The precise number of
Okenia
taxa inhabiting the Mediterranean Sea, as well as their general taxonomy, varies according to different specialists. So far, eight valid species have been reported from the area:
Okenia aspersa
(Alder & Hancock, 1845),
Okenia cupella
(Vogel & Schultz, 1970),
Okenia elegans
(Leuckart, 1828),
Okenia hispanica
Valdés & Ortea, 1995,
Okenia impexa
Er. Marcus, 1957,
Okenia leachii
(Alder & Hancock, 1854),
Okenia mediterranea
(Ihering, 1886), and
Okenia zoobotryon
(Smallwood, 1910). Of these, only three (
O
.
elegans
,
O
.
hispanica
, and
O
.
mediterranea
) have their type localities in the Mediterranean Sea, whereas the others were described from different biogeographic areas and later included in the Mediterranean biota. We carried out a review on Mediterranean
Okenia
species through an integrative approach, based on a wide literature search and a morphological and molecular analysis of available type material and samples collected recently. The present study confirmed the presence of
O
.
aspersa
,
O
.
elegans
,
O
.
hispanica
, and
O
.
mediterranea
in the Mediterranean Sea, although leaving remaining questions about some of those taxa. The distribution of
O
.
cupella
,
O
.
impexa
, and
O
.
zoobotryon
is limited to the western Atlantic, and of
O
.
leachii
to the eastern Atlantic. All specimens previously identified as
O
.
cupella
,
O
.
impexa
, and
O
.
zoobotryon
by different authors in the Mediterranean Sea were repeatedly misidentified. Thus, we describe
Okenia problematica
sp. nov. and
Okenia longiductis
sp. nov., from the “Mediterranean”
Okenia cupella/impexa
and
O
.
zoobotryon
. We also consider here
Okenia pusilla
Sordi, 1974 a
nomen dubium
and include a redescription of the holotype of
O
.
cupella
. A molecular phylogeny, including all the sequenced
Okenia
species, was performed in order to evaluate the evolutionary relationships of the newly described species with the other congeneric taxa.
