Mass mortality of diadematoid sea urchins in the Red Sea and Western Indian Ocean.

Sea urchins are primary herbivores on coral reefs, regulating algal biomass and facilitating coral settlement and growth.1,2,3,4,5,6,7,8,9,10,11,12 Recurring mass mortality events (MMEs) of Diadema species Gray, 1825 have been recorded globally,13,14,15,16,17,18,19,20,21,22,23 the most notorious and ecologically significant of which occurred in the Caribbean in 1983,14,17,19,20 contributing to the shift from coral to algal-dominated ecosystems.17,24,25 Recently, first evidence of Diadema setosum mass mortality was reported from the eastern Mediterranean Sea.23 Here, we report extensive mass mortalities of several diadematoid species inhabiting the Red Sea and Western Indian Ocean (WIO)26,27,28 including first evidence of mortalities in the genus Echinothrix Peters, 1853. Mortalities initiated in the Gulf of Aqaba on December 2022 and span the Red Sea, the Gulf of Oman, and the Western Indian Ocean (Réunion Island), with population declines reaching 100% at some sites. Infected individuals are characterized by spine loss and tissue necrosis, resulting in exposed skeletons (i.e., tests) and mortality. Molecular diagnostics of the 18S rRNA gene confirm the presence of a waterborne scuticociliate protozoan most closely related to Philaster apodigitiformis in infected specimens-identical to the pathogen found in the 2022 Caribbean mass mortality of Diadema antillarum.13,15,18 Collapse of these key benthic grazers in the Red Sea and Western Indian Ocean may lead to algal dominance over corals, threatening the stability of coral reefs on a regional scale.29,30,31,32 We issue a warning regarding the further expansion of mortalities and call for immediate monitoring and conservation efforts for these key ecological species.

Mysterious sea urchin plague is spreading rapidly.

Pathogen that kills victims within days leaps from Caribbean to Red Sea.

Bindin is essential for fertilization in the sea urchin.

Species-specific sperm-egg interactions are essential for sexual reproduction. Broadcast spawning of marine organisms is under particularly stringent conditions, since eggs released into the water column can be exposed to multiple different sperm. Bindin isolated from the sperm acrosome results in insoluble particles that cause homospecific eggs to aggregate, whereas no aggregation occurs with heterospecific eggs. Therefore, Bindin is concluded to play a critical role in fertilization, yet its function has never been tested. Here we report that Cas9-mediated inactivation of the bindin gene in a sea urchin results in perfectly normal-looking embryos, larvae, adults, and gametes in both males and females. What differed between the genotypes was that the bindin-/- sperm never fertilized an egg, functionally validating Bindin as an essential gamete interaction protein at the level of sperm-egg cell surface binding.

Gross and microscopic pathology of West Indian sea eggs (Tripneustes ventricosus).

In this study, we performed comprehensive pathology examinations on 83 Tripneustes ventricosus from 11 locations on St. Kitts to build baseline data necessary for disease diagnosis in this species. Gross abnormalities were observed in 23/83 (28%) urchins and included spine loss, visceral hyperpigmentation, test discoloration, and test ulceration. Ciliates were the only protists identified in this study via examination of tissue wet mounts and histology, documented in 50/83 (60%) urchins. Microscopic observations associated with visibly abnormal status included muscle necrosis, test and appendage inflammation, appendage (tube feet, spines, and pedicellariae) degeneration, severe coelomocytosis, and generalized hypermelanosis. Enterocyte intranuclear inclusion bodies, microbial aggregates, nerve pigmentation, enteric pigmentation, integument-associated crustaceans, and encysted metazoan parasites were of uncertain pathological significance. The etiology for any lesion was not microscopically apparent, contrasting literature implicating common marine bacteria in urchin diseases. This study highlights the importance of histopathology in urchin disease investigations and facilitates the recognition of disease in T. ventricosus.

Syndesmis François, 1886 (Rhabdocoela: Umagillidae): a revisitation, with a synopsis and an identification key to species, and new molecular evidence for ascertaining the phylogeny of the group.

Syndesmis François, 1886 is a genus of umagillid turbellarians comprising species which are typically endosymbionts of echinoids, i.e. sea urchins. This group is likely key in addressing the issue of transition between a free-living and a parasitic mode of life in the Platyhelminthes. Accordingly, its phylogeny should be considered for detailed analysis, namely by addressing molecular evidence for its different species. At the present time, a revisitation of Syndesmis is required and fully justified by the following lines of argument: (i) the body of knowledge on Syndesmis is large, but the information is scattered through many different works in the literature; (ii) for about 60 years, it was a common practice to assign the umagillids isolated from sea urchins as a single species, i.e. the type-species, Syndesmis echinorum François, 1886, which was later split into several species on morphological grounds; and (iii) the type-species - for which no molecular information is available - was redescribed and new species were described in recent years but the generic diagnosis of Syndesmis was not emended accordingly. The present state of art additionally justifies the necessity of (i) an updated synopsis of species and (ii) an identification key to the 26 species described from different hosts and geographical locations. All these aspects define the aims of the present study. It is proposed that S. antillarum is attributed to Stunkard & Corliss (1951) and not to Powers (1936).

Diversity and Evolution of Paramoeba spp. and their Kinetoplastid Endosymbionts.

Members of the genus Paramoeba (including Neoparamoeba) (Amoebozoa) are single-celled eukaryotes of economic and ecological importance because of their association with disease in a variety of marine animals including fish, sea urchins, and lobster. Interestingly, they harbor a eukaryotic endosymbiont of kinetoplastid ancestry, Perkinsela sp. To investigate the complex relationship between Paramoeba spp. and Perkinsela sp., as well as the relationships between different Paramoeba species, molecular data was obtained for four novel isolates. We also acquired new data from the urchin pathogen P. invadens. Comprehensive molecular phylogenetic analyses were carried out using 33 newly obtained 18S rDNA sequences from the host amoebae and 16 new 18S rDNA sequences from their corresponding Perkinsela sp., together with all publicly available 18S molecular data. Intra-isolate 18S rDNA nucleotide diversity was found to be surprisingly high within the various species of Paramoeba, but relatively low within their Perkinsela sp. endosymbionts. 18S rDNA phylogenies and ParaFit co-evolution analysis revealed a high degree of congruence between the Paramoeba and Perkinsela sp. tree topologies, strongly suggesting that a single endosymbiotic event occurred in the common ancestor of known Paramoeba species, and that the endosymbionts have been inherited vertically ever since.

Marine protected areas facilitate parasite populations among four fished host species of central Chile.

1. Parasites comprise a substantial proportion of global biodiversity and exert important ecological influences on hosts, communities and ecosystems, but our knowledge of how parasite populations respond to human impacts is in its infancy. 2. Here, we present the results of a natural experiment in which we used a system of highly successful marine protected areas and matched open-access areas in central Chile to assess the influence of fishing-driven biodiversity loss on parasites of exploited fish and invertebrate hosts. We measured the burden of gill parasites for two reef fishes (Cheilodactylus variegatus and Aplodactylus punctatus), trematode parasites for a keyhole limpet (Fissurella latimarginata), and pinnotherid pea crab parasites for a sea urchin (Loxechinus albus). We also measured host density for all four hosts. 3. We found that nearly all parasite species exhibited substantially greater density (# parasites m(-2)) in protected than in open-access areas, but only one parasite species (a gill monogenean of C. variegatus) was more abundant within hosts collected from protected relative to open-access areas. 4. These data indicate that fishing can drive declines in parasite abundance at the parasite population level by reducing the availability of habitat and resources for parasites, but less commonly affects the abundance of parasites at the infrapopulation level (within individual hosts). 5. Considering the substantial ecological role that many parasites play in marine communities, fishing and other human impacts could exert cryptic but important effects on marine community structure and ecosystem functioning via reductions in parasite abundance.

Validating the identity of Paramoeba invadens, the causative agent of recurrent mass mortality of sea urchins in Nova Scotia, Canada.

Green sea urchins Strongylocentrotus droebachiensis along the coast of Nova Scotia, Canada, suffer mass mortalities from infection by the pathogenic amoeba Paramoeba invadens Jones, 1985. It has been speculated that P. invadens could be a form of Neoparamoeba pemaquidensis, a species associated with disease in S. droebachiensis and lobsters in the northeast USA. During a disease outbreak in fall 2011, we isolated amoebae from moribund urchins collected from 4 locations along ~200 km of coastline. In laboratory infection trials, we found that timing and rate of morbidity corresponded to that of similar experiments conducted in the early 1980s, when P. invadens was first identified. All isolates had a similar size and morphology to the original description, including an absence of microscales. Sequences of nuclear SSU rDNA show that disease was caused by one 'species' of amoeba across the range sampled. Phylogenetic analyses prove that P. invadens is not conspecific with N. pemaquidensis, but is a distinct species most closely related to N. branchiphila, a suspected pathogen of sea urchins Diadema aff. antillarum in the Canary Islands, Spain. Morphology and closest phylogenetic affinities suggest that P. invadens would be assignable to the genus Neoparamoeba; however, nuclear SSU rDNA trees show that Neoparamoeba and Paramoeba are phylogenetically inseparable. Therefore, we treat Neoparamoeba as a junior synonym of Paramoeba, with P. invadens retaining that name, and N. pemaquidensis and N. aestuarina reverting to their original names (P. pemaquidensis and P. aestuarina), and with new combinations for N. branchiphila Dykova et al., 2005, and N. perurans Young et al., 2007, namely P. branchiphila comb. nov. and P. perurans comb. nov.

Neoparamoeba branchiphila infections in moribund sea urchins Diadema aff. antillarum in Tenerife, Canary Islands, Spain.

A total of 109 sea urchins from 3 species collected in 2 localities off the coast of Tenerife Island, Spain, were examined for the presence of free-living amoebae in their coelomic fluid. Amoeba trophozoites were isolated exclusively from moribund individuals of long-spined sea urchins Diadema aff. antillarum (Philippi) (Echinoidea, Echinodermata) that manifested lesions related to sea urchin bald disease on their tests (16 out of 56 examined). No amoebae were detected in Arbacia lixula (L.) and Paracentrotus lividus (Lamarck). From the former sea urchin species, 8 strains, established from 10 primary isolates, were identified as Neoparamoeba branchiphila Dyková et al., 2005 using morphological and molecular methods. Results of this study (limited to the screening for free-living amoebae) together with data on agents of sea urchin mortalities reported to date justify the hypothesis that free-living amoebae play an opportunistic role in D. aff. antillarum mortality. The enlargement of the dataset of SSU rDNA sequences brought new insight into the phylogeny of Neoparamoeba species.

Food webs and fishing affect parasitism of the sea urchin Eucidaris galapagensis in the Galápagos.

In the Galápagos Islands, two eulimid snails parasitize the common pencil sea urchin, Eucidaris galapagensis. Past work in the Galápagos suggests that fishing reduces lobster and fish densities and, due to this relaxation of predation pressure, indirectly increases urchin densities, creating the potential for complex indirect interactions between fishing and parasitic snails. To measure indirect effects of fishing on these parasitic snails, we investigated the spatial relationships among urchins, parasitic snails, commensal crabs, and large urchin predators (hogfish and lobsters). Parasitic snails had higher densities at sites where urchins were abundant, probably due to increased resource availability. Commensal crabs that shelter under urchin spines, particularly the endemic Mithrax nodosus, preyed on the parasitic snails in aquaria, and snails were less abundant at field sites where these crabs were common. In aquaria, hogfish and lobsters readily ate crabs, but crabs were protected from predation under urchin spines, leading to a facultative mutualism between commensal crabs and urchins. In the field, fishing appeared to indirectly increase the abundance of urchins and their commensal crabs by reducing predation pressure from fish and lobsters. Fished sites had fewer snails per urchin, probably due to increased predation from commensal crabs. However, because fished sites also tended to have more urchins, there was no significant net effect of fishing on the number of snails per square meter. These results suggest that fishing can have complex indirect effects on parasites by altering food webs.

Diversity and ecological remarks of ectocommensals and ectoparasites (Annelida, Crustacea, Mollusca) of echinoids (Echinoidea: Mellitidae) in the Sea of Cortez, Mexico.

