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![Metamorphosis in the Facetotecta and Rhizocephala. A. Y-cyprid (Facetotecta) has metamorphosed into the slug-shaped ypsigon stage, leaving behind the empty cuticle of the cyprid (adapted from [35]). B. A cyprid of Sacculina carcini (Rhizocephala) settled on a crab has formed an injection stylet through which it is now injecting the slug-shaped vermigon into the haemocoelic fluid of the host. The specimen was removed in vivo from the host cuticle which would have been at the dotted line. Photo by J. Høeg and H. Glenner.](profile/Keith-Crandall/publication/24305881/figure/fig2/AS:213384466309123@1427886229296/Metamorphosis-in-the-Facetotecta-and-Rhizocephala-A-Y-cyprid-Facetotecta-has.png)
Metamorphosis in the Facetotecta and Rhizocephala. A. Y-cyprid (Facetotecta) has metamorphosed into the slug-shaped ypsigon stage, leaving behind the empty cuticle of the cyprid (adapted from [35]). B. A cyprid of Sacculina carcini (Rhizocephala) settled on a crab has formed an injection stylet through which it is now injecting the slug-shaped vermigon into the haemocoelic fluid of the host. The specimen was removed in vivo from the host cuticle which would have been at the dotted line. Photo by J. Høeg and H. Glenner.
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The Thecostraca are arguably the most morphologically and biologically variable group within the Crustacea, including both suspension feeders (Cirripedia: Thoracica and Acrothoracica) and parasitic forms (Cirripedia: Rhizocephala, Ascothoracida and Facetotecta). Similarities between the metamorphosis found in the Facetotecta and Rhizocephala sugges...
Citations
... Y-adults and their manner of producing and liberating larvae still await discovery. It is thought that the ypsigon may be an infective stage, with the adults being endoparasitic in as yet unknown hosts; such a life cycle would be comparable to that of Rhizocephala, parasitic barnacles that infect other crustaceans (Glenner et al. 2008;Pérez-Losada et al. 2009). In fact, Facetotecta are firmly established, along with Ascothoracida, as relatives of Cirripedia (barnacles) within Thecostraca, based on both morphological and molecular data (Grygier 1985;Pérez-Losada et al. 2012;Petrunina et al. 2013). ...
The enigmatic ‘y-larvae’ (Thecostraca: Facetotecta) are microscopic marine planktonic crustaceans that were discovered more than a century ago, yet to this day their adults remain unknown. Despite occurring locally in large diversities, and therefore presumably being of ecological importance, only 17 species have been described globally, rendering it practically impossible to identify any y-larval specimen from any locality. The fact that species have been based on different life stages (nauplii and/or cyprids) further hampers identification. Y-larvae include many forms with planktotrophic (feeding) nauplii and even more with lecithotrophic (non-feeding) nauplii. At one coral-reef locality on the shore of Sesoko Island (Okinawa, Japan), extensive fieldwork in 2018 and 2019 confirmed an enormous taxonomic diversity of y-larvae there. Here, we present morphological diagnoses and an identification key for 34 lecithotrophic y-naupliar types (or morphospecies), which will correspond minimally to the same number of species when described. As a temporary measure, all are referred to by alphabetical parataxonomic designations, except for three that have been formally described already within the genus Hansenocaris Itô, 1985. To this should be added an additional 7–9 planktotrophic y-naupliar morphospecies, which are only treated briefly. Most often, y-larval taxonomy has been based on the cyprid stage, but the large morphological diversity of y-nauplii suggests that nauplii are at least as important for taxonomy. Lecithotrophic y-nauplii display a multitude of body shapes, the form-evolution of which is discussed here with reference to a recent molecular phylogeny of Facetotecta partly based on material from the same site. An indirect estimate of the relative abundances of all 34 lecithotrophic y-naupliar morphospecies is presented, based on laboratory-reared final-instar specimens. This treatment is intended as a step towards a proper taxonomy and a revised classification of Facetotecta, which will involve detailed descriptions of both nauplii and cyprids. Until such work progresses, the present overview of the y-naupliar fauna of a single Okinawan locality known to be a hotspot of y-larval diversity is offered as a baseline for further surveys of Facetotecta elsewhere in the Indo-West Pacific and beyond.
