There is growing concern about the effects of enhanced levels of solar ultraviolet radiation on the living components of the biosphere (i.e. cancer, loss of biodiversity and productivity, etc.). In shallow coastal environments, many benthic species release their gametes directly in the water column where fertilisation occurs and the planktonic larvae remain for several weeks. Any effects on these early life stages could significantly impair reproductive input or alter the fitness of the community. The purpose of this paper is to provide new insights into the mechanisms of UV toxicity on sea-urchin spermatozoa in a cytological context, and to address the question of the potential ecological consequences of the damage. The Mediterranean sea-urchin Sphaerechinus granularis (Lamarck) was chosen as a model to study the effects of ecologically relevant doses of UV-R on the spermatozoa of marine invertebrates. Structural damage was visualised by use of transmission electron microscopy and the single-cell gel electrophoresis (Comet) assay was used to assess chromatin integrity in spermatozoa. The present results provide experimental evidence that irradiation with UV induces structural and chromatin damage in sea-urchin sperm. Almost 90% of spermatozoa exhibited morphological alterations and DNA strand breakage increased 2-fold. The observed alterations of the acrosome, plasma membrane and mitochondria can explain the concomitant impairment of fertilisation (23% decrease of fertilisation rate), which in turn may affect reproductive success. On the other hand, how DNA damage and fertilisation rate correlate remains unclear; however, when not repaired genetic lesions can lead to abnormal development and/or the transmission of heritable damage. The 3-fold decrease of the frequency of 2-celled embryos indicates a delay or inhibition of the first cell division, which may be ascribed to impairment of nuclear chromatin and/or other cellular targets.