A rare kind of seahorse and a rainbow-coloured sea slug with a titillating name are among the creatures making a comeback in UK waters, according to an annual conservation review.

The coast around Britain is now home to more than 100 species of nudibranchs – brightly hued, soft-bodied marine molluscs that appear nude because of their lack of external shells. The Wildlife Trusts credited a big conservation push around the coast for their proliferation.

It was a good autumn for sightings of curled octopus, the trusts said, and basking sharks were seen in Cardigan Bay for the first time in three years. The Wildlife Trusts are a grassroots movement of 800,000 people who help survey shores to gather information and monitor marine protected areas.

In Dorset, fishermen have been reporting sightings of the extremely rare short-snouted seahorse off the Purbeck coast. This toothless breed has a short, upturned snout, which it uses to suck up its favourite prey of small shrimp and plankton. Not very good swimmers, they use their tails to cling on to seagrass or seaweed, and they face various threats, from trawlers scouring the seabed to yacht anchors, according to the Wildlife Trusts.

In Cornwall, the spiny lobster or crawfish is making a comeback from overfishing in the 1960s and 1970s, while undulate rays seem to be thriving along the south coast, though they are still considered endangered following over-exploitation.

According to reports plastic has been found in almost 100% of gannet nests on Alderney. Photograph: Alderney Wildlife Trust/PA

The little tern, one of the UK’s rarest breeding seabirds, has scored some successes with the help of conservation work. The bird successfully bred at Cumbria Wildlife Trust’s South Walney nature reserve for the first time in 33 years, and nested on Essex’s Tollesbury Wick nature reserve for the first time in 10 years.

Despite concerns in the spring that late snowfall in the Arctic would hit the breeding success of sanderlings, which migrate through the UK, there were record numbers at Gibraltar Point, Lincolnshire, in the autumn.

The findings were not all positive, however, with millions of creatures washed up on beaches along the North Sea coast after a storm in March. There were also sewage spills and storm drains dumping wet wipes and sanitary products on to beaches, while plastic pollution continues to be a major problem and beach cleans took tonnes of litter off the shoreline.

On Alderney, plastic mostly from fishing industry rope or line is now found in almost all gannet nests, posing a significant risk to birds and chicks.

Beach cleans on the Isle of Wight collected 400 bags of rubbish, while Welsh Wildlife Trust collections picked up 14,095 pieces of litter. In Kent, 2,892kg of rubbish and 60 shopping trolleys were collected from the Medway Estuary.

Dr Lissa Batey, senior living seas officer at the Wildlife Trusts, said: “This review of sightings and action from across the UK has given a glimpse, a mere taster, of the wonders of our marine wildlife – delightful species that everyone has the opportunity to encounter and learn more about.

“But it has also shown us the problems that remain and the challenges that our sea life faces. It’s not too late. We are already seeing recovery in some of our marine protected areas, but we don’t yet have a fully functioning network of nature reserves at sea, where wildlife has the opportunity to thrive.

“That’s why we are looking forward to the third designation of marine conservation zones in 2019 – with these we would have the potential to reverse current marine wildlife declines.”

• This article was amended on 4 December 2018 to clarify that conservation information about seahorses came from the Wildlife Trusts.