... These larvae develop through a series of dispersive stages (y-nauplii : Itô 1986;Kolbasov et al. 2021;Dreyer et al. 2023a) and a single putative attachment stage (y-cyprid: Kolbasov et al. 2022;Dreyer et al. 2023a). A subsequent stage called the ypsigon, which lacks segmented appendages, a gut and compound eyes, has been induced through in vivo exposure of y-cyprids to the crustacean moulting hormone (Glenner et al. 2008;Pérez-Losada et al. 2009;Dreyer et al. 2023a). Having shed the y-cyprid's cuticle, the ypsigon is tentatively regarded as an early instar or juvenile of a hypothetical endoparasitic adult stage (Pérez-Losada et al. 2009;Dreyer et al. 2023a). ...
... A subsequent stage called the ypsigon, which lacks segmented appendages, a gut and compound eyes, has been induced through in vivo exposure of y-cyprids to the crustacean moulting hormone (Glenner et al. 2008;Pérez-Losada et al. 2009;Dreyer et al. 2023a). Having shed the y-cyprid's cuticle, the ypsigon is tentatively regarded as an early instar or juvenile of a hypothetical endoparasitic adult stage (Pérez-Losada et al. 2009;Dreyer et al. 2023a). The y-larval life cycle thus envisaged is reminiscent of that observed in parasitic barnacles (Cirripedia: Rhizocephala; Pérez-Losada et al. 2009;Dreyer et al. 2023a). ...
... Having shed the y-cyprid's cuticle, the ypsigon is tentatively regarded as an early instar or juvenile of a hypothetical endoparasitic adult stage (Pérez-Losada et al. 2009;Dreyer et al. 2023a). The y-larval life cycle thus envisaged is reminiscent of that observed in parasitic barnacles (Cirripedia: Rhizocephala; Pérez-Losada et al. 2009;Dreyer et al. 2023a). ...
Despite discovery more than 100 years ago and documented global occurrence from shallow waters to the deep sea, the life cycle of the enigmatic crustacean y-larvae isincompletely understood and adult forms remain unknown. To date, only 2 of the 17 formally described species, all based on larval stages, have been investigated using an integrative taxonomic approach. This approach provided descriptions of the morphology of the naupliar and cyprid stages, and made use of exuvial voucher material and DNA barcodes. To improve our knowledge about the evolutionary history and ecological importance of y-larvae, we developed a novel protocol that maximises the amount of morpho-ecological and molecular data that can be harvested from single larval specimens. This includes single-specimen DNA barcoding and daily imaging of y-nauplii reared in culture dishes, mounting of the last naupliar exuviae on a slide as a reference voucher, live imaging of the y-cyprid instar that follows, and fixation, DNA extraction, amplification and sequencing of the y-cyprid specimen. Through development and testing of a suite of new primers for both nuclear and mitochondrial protein-coding and ribosomal genes, we showcase how new sequence data can be used to estimate the phylogeny of Facetotecta. We expect that our novel procedure will help to unravel the complex systematics of y-larvae and show how these fascinating larval forms have evolved. Moreover, we posit that our protocols should work on larval specimens from a diverse array of moulting marine invertebrate taxa.
... Moreover, depending on the variation in host types due to host-switching events between distantly related hosts, a wide range of morphological adaptations might evolve in the symbiont species, resulting in a relatively high degree of morphological variation in these clades (Ghalambor et al., 2007;Joy, 2013;Kise et al., 2023;Munday et al., 2004;Sapp, 1994). Convergent evolution may also play a role, if similar selection pressures are at play in different clades of symbionts, resulting in the same, or similar (analogous or homologous) adaptations (Goto et al., 2012;Horká et al., 2018;Kise et al., 2023;Li et al., 2018;Pérez-Losada et al., 2009;Poulin & Randhawa, 2015). ...
Symbiotic species, living within or on the surface of host organisms, may evolve a wide range of adaptations as a result of various selection pressures, host specificity of the symbiont and the nature of the symbiosis. In tropical marine coral reef ecosystems, palaemonid shrimps (Crustacea: Decapoda: Caridea) live in association with at least five different invertebrate phyla. Host switches between (distantly) related host groups, and the thereby associated selection pressures were found to play a major role in the diversification of these shrimp lineages, giving rise to various host-specific adaptations. Two lineages of palaemonid shrimp, which have switched from an ectosymbiotic association towards endosymbiosis, are studied for their morphological diversification and possible convergence. Special attention is given to the between-phyla host switches involving ascidian and bivalve hosts, which are characteristic for these lineages. Using landmark-based (phylo)morphospace analyses and Scanning Electron Microscopy, the walking leg dactylus shape and the microstructures on these dactyli are studied. No specific bivalve-or ascidian-associated morphotypes were found, but morphological convergence in dactylus morphology was found in various species within the two studied clades with similar host groups. In addition, multiple lineages of bivalve-associated species appear to be morphologically diverging more than their ascidian-associated relatives, with 'intermediate' morphotypes found near host-switching events.
