  In the holographic framework, a half BPS Wilson loop in ${\cal N}=4$
supersymmetric Yang-Mills in the fundamental, symmetric or antisymmetric
representation of SU(N), is best described by a fundamental string, a D3 brane
or a D5 brane with fluxes in their worldvolumes, respectively. We derive the
spectrum of excitations of such D3 brane in $AdS_5\times S^5$ explicitly,
considering its action in both the bosonic and the fermionic sectors, and
demonstrate that it is organized according to short multiplets of the
supergroup $OSp(4^*|4)$. We also show that the modes of the fundamental string
form an ultra-short multiplet of this supergroup. In the case of the D5 brane
the bosonic spectrum is only partially known but we argue that it also fills
representations of $OSp(4^*|4)$. This way we provide a step towards a unifying
picture for the description of holographic excitations of the circular and
straight supersymmetric Wilson loops in arbitrary representations.