The diversity of symbiotic invertebrates on intertidal and shallow water echinoids of the northwestern Gulf of California was studied. Five ectosymbionts were found: three pinnotherid crabs, Dissodactylus lockingtoni Glassell (1935), Dissodactylus nitidus Smith (1870), and Dissodactylus xanthusi Glassell (1936); one eulimid gastropod, Turveria encopendema Berry (1956); and one polychaete, Struwela sp. We discovered seasonal fluctuation in D. nitidus Smith (1870) and D. xanthusi Glassell (1936), which result in these species being undetectable during the winter season when the temperature in the head of the Gulf of California drops to 14-18 degrees C; the rest of the species are permanent residents in this area. An updated list of all external macrosymbionts of echinoids of the Mexican Pacific is presented.

Didymium-like myxogastrids (class Mycetozoa) as endocommensals of sea urchins (Sphaerechinus granularis).

This paper sums up the results of light microscopical, ultrastructural and molecular studies of five strains of amoeboid organisms isolated as endocommensals from coelomic fluid of sea urchins, Sphaerechinus granularis (Lamarck), collected in the Adriatic Sea. The organisms are reported as Didymium-like myxogastrids. Of the life-cycle stages, the attached amoeboids, flagellated trophozoites, cysts and biflagellated swarmers are described. Formation of fruiting bodies was not observed. Although phylogenetic analyses of SSU rDNA sequences indicated a close relationship with Hyperamoeba dachnaya, our sea-urchin strains have not been assigned to the genus Hyperamoeba Alexeieff, 1923. The presence of either one or two flagella reported in phylogenetically closely related organisms and mutually distant phylogenetic positions of strains declared as representatives of the genus Hyperamoeba justify our approach. Data obtained in this study may be useful in future analyses of relationships of the genera Didymium, Hyperamoeba, Physarum and Pseudodidymium as well as in higher-order phylogeny of Myxogastrea.

Morphological redescription of two endocommensal ciliates, Entorhipidium fukuii Uyemura, 1934 and Madsenia indomita (Madsen, 1931) Kahl, 1934 from digestive tracts of sea urchins of the Yellow Sea, China (Ciliophora; Scuticociliatida).

Definitions of the genera Entorhipidium and Madsenia have been updated on the basis of the results of studies on the living morphology and infraciliature of the endocommensal ciliates, Entorhipidium fukuii Uyemura, 1934 and Madsenia indomita (Madsen, 1931) Kahl, 1934, isolated from digestive tracts of the sea urchin, Hemicentrotus pulcherrimus. Entorhipidium are flattened marine endocommensal scuticociliates with a sigmoid body shape and conspicuous tail; the buccal cavity is in the anterior half of the cell and the buccal ciliature has a Uronema-like pattern; the somatic kineties form both apical and post-oral sutures. Because of this new definition, a new combination is suggested: Entorhipidium caudatum (Poljansky, 1951) nov. comb. (basionym: Cryptochilidium caudatum Poljansky, 1951). Madsenia are flattened, slender-bodied endocommensal scuticociliates, having a buccal cavity in the anterior half of the cell with M1 and M2 fused into a single structure and both M3 and the paroral membrane short; the somatic kineties form an anterior suture; no caudal bristle was observed.

Indirect food web interactions: sea otters and kelp forest fishes in the Aleutian archipelago.

Although trophic cascades-the effect of apex predators on progressively lower trophic level species through top-down forcing-have been demonstrated in diverse ecosystems, the broader potential influences of trophic cascades on other species and ecosystem processes are not well studied. We used the overexploitation, recovery and subsequent collapse of sea otter (Enhydra lutris) populations in the Aleutian archipelago to explore if and how the abundance and diet of kelp forest fishes are influenced by a trophic cascade linking sea otters with sea urchins and fleshy macroalgae. We measured the abundance of sea urchins (biomass density), kelp (numerical density) and fish (Catch per unit effort) at four islands in the mid-1980s (when otters were abundant at two of the islands and rare at the two others) and in 2000 (after otters had become rare at all four islands). Our fish studies focused on rock greenling (Hexagrammos lagocephalus), the numerically dominant species in this region. In the mid-1980s, the two islands with high-density otter populations supported dense kelp forests, relatively few urchins, and abundant rock greenling whereas the opposite pattern (abundant urchins, sparse kelp forests, and relatively few rock greenling) occurred at islands where otters were rare. In the 2000, the abundances of urchins, kelp and greenling were grossly unchanged at islands where otters were initially rare but had shifted to the characteristic pattern of otter-free systems at islands where otters were initially abundant. Significant changes in greenling diet occurred between the mid-1980s and the 2000 although the reasons for these changes were difficult to assess because of strong island-specific effects. Whereas urchin-dominated communities supported more diverse fish assemblages than kelp-dominated communities, this was not a simple effect of the otter-induced trophic cascade because all islands supported more diverse fish assemblages in 2000 than in the mid-1980s.

Population dynamics, infestation and host selection of Vexilla vexillum, an ectoparasitic muricid of echinoids, in Madagascar.

The symbiotic interaction, population and infestation dynamics of the muricid Vexilla vexillum (Gmelin, 1791) on 2 echinoid species, Tripneustes gratilla (Linnaeus, 1785) and Echinometra mathaei (Blainville, 1825), was investigated on the barrier reef off Toliara (Madagascar). V. vexillum is an ectoparasitic muricid which was exclusively found in association with sea urchins, on which it moves freely and browses over the integument. Host recovery from damage caused by muricid grazing was dependent on lesion size. Small lesions regenerated while larger ones were subjected to secondary infections, which led to host death. A 27 mo survey (2000 to 2003) of the muricid's population dynamics revealed annual recruitment episodes during the mid-summer season (December to January). Patterns of recruitment peaks were apparently linked to its reproductive cycle. Demographic parameters including growth and mortality rates of the muricid were estimated from analysis of size-frequency distributions. Growth was described by the von Bertalanffy function. The model predicts that V. vexillum is a fast-growing species in which asymptotic shell length (L infinity = 1.024 cm) is reached 6 to 7 mo after recruitment. The growth rate constant K, and shell length at settlement L0, were estimated from the model. Estimated mortality rate was 55% yr(-1); V. vexillum has a short lifespan. The observed high growth rate together with the high mortality rate suggest that V. vexillum is a semelparous species. A field survey of the infestation dynamics of V. vexillum was performed during 3 consecutive years, with seasonal variation in parasite prevalence on both echinoid host species. Although both T. gratilla and E. mathaei were infested, a preference towards T. gratilla was noted. This was attributed to T. gratilla's test morphology (which allows better accessibility for grazing), to the muricid's higher recognition capacity of T. gratilla (as determined by olfactory experiments) and to the high recruitment predictability of that particular host. This study provides novel information on the biology of V. vexillum, an echinoid epidermal grazer, and its relationship with 2 ecologically and economically important echinoid species.

Induction of metamorphosis in the sea urchin Holopneustes purpurascens by a metabolite complex from the algal host Delisea pulchra.

Most benthic invertebrates have complex life cycles with planktonic larvae that return to the substratum to settle and metamorphose into a benthic stage. Although naturally produced chemical cues have long been thought to be important for the settlement or metamorphosis of invertebrate larvae, few ecologically relevant chemical cues have been clearly identified. The marine echinoid Holopneustes purpurascens has a complex life cycle, with a planktonic, nonfeeding dispersive larva that metamorphoses into a benthic stage that lives in the canopy of subtidal benthic algae such as the red alga Delisea pulchra and the kelp Ecklonia radiata. Recently recruited juveniles are found primarily on D. pulchra, and we hypothesized that this was in response to a chemical cue produced by this alga. Competent larvae metamorphosed in the presence of D. pulchra, or seawater surrounding this alga, but not in response to the presence of E. radiata or its extracts. A cue for metamorphosis was isolated and characterized from D. pulchra and found to be a water-soluble complex of the sugar floridoside and isethionic acid in a 1:1 molar ratio. The floridoside-isethionic acid complex also triggered settlement in H. purpurascens; however, this response was less specific than metamorphosis and was reversible. Larvae of H. purpurascens also metamorphosed in the presence of several other species of red, but not brown or green, algae from their habitat. Floridoside is found only in red algae, suggesting that the floridoside-isethionic acid complex may be acting as a cue for metamorphosis in other red algae as well as in D. pulchra.

Experimental transmission of the nematode Echinomermella matsi to the sea urchin Strongylocentrotus droebachiensis in the laboratory.

