David versus Goliath: An interspecific comparison between small-sized Halicryptus spinulosus and large-sized Halicryptus higginsi (Priapulida)

Abstract

Halicryptus includes two priapulid species with highly different adult body sizes, small-sized Halicryptus spinulosus von Siebold, 1849 and large-sized Halicryptus higginsi Shirley and Storch, 1999. Due to ambiguity of diagnostic characters from juvenile to adult size and between species, a detailed morphological and molecular comparison is needed to resolve the distribution and characteristics of each species. We investigate an adult paratype specimen of H. higginsi, and young adult specimens of H. spinulosus from the Beaufort Sea and H. spinulosus from Kandalaksha Gulf, White Sea. Molecular analysis of the mitochondrial CO1 gene confirm the existence of two species of Halicryptus, a wide distribution of H. spinulosus across the Northern Hemisphere and the co-occurrence of both species in the Beaufort Sea. Our morphological analyses reveal undocumented characters for Halicryptus, such as the presence of three different sizes of buccal papillae, scattered small trunk papillae, and a loose ring of flosculus-tubulus-complexes on the posterior trunk. For both species, we revise characters, such as the number of long introvert retractors, pharynx protractors and lateral cusps of anterior teeth, and the maximum number of scalids in a row in H. spinulosus. We propose a total of ten reliable and potential diagnostic characters distinguishing the two species and revise the terminology of Halicryptus-specific characters due to inconsistency in the literature. In addition, we discuss the effect of body size on selected morphological traits.