Studies on the chemistry of the octopus renal system and an observation on the symbiotic relationship of the dicyemid Mesozoa

Common fear molecules induce defensive responses in marine prey across trophic levels

Predator-prey interactions are a key feature of ecosystems and often chemically mediated, whereby individuals detect molecules in their environment that inform whether they should attack or defend. These molecules are largely unidentified, and their discovery is important for determining their ecological role in complex trophic systems. Homarine and trigonelline are two previously identified blue crab (Callinectes sapidus) urinary metabolites that cause mud crabs (Panopeus herbstii) to seek refuge, but it was unknown whether these molecules influence other species within this oyster reef system. In the current study, homarine, trigonelline, and blue crab urine were tested on juvenile oysters (Crassostrea virginica) to ascertain if the same molecules known to alter mud crab behavior also affect juvenile oyster morphology, thus mediating interactions between a generalist predator, a mesopredator, and a basal prey species. Oyster juveniles strengthened their shells in response to blue crab urine and when exposed to homarine and trigonelline in combination, especially at higher concentrations. This study builds upon previous work to pinpoint specific molecules from a generalist predator's urine that induce defensive responses in two marine prey from different taxa and trophic levels, supporting the hypothesis that common fear molecules exist in ecological systems.

Gene expression profiles of dicyemid life-cycle stages may explain how dispersing larvae locate new hosts

Metazoans have evolved a great variety of life histories in response to environmental conditions. A unique example is encountered in dicyemid mesozoans. In addition to a highly simplified adult body comprising only ~ 30 cells, dicyemids exhibit a parasitic lifestyle that includes nematogens (asexual reproductive adults), rhombogens (sexual reproductive adults), vermiform larvae generated by nematogens, and infusoriform larvae generated by rhombogens. However, due to the difficulties of observing microscopic endoparasites, the complex life cycle and biological functions of life-cycle stages of dicyemids have remained mysterious. Taking advantage of the recently decoded genome of Dicyema japonicum, we examined genes that undergird this lifestyle. Using stage-specific gene expression profiles, we found that biological processes associated with molecular transport, developmental regulation, and sensory response are specified at different stages. Together with the expression of potential neurotransmitters, we further suggest that apical cells in infusoriform larva probably serve sensory functions, although dicyemids have no nervous system. Gene expression profiles show that more genes are expressed in free-living infusoriform larvae than in the other three stages, and that some of these genes are likely involved in locating new hosts. These data provide molecular information about the unique lifestyle of dicyemids and illustrate how an extremely simplified endoparasite adapted and retained gene sets and morphological characters to complete its life cycle.

SUPPLEMENTARY INFORMATION

Supplementary information accompanies this paper at 10.1186/s40851-019-0146-y.

Redescription of two Dicyemennea (Phylum: Dicyemida) from Rossia pacifica (Mollusca: Cephalopoda: Decapoda)

Two species of dicyemid mesozoan are redescribed from Rossia pacifica Berry, 1911, collected off Iwase in Toyama Bay, Honshu, Japan. Dicyemennea brevicephaloides Bogolepova-Dobrokhotova, 1962, is a large species that reaches about 4,000 microm in length. This species attaches to the surface of the branchial heart appendages. The vermiform stages are characterized as having 23 peripheral cells, a disc-shaped calotte, and an axial cell that extends to the propolar cells. An anterior abortive axial cell is absent in vermiform embryos. Infusoriform embryos consist of 35 cells; a single nucleus is present in each urn cell and the refringent bodies are absent. Dicyemennea rossiae Bogolepova-Dobrokhotova, 1962, is a medium species that reaches about 2000 microm in length. This species lives in folds of the renal appendages. The vermiform stages are characterized as having 29-34 peripheral cells, a conical calotte, and an axial cell that extends to the middle of the metapolar cells. An anterior abortive axial cell is present in vermiform embryos. Infusoriform embryos consist of 39 cells; 2 nuclei are present in each urn cell and the refringent bodies are solid.

Renal organs of cephalopods: a habitat for dicyemids and chromidinids

The renal organs of 32 species of cephalopods (renal appendage of all cephalopods, and renal and pancreatic appendages in decapods) were examined for parasite fauna and for histological comparison. Two phylogenetically distant organisms, dicyemid mesozoans and chromidinid ciliates, were found in 20 cephalopod species. Most benthic cephalopods (octopus and cuttlefish) were infected with dicyemids. Two pelagic cephalopod species, Sepioteuthis lessoniana and Todarodes pacificus, also harbored dicyemids. Chromidinid ciliates were found only in decapods (squid and cuttlefish). One dicyemid species was found in branchial heart appendages of Rossia pacifica. Dicyemids and chromidinids occasionally occurred simultaneously in Euprymna morsei, Sepia kobiensis, S. peterseni, and T. pacificus. The small-sized cephalopod species, Idiosepius paradoxus and Octopus parvus, harbored no parasites. Comparative histology revealed that the external surface of renal organs varies morphologically in various cephalopod species. The small-sized cephalopod species have a simple external surface. In contrast, the medium- to large-sized cephalopod species have a complex external surface. In the medium- to large-sized cephalopod species, their juveniles have a simple external surface of the renal organs. The external surface subsequently becomes complicated as they grow. Dicyemids and chromidinids attach their heads to epithelia or insert their heads into folds of renal appendages, pancreatic appendages, and branchial heart appendages. The rugged and convoluted external surface provides a foothold for dicyemids and chromidinids with a conical head. They apparently do not harm these tissues of their host cephalopods.