... Over the past 5 years, our research group has handled more than 10,000 y-larval specimens from various Pacific locations (primarily Okinawa, Japan; Dreyer et al. 2023). With the growing phylogenetic and genomic interest in barnacles (Kim et al. 2019;Nunez et al. 2021;Ip et al. 2021;Bernot et al. 2022), as well as the realisation that the biological diversity of y-larvae is markedly larger than what had previously been appreciated (Glenner et al. 2008;Pérez-Losada et al. 2009;Grygier et al. 2019;Dreyer et al. 2023; data herein), we believe the time is ripe for a review that synthesises current knowledge on this enigmatic group. We first present a short historical overview of the study of y-larvae, then review their systematics before outlining in detail the structural features of each known functional phase of the life history in the order in which they appear during development (y-nauplii, y-cyprid, ypsigon; Figure 3). ...
... A duct extends from at least one large glandular system to a large pore located on the ventral side of the wider basal part of the labrum ( Figure 7H; see also Grygier 1987a), similarly to a labral gland and pore in y-nauplii (Elofsson 1971). The function of the glandular secretion is unknown, but inasmuch as the ypsigon stage (see below) exits the cyprid cuticle exactly at the mid-point between the antennules and the labrum's hooked spines (Figure 7; see also Glenner et al. 2008;Pérez-Losada et al. 2009;Dreyer et al. 2021), the production of enzymes that dissolve or soften the host integument would be in line with the general belief that those spines play a role in piercing and penetrating the host. ...
... Despite conflicting tree topologies, parasitism likely arose independently several times in Thecostraca Rees et al. 2014). The putative adult stage of the y-larvae has never been found or recognised, but accumulated evidence about the facetotectan life cycle suggests that the adults are likely to be highly specialised (endo)parasites in unknown hosts (Glenner et al. 2008;Pérez-Losada et al. 2009;data herein). A detailed comparison of the life-history stages involved in the transition from a planktonic to a parasitic existence between facetotectans and other parasitic thecostracans could have wide implications for understanding the evolution of parasitism in barnacles. ...
Resolution of recalcitrant nodes in the Tree of Life has been substantially eased recently by increased worldwide sampling and advancements in sequencing technology. It has become routine to use molecular data to characterise and taxonomically allocate tens to hundreds of taxa based on the DNA occurring in a few drops of water. Despite this, the adult stages of one invertebrate taxon, the enigmatic crustacean ‘y-larvae’ (Thecostraca: Facetotecta), have never been found, and the true diversity of this group has long remained unknown. Here, we review the current state of our knowledge concerning these mysterious larval forms and provide a significant body of new morphological and ecological data to make y-larvae accessible to a wider community of biologists. After summarising the history of y-larva studies, we review the current state of facetotectan systematics and outline in detail the structural fea- tures of each known phase in the life history, from y-nauplius through y-cypris to ypsigon. In particular, we document a suite of new ultrastructural details of the ypsigon, the putatively invasive phase if, as suspected, the adults are parasitic. Scanning and transmission electron microscopy are used to show that after a moult the ypsigon inherits numerous structures directly from the preceding y-cypris larva, including parts of the nervous system, sensory organs, pores, and antennal musculature. A comparison of the ypsigon to the very similar, yet surely independently evolved and thus merely analogous invasive stages of parasitic barnacles (Rhizocephala) provides a broader phylogenetic and functional context for these findings. These structural traits agree with molecular phylogenetic data placing Facetotecta as an early- branching sister clade to the cirripede barnacles, including Rhizocephala. We then review the ecology and biogeography of y-larvae in the global plankton and offer a comprehensive map and list of recorded localities. Finally, we statistically test the abundance of the different life-history stages of y-larvae in the surface plankton at one of our primary study sites (Sesoko Island, Okinawa, Japan) in relation to various environmental factors that may drive their occurrence there. We found evidence of crepuscular emer- gence around dawn (7 am–9 am) and dusk (5 pm–7 pm). This review is the most comprehensive synthesis of information on Facetotecta to date. Despite our continued ignorance of the adults, we hope it will serve both as a starting point for future scientists embarking on studies of this challenging group of crustaceans and as an inspiration for those working on other kinds of planktonic larvae.
