We study the nuclear star clusters in spiral galaxies of various Hubble types using spectra obtained with STIS on-board HST . We observed the nuclear clusters in 40 galaxies , selected from two previous HST/WFPC2 imaging surveys . At a spatial resolution of \sim 0 \farcs 2 the spectra provide a better separation of cluster light from underlying galaxy light than is possible with ground-based spectra . Approximately half of the spectra have a sufficiently high signal-to-noise ratio for detailed stellar population analysis . For the other half we only measure the continuum slope , as quantified by the B - V color . To infer the star formation history , metallicity and dust extinction , we fit weighted superpositions of single-age stellar population templates to the high signal-to-noise spectra . We use the results to determine the luminosity-weighted age , mass-to-light ratio , and masses of the clusters . The models provide excellent fits to the data and generally require a mixture of populations of different ages . Approximately half of the sample clusters contain a population younger than 1 Gyr . The luminosity-weighted age ranges from 10 Myrs to 10 Gyrs . The stellar populations of NCs are generally best fit as a mixture of populations of different ages . This indicates that NCs did not form in a single event , but instead they had additional star formation long after the oldest stars formed . On average , the sample clusters in late-type spirals have a younger luminosity-weighted mean age than those in early-type spirals ( \overline { \langle \log \tau \rangle _ { L } } = 8.37 \pm 0.25 vs . 9.23 \pm 0.21 ) . The average mass-weighted ages are older by \sim 0.7 dex , indicating that there often is an underlying older population that does not contribute much light , but does contain most of the mass . The average cluster masses are smaller in late-type spirals than in early-type spirals ( \overline { \log M } = 6.25 \pm 0.21 vs . 7.63 \pm 0.24 ) , and exceed the masses typical of globular clusters . The cluster mass correlates loosely with total galaxy luminosity . It correlates more strongly with both the Hubble type of the host galaxy and the luminosity of its bulge . The latter correlation has the same slope as the well-known correlation between supermassive black hole mass and bulge luminosity . The properties of both nuclear clusters and black holes in the centers of spiral galaxies are therefore intimately connected to the properties of the host galaxy , and in particular its bulge component . Plausible formation scenarios will have to account for this . We discuss various possible selection biases in our results , but conclude that none of them can explain the differences seen between clusters in early- and late-type spirals . The inability to infer spectroscopically the populations of faint clusters does introduce a bias towards younger ages , but not necessarily towards higher masses .