An analogue of the Mukai map $$m_g: {\mathcal {P}}_g \rightarrow {\mathcal {M}}_g$$
m
g
:
P
g
→
M
g
is studied for the moduli $${\mathcal {R}}_{g, \ell }$$
R
g
,
ℓ
of genus g curves C with a level $$\ell $$
ℓ
structure. Let $${\mathcal {P}}^{\perp }_{g, \ell }$$
P
g
,
ℓ
⊥
be the moduli space of 4-tuples $$(S, {\mathcal {L}}, {\mathcal {E}}, C)$$
(
S
,
L
,
E
,
C
)
so that $$(S, {\mathcal {L}})$$
(
S
,
L
)
is a polarized K3 surface of genus g, $${\mathcal {E}}$$
E
is orthogonal to $${\mathcal {L}}$$
L
in $${{\,\mathrm{Pic}\,}}S$$
Pic
S
and defines a standard degree $$\ell $$
ℓ
K3 cyclic cover of S, $$C \in \vert {\mathcal {L}} \vert $$
C
∈
|
L
|
. We say that $$(S, {\mathcal {L}}, {\mathcal {E}})$$
(
S
,
L
,
E
)
is a level $$\ell $$
ℓ
K3 surface. These exist for $$\ell \le 8$$
ℓ
≤
8
and their families are known. We define a level $$\ell $$
ℓ
Mukai map $$r_{g, \ell }: {\mathcal {P}}^{\perp }_{g, \ell } \rightarrow {\mathcal {R}}_{g, \ell }$$
r
g
,
ℓ
:
P
g
,
ℓ
⊥
→
R
g
,
ℓ
, induced by the assignment of $$(S, {\mathcal {L}}, {\mathcal {E}}, C)$$
(
S
,
L
,
E
,
C
)
to $$ (C, {\mathcal {E}} \otimes {\mathcal {O}}_C)$$
(
C
,
E
⊗
O
C
)
. We investigate a curious possible analogy between $$m_g$$
m
g
and $$r_{g, \ell }$$
r
g
,
ℓ
, that is, the failure of the maximal rank of $$r_{g, \ell }$$
r
g
,
ℓ
for $$g = g_{\ell } \pm 1$$
g
=
g
ℓ
±
1
, where $$g_{\ell }$$
g
ℓ
is the value of g such that $$\dim {\mathcal {P}}^{\perp }_{g, \ell } = \dim {\mathcal {R}}_{g,\ell }$$
dim
P
g
,
ℓ
⊥
=
dim
R
g
,
ℓ
. This is proven here for $$\ell = 3$$
ℓ
=
3
. As a related open problem we discuss Fano threefolds whose hyperplane sections are level $$\ell $$
ℓ
K3 surfaces and their classification.