A cryptic radiation of Caribbean sea slugs revealed by integrative analysis: Cyerce ‘ antillensis’ (Sacoglossa: Caliphyllidae) is six distinct species Author Moreno, Karina Department of Biological Sciences, California State Polytechnic University Pomona, Pomona, CA 91768, USA Author Rico, Diane M. Department of Biological Sciences, California State University, Los Angeles, CA 90032 - 8201, USA Author Middlebrooks, Michael Department of Biology, University of Tampa, Tampa, FL 33609, USA Author Medrano, Sabrina Department of Biological Sciences, California State Polytechnic University Pomona, Pomona, CA 91768, USA Author Valdés, Ángel A. Department of Biological Sciences, California State Polytechnic University Pomona, Pomona, CA 91768, USA Author Krug, Patrick J. Department of Biological Sciences, California State University, Los Angeles, CA 90032 - 8201, USA pkrug@calstatela.edu text Zoological Journal of the Linnean Society 2024 2023-10-12 200 4 940 979 http://dx.doi.org/10.1093/zoolinnean/zlad111 journal article 10.1093/zoolinnean/zlad111 0024-4082 PMC10983082 38566915 11240925 E8CC81A3-E625-4C48-B783-29AA9BFC83C3C Cyerce willetteorum sp.nov. ( Figs 1–3 , 5 , 8F , 17 ) ZooBank registration: lsid:zoobank. org:act: B2309FF2-4BF7-4317-BE5F-5398AB298F30 Type material Holotype : Stirrup Cay , Bahamas , July 2001 , 2 mm preserved length ( LACM 3844 ; isolate 07Stir01). Range Great Stirrup Cay, Bahamas (present study). Description External morphology: No photograph was taken of the live specimen, hence no description of external morphology is possible at this time. Internal morphology: Radula of holotype ( LACM 3844 , isolate 07Stir01) with six teeth on ascending limb and seven teeth on descending limb ( Fig. 17A ). Teeth shallow, bent at an angle of 18°, tapering to pointed tip; 97 µm in length ( Fig. 17B ). Row of nine denticles along each cutting edge, irregular in shape and width; narrow at base, widening to a curved edge, ≤ 2.5 μm wide. Denticles narrower, more rectangular near tip ( Fig. 17B ). Partial ascus with four pre-radular teeth ( Fig. 17C ). Penis with curved, wide stylet, narrowing gradually from embedded base, 120 µm long; oval opening at wide tip ( Fig. 8F ). Ecology Collected from Pe. capitatus . Etymology Named in joint honour of Dr Demian Willette, who contributed specimens to this work and helped P.J.K. to initiate studies of sacoglossan biodiversity, and his brother Darian Willette, a devoted naturalist lost at too young an age. Figure 16. Cyerce ellingsonorum , radular scanning electron micrographs of LACM 3841 (isolate 06JAM09). A, complete radula with ascus. B, close-up view of tissue-free teeth from ascending limb, showing rectangular denticles. C, close-up view of ascus. Remarks Only a single exemplar of C. willetteorum was sampled over the 15-year duration of this study of Western Atlantic sacoglossans. However, the lone specimen was supported as a distinct species by all methods of molecular species delimitation. Although the external morphology could not be described at this time, the penial stylet was distinct from the other complex members, less curved and shorter than C. nicholasi but more curved and longer than any other newly described species. The radular denticles were also distinctive, with an irregular shape and curved edge not observed in any other species. Given the genetic support for the holotype as a separate species and the distinctive characteristics of the internal anatomy, we elected to name the species to clarify the diversity of the Caribbean Cyerce spp. and to facilitate future studies of the biology of this rare taxon. This was the only species collected from Pe. capitatus , although C. cf. cristallina associates transiently with Pe. capitatus at night (M. Charteris, unpublished data). Algal congeners Pe. dumetosus and Pe. lamourouxii were used as hosts by four other complex members and have thicker filaments than Pe. capitatus on the ‘shaving brush’ portion of the algal thallus. However, extensive collections of Pe. capitatus made elsewhere in the Caribbean did not yield specimens of Cyerce , leaving it unclear whether Pe. capitatus is a preferred host of C. willetteorum or was merely the more locally abundant Penicillus sp. at the type locality. Further study is needed to understand the basis of host specificity within the C. antillensis complex.