Researcher Information

KAKUI Keiichi


Diversity in invertebrates

Department of Biological Sciences, Biodiversity


Taxonomy and biology of marine invertebrates, especially the crustacean order Tanaidacea

FieldSystematic zoology, Reproductive biology, Morphology, Zoology
KeywordMarine invertebrates, Arthropoda, Crustacea, Tanaidacea, Benthos, Taxonomy, New species, Molecular phylogeny, Reproduction, Sexual mode, Morphology, Life cycle, Evolution

Introduction of Research

I am interested in the taxonomy, phylogeny, reproductive biology, morphology, and evolution of invertebrate animals. The main focus of my research is the order Tanaidacea, a group of tiny aquatic crustaceans, with around 1500 species. However, past or ongoing collaborative projects in taxonomy and phylogeny have dealt with animal groups other than tanaidaceans, including isopods, amphipods, ingolfiellid, decapods, ostracods, copepods, mite, sea spiders, nematodes, nematomorph, entoproct, nemertean, annelids, platyhelminthes, feather star, and xenoturbelid.

Various tanaidaceans
A parasitic copepod (arrow) on a tanaidacean
A tanaidacean which can self-fertilize
A deep-sea tanaidacean with a wide distributional range spanning at least 3700 km
A tanaidacean described by my student, one collaborator, and me; it spins threads to make its tube-like nest (cf. Kakui et al. 2021)
A hermit-crab-like tanaidacean. Movie available at

Representative Achievements

Kakui K, Fukuchi J, Ohta M (2023) Diexanthema hakuhomaruae sp. nov. (Copepoda: Siphonostomatoida: Nicothoidae) from the hadal zone in the northwestern Pacific, with an 18S molecular phylogeny. Acta Parasitologica.
Kakui K, Hiruta C (2022) Protogynous hermaphroditism in Crustacea: a new example from Tanaidacea. Canadian Journal of Zoology 100: 481–487.
Kakui K, Shimada D (2022) Dive into the sea: first molecular phylogenetic evidence of host expansion from terrestrial/freshwater to marine organisms in Mermithidae (Nematoda: Mermithida). Journal of Helminthology 96: e33.
Kakui K, Hiruta C (2022) Description of a new Hamatipeda species, with an 18S molecular phylogeny (Crustacea: Tanaidacea: Typhlotanaidae). Zoological Science 39: 140–146.
Kakui K, Fleming JF, Mori M, Fujiwara Y, Arakawa K (2021) Comprehensive transcriptome sequencing of Tanaidacea with proteomic evidences for their silk. Genome Biology and Evolution 13: evab281.
Academic degreePh.D.

Department of Biological Sciences, Biodiversity

KAKUI Keiichi


What is your dream that you want to achieve through your research?

Do you know what tanaidaceans are? I think perhaps not. I want to change this situation and popularize this group of small aquatic animals. Tanaidaceans are scientifically interesting; some of them can spin silk in sea water, others behave like hermit-crabs and carry empty gastropod shells, and some can even change their sex. I study various aspects of tanaidacean biology, including their taxonomy, systematics, reproductive biology, and so forth, both for their benefit and mankind’s.

Various tanaidaceans. From left to right: two thread-producing species; gastropod-shell carrier; sex-changing species (it also produces thread). Scales: 0.5 mm.
What made you decide to become a researcher?

I can’t remember any specific event. When I was a PhD candidate, I realized that I liked research and wanted to do more of it (at least for some more years). Being a researcher became one of the options for my life’s work.

Please tell us about yourself, things you are good at, your favorites, hobbies, and daily routines.

One of my favorite things is taking photos of small creatures. There are many interesting creatures around us, and well-taken photos motivate me in writing manuscripts. I also enjoy encountering and learning about animal groups that I’m not familiar with. I like rearing animals. Aside from science, I get a lot of pleasure in traveling to new places and eating the local food. Finally, listening to music and playing with my family add a lot to my life.

Various animals I have ever met.