... Although previous workers have hypothesized a diversity extending to >100 species globally (Hansen, 1899), or 40 or more species from Okinawa alone (Glenner et al., 2008), a mere 17 species of Facetotecta have been formally described , all within the genus Hansenocaris. Pérez-Losada et al. (2009) sequenced three nuclear markers of seven unvouchered putative species, and while this and other studies have demonstrated a close affinity of y-larvae with the barnacles (Grygier, 1987;Høeg et al., 2009;Chan et al., 2021), the lack of voucher specimens has impeded rigorous taxonomic resolution within Facetotecta. ...
... The ypsigon is a worm-like, unsegmented larval stage that lacks typical arthropodal characteristics except for an exceedingly thin epicuticle bearing spinules and setae (Glenner et al., 2008;Dreyer et al., in press). Ypsigons can crawl only for short distances and are never found in plankton samples, which together with the similarities to the infective vermigon stage of rhizocephalans, lends credence to the notion that y-larvae become specialized endoparasites in unknown hosts (Glenner et al., 2008;Pérez-Losada et al., 2009;Dreyer et al., in press). ...
... The cyprid stage has featured prominently in resolving phylogenies of other barnacles Høeg and Rybakov, 2007;Glenner et al., 2010) and has been used to considerably greater effect than nauplii as the basis for formal taxonomy in Facetotecta . Y-cyprids exhibit a phenotypically complex morphology with potentially a greater number of informative characters than nauplii, but morphology-based cladistic analyses of y-cyprid characters have until now failed to offer sufficient resolution to resolve species-level splits in Facetotecta Pérez-Losada et al., 2009). The extent to which this instar offers supporting/diagnostic characters for the larger clades revealed here by molecular systematic means should be explored, since y-nauplii do not offer much morphological corroboration of these clades. ...
Resolving the evolutionary history of organisms is a major goal in biology. Yet for some taxa the diversity, phylogeny, and even adult stages remain unknown. The enigmatic crustacean "y-larvae" (Facetotecta) is one particularly striking example. Here we use extensive video-imaging and single-specimen molecular sequencing of >200 y-larval specimens to comprehensively explore for the first time their evolutionary history and diversity. This integrative approach revealed five major clades of Facetotecta, four of which encompass a considerable larval diversity. Whereas morphological analyses recognized 35 y-naupliar "morphospecies", molecular species delimitation analyses suggested the existence of between 88 and 127 species. The phenotypic and genetic diversity between the morphospecies suggests that a more elaborate classification than the current one-genus approach is needed. Morphology and molecular data were highly congruent at shallower phylogenetic levels, but no morphological synapomorphies could be unambiguously identified for major clades, which mostly comprise both planktotrophic and lecithotrophic y-nauplii. We argue that lecithotrophy arose several times independently whereas planktotrophic y-nauplii, which are structurally more similar across clades, most likely display the ancestral feeding mode of Facetotecta. We document a remarkably complex and highly diverse phylogenetic backbone for a taxon of marine crustaceans, the full life cycle of which remains a mystery.
... Morphological studies based on wild-caught specimens have proven unsuccessful in delimiting species within geographically widespread "types" such as type IV (Hansen, 1899;Schram, 1972), "Pacific Type I" (Itô, 1986), and type VIII with its three subtypes VIII-a, -b, and -c (Itô, 1987), which are each practically identical wherever found. Formal cladistic analyses using morphological characters have not resolved facetotectan phylogeny (Pérez-Losada et al., 2009;Kolbasov et al., 2022), and the need for DNA sequences of live-imaged and properly vouchered material is inescapable . Phylogenetic analyses show that y-larvae form a distinct and monophyletic group (Grygier, 1987 Dreyer et al., 2022). ...
