrans Remm indicate they are modified mechanoreceptors (sensilla trichodea) in which the trichogen cell is a glandular cell producing a hygroscopic secretion. The cytoplasm of the glandular trichogen cell fills the lumen of a secretory seta, which shows one or more pores on the apex. The cytoplasm contains numerous microtubules responsible for transportation of proteinaceous vesicles, and an extremely large polyploid nucleus typical of gland cells. The main role of the hygroscopic secretion is to moist the body and thus facilitate cuticular respiration.