custom plural for the short form of an acronym using glossaries

Question

I'm using glossaries and would like to use a custom short form plural (it should be the same as the short form singular).

I've tried using \glsshortpluralkey, but this only affects how the short form plural appears the first time the acronym is used (MWE below). I've also tried using \glsshortpluralaccess and \glsshortplural, but both are 'undefined control sequences'. I'm using TeX Live 2013.

\documentclass{article}
\usepackage[colorlinks]{hyperref}
\usepackage[acronym]{glossaries}

\makeglossaries

\newacronym[\glsshortpluralkey=cM]{cM}{cM}{centiMorgan}

\begin{document}

First use: \glspl{cM}. Second use: \glspl{cM}.

\printglossary[type=acronym,title=List of Abbreviations]

\end{document}

Edited to clarify: the desired output is:

First use: centiMorgans (cM). Second use: cM.

Answer 1

In chapter 4 of the user manual the two options are plural and firstplural.

The change is simply: \newacronym[plural=cM,firstplural=centiMorgans (cM)]{cM}{cM}{centiMorgan} and the mwe with this change is:

\documentclass{article}
\usepackage[colorlinks]{hyperref}
\usepackage[acronym]{glossaries}

\makeglossaries

\newacronym[plural=cM,firstplural=centiMorgans (cM)]{cM}{cM}{centiMorgan}

\begin{document}

First use: \glspl{cM}. Second use: \glspl{cM}.

\printglossary[type=acronym,title=List of Abbreviations]

\end{document}

This results in: