Histopathology of Antarctic krill, Euphausia superba, bearing black spots

Microbial composition of carapace, feces, and water column in captive juvenile green sea turtles with carapacial ulcers

Introduction

Green sea turtles are endangered marine reptiles. Carapacial ulcers will develop on juvenile green sea turtles during artificial rescue, seriously affecting their health and potentially leading to death.

Methods

To determine the pathogens causing ulcerative carapacial disease, we performed 16S and ITS high-throughput sequencing, and microbial diversity analysis on samples from carapacial ulcers, healthy carapaces, feces, and seawater of juvenile green sea turtles.

Results

Our analysis showed that changes in microbial diversity of green sea turtle feces and seawater were not significantly associated with ulcerative carapacial disease.

Discussion

Psychrobacter sp. is the dominant species in the carapacial ulcers of green sea turtles. The bacterium is present in both healthy turtles and seawater where carapacial ulcers did not occur and decreasing seawater temperatures are likely responsible for the infection of juvenile green turtles with Psychrobacter sp. This is the first study on carapacial ulcers in captive juvenile green sea turtles. Our research provides theoretical guidance for the prevention and control of carapacial ulcers in captive juvenile green sea turtles.

Krill Nyctiphanes simplex gonad affection associated with acute-intensity phyllobothriid plerocercoid infection

This is the first acute-intensity record of helminths parasitizing the subtropical krill Nyctiphanes simplex Hansen, 1911. We briefly describe the pathology of infection of Phyllobothriidae gen. sp. plerocercoids parasitizing N. simplex in the Gulf of California. Infection occurred with a very low prevalence (P = 0.06%, n = 1563 specimens), although acute-intensity exceeded several hundred plerocercoids crowding the hemocoel in one female host. Nyctiphanes simplex showed inflammatory response of hemocyte-based infiltration, nodule formation, and presumptive melanization. Remarkably, cestodes invade and supplant the gonad, causing atretic oocytes and severe tissue destruction in the gonad likely leading to castration and cell death in connective tissue of the infected organs suggesting that acute-intensity infection exceeds the krill's reaction capacity. Thus, Phyllobothriidae gen. sp. negatively affects the host by depleting its fitness, leading to total castration to prevent/block host reproduction.

Parasites and Diseases

The Antarctic krill Euphausia superba is among the most studied species of the Order Euphausiacea in biological and ecological aspects; however, reports of their parasites and diseases are relatively scarce. A worldwide overview of all parasites known for 48 out 86 extant euphausiid species includes 17 distinct types of epibionts, pathogens, parasites, and parasitoids. So far, only seven of them have been reported interacting with E. superba [epibionts: exuviotrophic ciliates (Foettingeriidae) and microplanktophagous ciliates (Suctoridae, Ephelota), pathogens: chitinoclastic bacteria and fungi; and trophically transmitted endoparasites: Apicomplexans (Gregarinidae, Cephaloidophora), nematode infecting krill’s eggs (under laboratory conditions), and histophagous parasites: Apostomatida ciliates of the family Pseudocollinidae]. The epibionts have interspecific associations that strongly depend on the krill’s moult cycle, discarding them at each moulting event. Their colonization and intensity show a remarkable synchronization with the krill moulting process at individual, school, and population levels. The social and sometimes highly dense swarms and schools of E. superba, its keystone trophic function (both as voracious predator and as prey to multiple predators) should make it a critical vector for trophically transmitted parasites in the food web. However, E. superba interacts with a relatively low diversity of epibionts, pathogens, and parasites, in comparison with parasite diversity known for relatively well-studied temperate (Meganyctiphanes norvegica, Euphausia pacifica) and subtropical (Nyctiphanes simplex) euphausiid species. The apparently low parasite diversity of E. superba is likely associated with its Antarctic zoogeographic pattern; where, parasites have not invaded the Antarctic krill with the same evolutionary success as have occurred with other euphausiid species from tropical, subtropical, temperate, and even Arctic ecosystems.

Histophagous ciliate Pseudocollinia brintoni and bacterial assemblage interaction with krill Nyctiphanes simplex. I. Transmission process

Histophagous ciliates of the genus Pseudocollinia cause epizootic events that kill adult female krill (Euphausiacea), but their mode of transmission is unknown. We compared 16S rRNA sequences of bacterial strains isolated from stomachs of healthy krill Nyctiphanes simplex specimens with sequences of bacterial isolates and sequences of natural bacterial communities from the hemocoel of N. simplex specimens infected with P. brintoni to determine possible transmission pathways. All P. brintoni endoparasitic life stages and the transmission tomite stage (outside the host) were associated with bacterial assemblages. 16S rRNA sequences from isolated bacterial strains showed that Photobacterium spp. and Pseudoalteromonas spp. were dominant members of the bacterial assemblages during all life phases of P. brintoni and potential pathobionts. They were apparently unaffected by the krill's immune system or the histophagous activity of P. brintoni. However, other bacterial strains were found only in certain P. brintoni life phases, indicating that as the infection progressed, microhabitat conditions and microbial interactions may have become unfavorable for some strains of bacteria. Trophic infection is the most parsimonious explanation for how P. brintoni infects krill. We estimated N. simplex vulnerability to P. brintoni infection during more than three-fourths of their life span, infecting mostly adult females. The ciliates have relatively high prevalence levels (albeit at <10% of sampled stations) and a short life cycle (estimated <7 d). Histophagous ciliate-krill interactions may occur in other krill species, particularly those that form dense swarms and attain high population densities that potentially enhance trophic transmission and allow completion of the Pseudocollinia spp. life cycle.

Histophagous ciliate Pseudocollinia brintoni and bacterial assemblage interaction with krill Nyctiphanes simplex. II. Host responses

Unlike decapod crustaceans of commercial interest, the krill defense system and its response to parasites and pathogens is virtually unknown. Histophagous ciliates of the genus Pseudocollinia interact with at least 7 krill species in the northeastern Pacific. Although they can cause epizootic events, the physiology of the histophagous ciliate-host interaction and krill (host) defenses remain unknown. From 1 oceanographic survey along the southwestern coast of the Baja California Peninsula near Bahía Magdalena and 2 in the Gulf of California, we investigated parasitoid-host physiological responses (fatty acid and oxidative stress indicators) of the subtropical krill Nyctiphanes simplex infected with the ciliate P. brintoni. All life stages of P. brintoni were associated with opportunistic bacterial assemblages that have not been explicitly investigated in other Pseudocollinia species (P. beringensis, P. oregonensis, and P. similis). Parasitoid ciliates exclusively infected adult females, which showed increased lipid content during gonad development. As the infection progressed, omega-3 eicosapentaenoic and docosahexaenoic fatty acids, which may act as energy sources to produce high numbers of ciliate transmission stages, were quickly depleted. Antioxidant enzymes, components of the crustacean defense system, varied throughout infection, but without inhibiting Pseudocollinia infection, i.e. higher levels of lipid oxidative damage were detected in late stages of infection. The ineffective response of the krill antioxidant defense system against histophagous ciliates and the bacteria associated with the ciliates suggests that Pseudocollinia ciliates are functionally analogous to krill predators and may have a strong influence on the population dynamics of krill.