Diverse pereopodal secretory systems implicated in thread production in an apseudomorph tanaidacean crustacean

Among arthropods, various insects, spiders, and crustaceans produce thread. The crustacean Tanaidacea include species that use thread mainly to construct dwelling tubes. While thread production was previously known only in Tanaoidea and Paratanaoidea, it was recently discovered in two species in Kalliapseudidae (Apseudoidea), although information on the morphology of the thread-producing system was lacking. Using histology, light and scanning electron microscopy, we found that the kalliapseudid Phoxokalliapseudes tomiokaensis comb. nov. lacks the sort of glandular structures associated with thread production in the pereonites, but has these structures in pereopods 1-6. We observed four types of glandular systems defined by the types and distribution of glands they contain: Type A (pereopod 1), Type B (pereopods 2 and 3), Type C (pereopods 4 and 5), and Type D (pereopod 6). All types have small rosette glands and lobed glands; Type A additionally has large rosette glands. The inferred thread-producing apparatus in P. tomiokaensis is very different from that in Tanaoidea and Paratanaoidea, suggesting that kalliapseudids evolved thread production independently from the latter two groups.