A seahorse carries the burden of cuttlefish eggsIt is the curious incident of the cuttlefish and seahorse in the daytime.Biologists diving in the seas off Spain witnessed a European cuttlefish laying its eggs on a passing seahorse, which then swims away, eggs in tow. The researchers believe they have filmed the European cuttlefish laying eggs in the wild for the first time. However, during filming, one cuttlefish appears to confuse the seahorse with seaweed, laying its eggs on the passing fish despite the water being clear. The startling behaviour was captured on film by Manuel Enrique Garcia Blanco, an underwater cameraman and photographer, who works with Ms Fiona Read, a PhD student at the Marine Research Institute in Vigo, Galicia, Spain. Together, the researchers are investigating the sustainability of fisheries in the northwest Iberian Peninsula, including an old type of fishing for cuttlefish still practised by fishers living in a few villages within Galicia. As part of the research, Mr Blanco was diving off the island of Toralla in the Ria de Vigo, where cuttlefish arrive in the winter to breed and spawn. During one dive, he filmed a pair of European cuttlefish (Sepia officinalis) mating and laying eggs in the seagrass, which to the best of the researchers' knowledge is the first time such behaviour has been recorded in the wild. Over two hours, he recorded the cuttlefish laying 15 eggs at two different sites. The cuttlefish attached the eggs onto seagrass, which holds the eggs in the current, laying an egg at each site around three minutes apart. This is similar to what other European cuttlefish do when reproducing in captivity, behaviour observed by the researchers the year before in an aquarium. However, Mr Blanco also captured another, much more improbable first: a seahorse swimming along with two cuttlefish eggs attached to its tail. "The cuttlefish had laid the eggs on the seahorse rather than the seagrass," Ms Read told the BBC. "From the egg-laying behaviour, it is almost certain the cuttlefish put the eggs on when the seahorse was sitting in the seagrass." Rather than let the seahorse be compromised by the eggs, Mr Blanco decided to intervene. "The eggs were really tight round the seahorse's tail and it was thought to effect the survival of the seahorse," explains Ms Read. "So the seahorse was captured and the eggs were cut from its tail. "The seahorse is expected to survive but the eggs were damaged during the dissection."