dark central cerata
groups, in field
with egg mass
Costasiella formicaria (Baba, 1959)
|Maximum size: about 15 mm.
has a cream body frosted with dusky gray and flecked
with white. The gray pigment is densest on the edges of the rhinophores
and head but varies in intensity and distribution. The cardiac sack is
branches of the digestive gland in the cerata range from olive green to
golden brown, again with variation in intensity. It may become darker
with age and the central cerata may be markedly darker than the lateral
Natural history: Prior to 2020, Costasiella
formicaria was known from only three animals found in highly
protected to moderately exposed locations. The one Maui
animal came from a moderately exposed rocky area at 3-7 m (10-28 ft). In
2020 and 2021, many animals were found at depths of 6-12 m (19-40 ft) on open
sand and in Halimeda kanaloana
beds where it appears to feed on a sand-dwelling, filamentous
green alga. Japanese animals fed on a similar-appearing algae identified
as Pseudochlorodesmis furcellata (Jensen, et. al., 2014). It lays a white, collar-shaped egg mass and its cerata are
autotomized. During population blooms they may form mating, resting and/or migratory aggregations. (Note 1)
Big Island, Maui, Oahu and Midway: also known from Japan.
It was first
recorded in Hawaii from Kaneohe Bay, Oahu by Terry Gosliner.
Midway Atoll; May 30, 1993.
Observations and comments:
1: Kelly McCaffrey (and others) reported seeing 50 to 100 animals on multiple occasions on open sand and on sand between Halimeda kanaloana
patches (crawling, by day). He reported sizes ranging from
7-8 mm to about 15 mm with larger animals being darker. Animals were
often seen in moderately large groups suggesting that they form mating
aggregations and/or migrate between patches of algae as local food is
exhausted. For a video of an aggregation (by Kelly Mccaffrey) click here. Pam Madden also found dense clumps of resting animals at night suggesting that they may stay together for prolonged periods. (see photos)