​Shark-cetacean aggregations and associations of false kill- er whales (Pseudorca crassidens) with other cetaceans have been reported previously (e.g., Heithaus 2001; Baird et al. 2008). However, this is the first description of non- aggressive associations of P. crassidens with non- cetaceans.
Here we report encounters of P. crassidens associated with silky sharks (Carcharhinus falciformis) at different reef sites of the Farasan Banks, Red Sea, Saudi Arabia in 2014. On August 13, a group of 17+ P. crassidens (2-4 m in body length; including one juvenile, <2 m; Fig. 1a) approached a dive boat and spent approximately 45 minutes interacting with ~20 snorkelers at BDohra Island Reef^ (19° 49'16.80"N, 39° 54'6.75"E). The group was accompanied by 6+ C. falciformis (1.6-2 m; Fig. 1b), which remained in close proximity (~15 m) to snorkelers and whales for approximately 20 minutes (Fig. 1c).
On August 19, a group of 15 divers was approached by eight P. crassidens individuals (2-4 m; including 1 juvenile <2 m), followed by two C. falciformis (1.6-2 m) at BOld Fan- tasy Island Reef^ (19° 38'36.39"N, 40° 1'10.66"E). Both en- counters lasted for 10 and 2 minutes, respectively. During both occasions neither whales nor sharks displayed signs of distress or rejection towards each other’s presence. An additional sighting of a P. crassidens group accompanied by C. falciformis on June 18 in the southern Farasan Banks was reported by the dive base (Dream-Divers, KSA), but no details are available.
The possible benefits of associations between false killer whales and silky sharks remain unclear and the aggregations might be random without direct benefits for the different spe- cies. However, shared feeding grounds might provide an ex- planation for the presence of multi-species groups. On August 13, an individual of P. crassidens carried a prey fish (supple- mentary video). Even though no feeding was directly ob- served, prey indicates a previous hunting event and may sug- gest the possibility of a foraging association (sensu Lukoschek and McCormick 2002).