Beu: Evolution of Janthina and Reduzia 
121 
Figure 1. Living specimens of Janthina janthina (Linnaeus) in life position in an aquarium, suspended beneath their floats, redrawn from 
photographs in Wilson & Wilson (1956: pi. l,figs 1-2) and Yonge & Thomson (1976: pi. 5, lower); bubbles in float shown schematically. 
(A) left (anatomically right) lateral view, showing float, attached to mesopodium and located against propodium (at left end of float); 
partially extended snout; black-tipped right cephalic tentacle, not fully visible here; smooth, almost circular right epipodium (centre); 
digitate posterior edge of left epipodium, at rear between shell and mesopodium; and rear end of float disintegrating. (B) dorsal (anatomically 
ventral) view of another specimen forming a new float bubble, showing odontophore and radula partly everted out of mouth at tip of 
snout; black-tipped, forked cephalic tentacles; cupped propodium with black lining, in process of enclosing an air bubble; shell (right 
edge); and epipodium (left edge; anatomically on right). Scale bar 10 mm. 
Introduction 
Janthina and Reduzia species are holoplanktonic gastropods, 
the only molluscs with relatively large, almost completely 
enclosing shells 20-40 mm in diameter that are members 
of the marine neustonic community (David, 1965; Cheng, 
1975). They float at the surface of the open ocean suspended 
beneath a float constructed from bubbles of air coated with 
mucus. Consequently, all living species are cosmopolitan 
in temperate and tropical seas. Their calcitic shells are thin 
and brittle. The description of a brown outer teleoconch 
layer in Janthina chavani (Ludbrook, 1978) by Kaim et al. 
(2012: 428, figs 5C-D; identified as Kaneconcha knorri 
Kaim, Tucholke & Waren, 2012) demonstrates that the 
calcitic outer shell layer is only 25 pm thick, although the 
inner aragonitic layer can be up to 175 pm thick in some 
specimens. Janthina and Reduzia species float with the shell 
hanging spire-down, upside-down compared with benthic 
gastropods (Fig. 1). Some of the most accurate and useful 
drawings of living specimens of Janthinajanthina (Linnaeus, 
1758) and J. globosa (Swainson, 1821) were published by 
Okutani (1956), clearly illustrating the relationship to each 
other of the shell, epipodium, propodium and float. The 
other members of this small community of floaters at the 
interface of the sea and air are colonial cnidarians, including 
mainly the “Portuguese man-o-war” siphonophore Physalia 
physalis (Linnaeus, 1758) and the chondrophores Porpita 
and Velella , which form the main prey of Janthina ; floating 
anemones of the family Minyadidae, which form the main 
prey of Reduzia (Abbott, 1968: 93, text-fig.); other floating 
anemones oftheAbylidae (David, 1965: fig. 9); blue juvenile 
specimens of the fish Nomeus gronovii (Gmelin, 1789), 
which shelter within the tentacles of Physalia and feed on 
it, as well as on other jellyfish and plankton (the brown 
adults are demersal; Bailly, 2008); the nudibranch gastropod 
Glaucus (Batson 2003: 50), blue and free-floating, feeding 
on Physalia , Porpita and Velella , and the semi-neustonic 
nudibranch Fionapinnata (Eschscholtz, 1831), creeping on 
chondrophores and other floating objects; small blue crabs 
of the genera Planes and Pachygrapsus found clinging to 
siphonophores, turtles, sea-snakes and other floating objects 
(Chace, 1951; Davenport, 1992; FransenandTiirkay, 2007; 
Pons et al. , 2011); and several species of stalked barnacles 
(Lepadidae) that attach to Janthina shells and other floating 
objects and one species, Dosima fascicularis (Ellis & 
Solander, 1786), which has a thin, translucent shell, forms 
a bubble float and floats independently at the sea surface. 
Some individuals of D. fascicularis share a common float 
up to 15 cm across (Cheng, 1975: 199, fig. 14). Barnacles of 
New Zealand, including Lepadidae, were revised by Foster 
(1979) and those of Australia were reviewed by Jones (2012); 
most Lepadidae are very widespread and common to both 
countries. One species of polychaete, Hipponoegaudichaudi 
Audoin & Milne-Edwards, 1830, associates with neustonic 
animals and commonly shelters on Dosima fascicularis in the 
tropical Pacific (Cheng, 1975: 201). Some authors have used 
the term pleuston for this fauna (e.g., Savilov, 1969; Cheng, 
1975), but the more general term neuston is preferred here. 
Most obligate neustonic taxa are blue, an obvious 
protection at the sea surface. Janthina shells are bluish violet, 
counter-shaded, with a paler dorsal shell surface seen against 
the sky from under the living animal, and a darker ventral 
surface seen from above against the dark blue of the ocean. 
The cosmopolitan planktotrophic cerithioid gastropod Litiopa 
melanostoma Rang, 1829 (Litiopidae) is benthic but lives on 
floating Sargassum algae and on flotsam such as wood and 
pumice, so Litiopa is not part of the neuston, although it is 
rarely found in situations where it is not floating. Species of 
