Either spelling is acceptable.  Columellae is the latin plural, in
recognition of the fact that columella is a Latin noun; and columellas is
the english plural, which is also appropriate since this Latin word has been
incorporated unchanged into the english vernacular.  There are many such
words, where either form of pluralization is acceptable -
operculums-opercula; antennas-antennae; abacuses-abaci; formulas-formulae;
isthmuses-isthmi; memorandums-memoranda; tibias-tibiae; etc.  In some cases,
the Latin form is more likely to be used in scientific parlance (formulae;
antennae) while the english plurals (formulas; antennas) are likely to be
used in everyday conversation or non-technical writing.
Columellae-columellas probably falls into this category.

        Strictly scientific terms of Latin origin tend to maintain their
Latin form in technical usage, but when such words become incorporated into
the vernacular , the Latin form tends to gradually fall into disuse.  And
so, you would get strange looks indeed if you used the Latin plural for such
Latin words as crater, color, triton, tuba, vortex, tumor, sinus, atrium,
anus, census, dictator, forum, opera, aura, sector, etc.

        Paul M.