The sea urchin is an echinoderm of growing interest as a marine resource. In some nations, such as Japan, sea urchin gonads, commonly known as "uni," are an integral component of local culture and cuisine. With almost 126 million residents, Japan is the greatest consumer of sea urchins in the world, consuming 80-90% of the world's sea urchin production [
1]. Meanwhile, Chile, with about 6,435 km of coastline, is the largest supplier of sea urchins, with more than 55,000 tons per year [
2]. The purple sea urchin
Paracentrotus lividus is extensively dispersed in the Mediterranean Sea and throughout the northeastern Atlantic coast, from Scotland and Ireland to southern Morocco [
3]. It is a commercially significant species with a strong market demand for its roe, notably in the Mediterranean Basin [
4] and in other European non-Mediterranean locations [
5,
6]. Interest in echinoculture arose because of the growing demand and the unsustainability of capturing wild sea urchins [
7,
8,
9,
10]. Echinoculture, in addition to responding to the demand increase, has helped to relieve pressure from fishing in wild populations [
11]. Good control of gonad development in sea urchin farming is extremely important to control the reproductive cycle of captive broodstock, but also to control the quality of the gonads when preparing them for collection and marketing for consumption. In Portugal,
P. lividus reproduces yearly, with gonad maturity and gamete release occurring at the same time for both sexes. The autumn-winter season marked the gonads' maturity, and the spring/summer season was a single, extended spawning season that followed [
12,
13]. The Mediterranean Sea has a varied reproductive cycle, with a first vigorous spawning in spring and a second, less intense spawning in October [
14]. Variations in environmental conditions, primarily seawater temperature, affects gonadal development and nutrient accumulation and also influences spawning duration, gametogenesis, and spawning are known to vary between geographical locations along a latitudinal gradient [
12,
15,
16]. Nevertheless, the reproductive cycle might also vary significantly from year to year according to how the environment (primarily temperature and photoperiod, but also hydrodynamics) affects
P. lividus population dynamics, reproductive biology, and secondary production [
12]. The quality of the food also plays a major role in the development of the gonads, and can strongly influence the gonadosomatic indices and biochemical characteristics of the gonads [
9,
12,
13,
14,
15,
16,
17]. The main objectives of this work were to understand the influence of two environmental parameters and feeding on the development and maturation of the gonads. To do this, two different seasonal periods were produced by adjusting the temperature and photoperiod. Additionally, two diets were tested: an inert feed formulated for sea urchins and a natural food (the macroalga
Ulva spp.).