The enigmatic “y-larvae” (Pancrustacea: Facetotecta) still have an incompletely understood lifecycle, and their adult forms remain unknown despite their discovery more than 100 years ago and their documented global occurrence from shallow waters to the deep-sea. Only two of the 17 formally described species, all based on larval stages, have been investigated using an integrative taxonomic approach that, besides providing descriptions of the morphology of the naupliar and cyprid stages, also made use of exuvial voucher material and DNA barcodes. To improve our knowledge about the systematics and phylogenetics of y-larvae, we developed a novel protocol that maximizes the amount of morphological, ecological, and molecular data that can be harvested from single individuals of these tiny larvae. This revolves around single larva barcoding, and includes daily imaging of y-nauplii reared in culture dishes, mounting of their last naupliar exuviae on a slide as a reference voucher, live imaging of the y-cyprid instar that follows, and fixation, DNA extraction, amplification, and sequencing of the y-cyprid specimen. By developing and testing a suite of new primers for both nuclear and mitochondrial protein-coding and ribosomal genes, we estimated the most comprehensive phylogeny of Facetotecta to date. We expect that our novel procedure will help to unravel the complex systematics of y-larvae and show how these fascinating larval forms have evolved. Moreover, we posit that our protocols should work on larval specimens of a diverse array of molting marine invertebrate taxa.
... Treatment with the crustacean molting hormone 20-hydroxy ecdysone has been shown to induce y-cyprids to molt into a unique minute, slug-like stage, called the ypsigon (Glenner et al., 2008). The morphology of both the y-cyprid and the ypsigon suggest that unknown adult stages are advanced endoparasites in still to be identified hosts (Glenner et al., 2008;Pérez-Losada et al., 2009). Thus, the incompletely known life cycle of Facetotecta includes free-swimming naupliar stages, a cypridiform larva specialized for attachment and an ypsigon with an unknown role (Høeg et al., 2014;Pérez-Losada et al., 2009). ...
... The morphology of both the y-cyprid and the ypsigon suggest that unknown adult stages are advanced endoparasites in still to be identified hosts (Glenner et al., 2008;Pérez-Losada et al., 2009). Thus, the incompletely known life cycle of Facetotecta includes free-swimming naupliar stages, a cypridiform larva specialized for attachment and an ypsigon with an unknown role (Høeg et al., 2014;Pérez-Losada et al., 2009). The y-nauplii are either planktotrophic (feeding) or lecithotrophic (nonfeeding), but the y-cyprid is always nonfeeding. ...
... Treatment with the crustacean molting hormone 20-hydroxy ecdysone has been shown to induce y-cyprids to molt into a unique minute, slug-like stage, called the ypsigon (Glenner et al., 2008). The morphology of both the y-cyprid and the ypsigon suggest that unknown adult stages are advanced endoparasites in still to be identified hosts (Glenner et al., 2008;Pérez-Losada et al., 2009). Thus, the incompletely known life cycle of Facetotecta includes free-swimming naupliar stages, a cypridiform larva specialized for attachment and an ypsigon with an unknown role (Høeg et al., 2014;Pérez-Losada et al., 2009). ...
... The morphology of both the y-cyprid and the ypsigon suggest that unknown adult stages are advanced endoparasites in still to be identified hosts (Glenner et al., 2008;Pérez-Losada et al., 2009). Thus, the incompletely known life cycle of Facetotecta includes free-swimming naupliar stages, a cypridiform larva specialized for attachment and an ypsigon with an unknown role (Høeg et al., 2014;Pérez-Losada et al., 2009). The y-nauplii are either planktotrophic (feeding) or lecithotrophic (nonfeeding), but the y-cyprid is always nonfeeding. ...
Although the naupliar and cypridiform stages of the enigmatic y-larvae of Facetotecta have been found in the marine plankton worldwide, they still represent the last significant group of crustaceans for which the adult forms are still unknown. From a number of y-cyprids representing different taxa from different locations, we employ scanning electron microscopy to describe fine morphological details of all external structures of this unique larval form. We document different segmentation patterns of the abdomen and presence/absence of the labrum and structural differences in the antennules, labrum, paraocular process, thoracopods, and telson lend support for the erection of several new genera as opposed to the single Hansenocaris. The data presented here emphasize the morphological limits of the genus Hansenocaris and the "bauplan" of cyprydiform larvae of Facetotecta. Although the optimum pathway is a joint analysis of both molecular and morphological characters, we use the morphological characters of y-cyprids to align them cladistically and determine the limits of the genus Hansenocaris s.s. and describe common characters for all y-cyprids including six pairs of the lattice organs instead five pairs considered as a ground pattern for all Thecostraca. We also determine plesiomorphic and apomorphic characters of all known y-cyprids and separate them from other thecostracan cypridiform larvae.
