Trivirostra hordacea (Kiener, 1845)
|Maximum size: 7 mm shell length (Severns, 2011).
In mature animals, the mantle is light brown flecked with
yellow-white and darker brown. It's studded with simple tubercles tipped
with yellow (more prominent near the mid-line). The body is
translucent cream and its upper surface is irregularly striated with
white. Shells are white and
typically smaller (and with a lower profile) than in the
Trivirostra edgari. The apical margin is usually straight to slightly convex. Size at
maturity is variable (3-7 mm shell length based on my material and Severns, 2011).
Natural history: Trivirostra hordacea
is a moderately common species in
moderately exposed to exposed rocky habitats and Halimeda kanaloana beds from 3-30 m (10-98 ft).
Big Island, Maui, Oahu, Kauai and Maro Reef: widely distributed in the Indo-Pacific.
It's listed in Kay, 1979, as Trivia hordacea. It may also be listed in Kay, 1979 as Trivia pellucidula. (Note 1) In addition, it may be listed in Severns, 2011 as Trivirostra cf. hyalina, Trivirostra ginae, Trivirostra sp., Trivirostra shawi and Trivirostra thaanumi. (Note 2)
Photo: CP: Maalaea Bay, Maui; April 26, 2003.
Note 1: The primary difference between T. hordacea and T. pellucidula
as discussed and illustrated in Kay, 1979, is the absence of a dorsal
sulcus in the latter. Otherwise, they seem identical. Since the dorsal
sulcus appears to develop as the animal matures in T. hordacea and maturity occurs over a wide size range it seems likely that Hawaiian shells attributed to T. pellucidula are actually slightly immature T. hordacea. This is not intended to address the question of whether or not the material in the original description is distinct. (see photo)
Note 2: We have seen many living animals of both T. hordacea and T. edgari
with the characteristics of the living animals being consistent.
We have not seen any animals that would suggest the presence
of other species in the genus. Shells are much more ambiguous. Both
species show a wide size range at maturity with some overlap. (see photo) And, they
show substantial variation based on ontogeny and size at maturity.
Therefore, it seems more parsimonious to assign the Hawaiian material to
two variable species rather than to a multitude of more narrowly
defined species as suggested by Severns, 2011. We have tentatively
assigned the illustrations in that source to either T. hordacea or T. edgari
based on their dorsal margins, profiles and axial sculpture. Again,
this is not intended to address the question of whether or not the
material in the original descriptions of those species is distinct.