We constructed unique settlement plates, each representing six different niches, to assess ascidian niche breadth, and deployed them in similar habitats in the Red Sea and the Mediterranean. Settlement plate design: To estimate and compare habitat niche breadth between species, we designed settlement plates each representing different niches, based on six substrate squares measures 10 x 10 cm as our basic units. The base of the plate was built from stainless steel, and the substrate squares were glued to the base. We used three substrate types within two current /light regimes to allocate six different "niches” to each settlement plate, with total measurement of 30 x 20 cm. The three substrate types were constructed from three materials: cement with sea shells, non-glazed ceramic and recycled plastic. Since substrate is a major factor for fouling species, we selected three materials that differ substantially from each other: plastic is smooth, the non-glazed ceramic is rough but homogenous, and the cement with sea shells is complex and heterogeneous. The two current and light regimes were achieved by placing the plates facing the pillars, and leaving the upper part of the plate open from three directions (from above and from either side) while the lower part remained open from the bottom only and was thus dark and with restricted water flow. We were interested in the relative patterns across species to these niche differences, and hence do not attempt to quantify the exact differences in light and flow between the upper and lower sections. Nevertheless, light measurements confirm the lower side was ~10 time darker than the upper side. We used all possible permutations (36 in total) of substrate type ordering within each current and light regime treatment in constructing the settlement plates. Study sites: The same experimental designs were used in the eastern Mediterranean Sea and northern Red Sea to facilitate direct comparisons. In the Mediterranean, the settlement plates were deployed on three pillars of the Israel Electric Company pier (32°28′ N 34°53 E), and in the Red Sea, on three pillars of the Israeli oil port (29°31′ N 34°56′ E), with minimal distance of 10 meters between pillars. At both sites, the plates were deployed at about 15 meters depth, with the seabed at 20 meters, in order to avoid any bottom effects such as sedimentation. The depth of 15 meters was chosen in order to minimize disturbance to the experiment by the strong winter storms, while still providing sufficient underwater work time when using scuba. As public entrance to both sites is prohibited, the experiment was subjected to minimal human disturbance. Pillars at both sites are located in the open sea (as opposed to closed harbors) and are >30 years old, therefore the fauna found on them represent a climax community. Study design: The experiment lasted one year, from February 2014 to February 2015. For analyses, we combined two types of plates, seasonal and full-year. We deployed 15 full-year plates at each site and these remained undisturbed for the entire year of the experiment. In addition, 10 seasonal plates were replaced every three months, totaling 40 seasonal settlement plates at each site. At the end of each experiment, the settlement plates were removed and taken to the lab for taxonomic identification (using Nikon SMZ18 stereomicroscope and dissection tools). Solitary ascidians were counted and identified to species level where possible (649 out of 658 individuals). In addition, we took monthly underwater photos of all settlement plates. Plates were photographed from the exact same distance and angle using a custom-made tripod. These photos were used in order to identify individuals that were present on the plates during the experiment but did not survive to the point of plate removal, in order to increase sample size for the niche breadth calculations. Unfortunately, only four individuals were added using these photos. We categorized the species found in the Red Sea as Lessepsian (species that are known to establish populations in the Mediterranean) or non-Lessepsian species (species that have not yet been recorded in the Mediterranean, i.e., non-invaders). In the Mediterranean we categorized the species as non-indigenous or indigenous. We included Styela plicata with the indigenous species of the Mediterranean for analysis although it possibly invaded from the Atlantic Ocean (Pineda et al., 2011; Maltagliati et al., 2015) as it is clearly not of tropical origin, unlike the rest of the non-indigenous species, and has been found in the Mediterranean for at least a century (de Barros et al., 2009). Data file structure: plate – id for each settlement plate (reminder: the six different habitats described above are within each plate). site – Mediterranean or Red Sea species type – ‘Lessepsian’ and ‘non Lessepsian’ for the species found on plates in the Red Sea site. ‘indigenous’ and ‘non-indigenous’ for species found on plates in the Mediterranean site. season – ‘spring’, ‘summer’, ‘autumn’, ‘winter’ for the seasonality plates. ‘long term’ for the long term (full year) plates. And ‘added by pictures’ for three records of P. nigra we added in order to increase its sample size for our niche breadth calculations. niche – ‘su’ for shells-up, ‘cu’ for ceramics-up, ‘pu’ for plastic-up, ‘sd’ for shells down etc… substrate – shells, ceramics or plastic light – up – upper part of the plate or down – the lower part of the plate. number of individuals – the actual data – number of individuals found