... The Facetotecta are marine planktonic crustacean larvae whose adult forms remain unknown (Grygier 1996;Glenner et al. 2008;Pérez-Losada et al. 2009;Høeg et al. 2014). Having at first been mistaken for nauplii of corycaeid copepods (Hensen 1887), their distinctness and likely affinity with the Cirripedia were first recognized by Hansen (1899) on the basis of his study of several forms of "Larven vom Typus y" (Nauplius I-V) from various parts of the Atlantic Ocean. ...
... " Later, a somewhat Ascothoracidalike subsequent larval phase comprising just one instar, the 'cypris y, ' was added to the life-cycle (Bresciani 1965), and eventually a slug-like, putatively host-infective juvenile phase, the 'ypsigon, ' was discovered as well (Glenner et al. 2008). The morphology of both the cypris y and ypsigon suggests that the still unknown adults are likely to be para-sitic (Scholtz 2008;Pérez-Losada et al. 2009;Høeg et al. 2014). Dreyer et al. (in press) provided a heavily illustrated comprehensive review of the different known phases of the facetotectan life cycle, partly based on new data, comparisons to the corresponding larval and juvenile stages of, especially, parasitic barnacles (Rhizocephala), and detailed profiles of the ecology and geographic distribution of y-larvae. ...
Two large (ca. 0.5 mm long), rare, and probably closely related species of Facetotecta (y-larvae), Hansenocaris crista-labri sp. nov. and Hansenocaris aquila sp. nov., are described on the basis of last-stage lecithotrophic nauplii reared from plankton at Sesoko Island, Okinawa, Japan. The two species resemble each other in having a labrum with a row of spines and a long, attenuate trunk region that terminates in a long, heavily spinose dorsocaudal spine. The labrum of H. cristalabri sp. nov. has an enormous, cockscomb-like ventral process that bears a row of distally directed, dagger-like spines along its anterior side, while the spine-bearing keel of the labrum of H. aquila sp. nov. extends posteriorly into a robust, eagle-like beak. The labral "crest" of H. cristalabri sp. nov. has no equivalent in any other described y-larva, nor in any other crustacean nauplius; its possible functions are discussed. Another diagnostic feature of H. cristalabri sp. nov., absent in H. aquila sp. nov., is a pair of shallow, rounded notches bounded by sharp spinules on the far posteriolateral margins of the cephalic shield. Both new species have longitudinal spine rows on the trunk dorsum, two rows in H. cristalabri sp. nov. and four in H. aquila sp. nov., something not previously documented for y-nauplii. The plate arrangement of the cephalic shield in H. cristalabri sp. nov. is described in detail, with an attempt to homologize the pattern with that of other y-nauplii (especially Hansenocaris furcifera Itô, 1989). The body surface of H. cristalabri sp. nov. has fewer setae and pores than any other late-or last-stage facetotectan nauplius described to date, suggesting paedomorphic development. A formal diagnosis is presented for the family-group taxon Hansenocarididae fam. nov.; this name, while already in use, has until now been nomenclaturally unavailable.
... Morphologically, they can be identified as barnacles by their nauplius and cyprid stages. The clade is monophyletic and comprised of two orders, Kentrogonida and Akentrogonida, consisting of 10 families and 288 species (Pérez-Losada et al. 2009;Glenner et al. 2010;Boxshall and Hayes 2019;Høeg et al. 2019). The Sacculinidae is the largest family with over 175 described species that are mostly parasitic on brachyuran crabs. ...
As with all animals, crustaceans serve as hosts to a very diverse taxa of parasites. These parasites range from unusual dinoflagellates that parasitise the hemocoels or eggs of their hosts, to classical helminths that use crustaceans as intermediate hosts, and to the bizarrely adapted tantulocarid and rhizocephalan crustaceans with their highly derived life styles. Here, I review the major parasitic taxa that use Crustacea as hosts. The parasites of decapods, particularly those in shrimps, crabs, and lobsters are the best known, primarily because of their impact on populations of their commercially important hosts. Several of these parasites are outright pathogens that cause widespread mortality, feminisation, and stunting in their host populations. Other parasites, particularly those in copepods, cladocerans, and amphipods have also received attention because of the ecological importance of these hosts in food webs. They have received notable studies on vertical transmission, the influence of cryptic species complexes (both host and parasite), as well as the emergence of new pathogens in these hosts. A few parasites are also known from brine shrimp (Anostraca) and barnacles (Cirripedia) which have served as laboratory or ecological models, respectively, but few of these parasites have received much study other than their initial taxonomic descriptions and systematic placement. Although molecular tools have revealed the systematics of many of the parasitic taxa, their biology and ecology remain poorly known.