Three experiments were conducted to investigate the possibility of maintaining the Echinomermella matsi-Strongylocentrotus droebachiensis system in the laboratory. The experiments were performed by injecting E. matsi larvae taken directly from gravid female nematodes into the mouths of sea urchins. In all experiments, this treatment resulted in a higher infection in treated animals than in unmanipulated controls. The successful establishment of larvae indicates that E. matsi has a monoxenous life cycle. The growth of larvae in experimentally infected hosts was slow, indicating that the generation time of the parasite is of the same magnitude as the life expectancy of the host, 1-2 yr. This slow growth rate suggests that considerable resources will be needed to maintain the system in the laboratory.

Intracytoplasmic ciliary elements in epidermal cells of Syndesmis echinorum and Paravortex cardii (Platyhelminthes, Dalyellioida).

Epidermal cells of Syndesmis echinorum and Paravortex cardii contain many intracytoplasmic ciliary components: clusters of centrioles disorganized and incomplete short axonemes composed of loosely organized microtubules of irregular lengths, fully formed axonemes though some with fewer than nine doublets, and ciliary rootlets. Furthermore, conspicuous dense granules are found in solitary groups in the cytoplasm. Clusters of dense granules are also closely associated with Golgi complexes and developing axonemal microtubules. Since the dense granules decrease in number as the axonemes increase, it is likely that the granules are involved in the formation of axonemal microtubules. Ciliary elements are especially abundant in epidermal cells of Paravortex cardii embryos, some of them resembling those previously described by several authors in differentiating ciliated cells engaged in centriologenesis and ciliogenesis. Attention has been focused on the relative proportion and position of these elements, as well as the different morphology and several assembling states that they exhibit in epidermal cells of adult S. echinorum and adults and embryos of P. cardii. A functional interpretation of some of the findings is given, which allows us to suggest a sequence of ciliogenetic events that occur in epidermal cells of both species.

Redescription of Syndesmis echinorum François, 1886 (Turbellaria: Neorhabdocoela: Umagillidae), with comments on distinctions between Syndesmis and Syndisyrinx.

Syndesmis echinorum is redescribed from specimens taken from the intestine of the sea urchin Paracentrotus lividus at the type locality, Banyuls-sur-Mer, France. This umagillid is shown to have a seminal bursa-bursal valve complex comparable to that of Syndisyrinx franciscanus. Thus, it is suggested that the genera Syndesmis and Syndisyrinx be distinguished on the basis of the form and proportions of the ejaculatory duct, male antrum, and penis stylet, rather than on the basis of the presence or absence of a seminal bursa and bursal valve. The sea urchin Sphaerechinus granularis at Banyuls harbors a worm that is, on the average, slightly more robust than S. echinorum from P. lividus; its morphology, however, agrees with that of S. echinorum. In addition to S. echinorum, P. lividus harbors at least 1 undescribed umagillid. Although 2 species found in Echinus esculentus have been referred to S. echinorum by previous workers, neither is conspecific with it.

Egg capsules of a parasitic turbellarian flatworm: ultrastructure of hatching sutures.

Egg capsules of Syndisyrinx franciscanus, an intestinal parasite of sea urchins (Strongylocentrotus spp.), consist of a bulb, which contains the embryos, and a stalk-like filament. The wall of the bulb is about 12 microns thick and is composed of sclerotized proteins. The end of the bulb opposite the attachment of the filament bears a reticulum of hatching sutures. Transmission electron microscopy discloses that hatching sutures traverse the entire thickness of the capsule wall. The inner 9-10 microns of sutures are a uniform 20 nm in width and contain a trilaminar cementum. The outer 2-3 microns of sutures are 15 nm to more than 500 nm in width and contain an electron-lucent cementum. The latter may contain an irregular, median, electron-dense layer or, more commonly, electron-dense granules. The outside of some capsules is partially covered by a thin, electron-dense material. A previous study showed that sutures in intact capsules of Syndisyrinx franciscanus are not affected by host digestive fluids, but are severely weakened immediately prior to hatching owing to activities of the embryos. The hypothesis that the embryos secrete a hatching enzyme is supported by findings that sutures of intact capsules are not affected by externally applied trypsin, but become weakened when capsules are cut open and then incubated in trypsin. Scanning electron microscopy reveals that the outer parts of sutures often remain intact after hatching. We hypothesize that the ability of sutures to resist enzymatic attack from the outside, but not the inside, results from differences in the chemical properties of the cementums in outer and inner parts of sutures.