Global patterns of parasite diversity in cephalopods

We compiled an updated global catalogue of parasites in cephalopods. Data were used to assess changes in taxonomic distinctness of parasites over two centuries and across the world’s oceans, to quantify turnover and nestedness components of parasite β-diversity, and to attempt estimating their γ-diversity at a global scale. A total of 309 parasites infecting 164 cephalopods were found. We hypothesize that this diversity counts for less than half the potential parasite richness in this molluscan taxon. Taxonomic breadth of parasites was significantly above expectations from null models for Mediterranean Sea and NE Atlantic Ocean, whereas the opposite occurred for NW Pacific Ocean, where a few closely related genera characterized the parasite pool. β-diversity of parasites was very high and dominated by turnover, except for the Atlantic Ocean where a nested pattern among sub-basins emerged. Taxonomic relatedness of parasite species remained substantially unchanged through time, but species replacements largely occurred over the last two centuries. Our findings highlighted potential hotspots of taxonomic distinctness in cephalopod parasites, geographic regions deserving future research, and the need for a deeper understanding of the magnitude of marine parasite diversity, their biogeography, and their role in marine ecosystems. Our global overview may represent a baseline step for future advances in this direction.

A Perspective Around Cephalopods and Their Parasites, and Suggestions on How to Increase Knowledge in the Field

Although interest in several areas of cephalopod research has emerged over the last decades (e.g., neurobiology, aquaculture, genetics, and welfare), especially following their 2010 inclusion in the EU Directive on the use of animals for experimental purposes, knowledge regarding the parasites of cephalopods is lacking. Cephalopods can be intermediate, paratenic, or definitive hosts to a range of parasites with a wide variety of life cycle strategies. Here, we briefly review the current knowledge in cephalopod parasitological research, summarizing the main parasite groups that affect these animals. We also emphasize some topics that, in our view, should be addressed in future research, including: (i) better understanding of life cycles and transmission pathways of common cephalopod parasites; (ii) improve knowledge of all phases of the life cycle (i.e., paralarvae, juveniles, adults and senescent animals) and on species from polar deep sea regions; (iii) exploration of the potential of using cephalopod-parasite specificity to assess population boundaries of both, hosts and parasites; (iv) risk evaluation of the potential of standard aquacultural practices to result in parasite outbreaks; (v) evaluation and description of the physiological and behavioral effects of parasites on their cephalopod hosts; (vi) standardization of the methods for accurate parasite sampling and identification; (vii) implementation of the latest molecular methods to facilitate and enable research in above mentioned areas; (viii) sharing of information and samples among researchers and aquaculturists. In our view, addressing these topics would allow us to better understand complex host-parasite interactions, yield insights into cephalopod life history, and help improve the rearing and welfare of these animals in captivity.

First comparative insight into the architecture of COI mitochondrial minicircle molecules of dicyemids reveals marked inter-species variation

Dicyemids, poorly known parasites of benthic cephalopods, are one of the few phyla in which mitochondrial (mt) genome architecture departs from the typical ~16 kb circular metazoan genome. In addition to a putative circular genome, a series of mt minicircles that each comprises the mt encoded units (I-III) of the cytochrome c oxidase complex have been reported. Whether the structure of the mt minicircles is a consistent feature among dicyemid species is unknown. Here we analyse the complete cytochrome c oxidase subunit I (COI) minicircle molecule, containing the COI gene and an associated non-coding region (NCR), for ten dicyemid species, allowing for first time comparisons between species of minicircle architecture, NCR function and inferences of minicircle replication. Divergence in COI nucleotide sequences between dicyemid species was high (average net divergence = 31.6%) while within species diversity was lower (average net divergence = 0.2%). The NCR and putative 5' section of the COI gene were highly divergent between dicyemid species (average net nucleotide divergence of putative 5' COI section = 61.1%). No tRNA genes were found in the NCR, although palindrome sequences with the potential to form stem-loop structures were identified in some species, which may play a role in transcription or other biological processes.