Dyma'r ail lyfr yng nghyfres Cynefin. Mae hwn yn llyfr sy'n trafod cynefin difyr glan y mr - yr anifeiliaid, yr adar, y pryfetach, ymlusgiaid, amffibiaid, pysgod, molysgiaid, cramenogion, cen, planhigion, gwymon a chregyn. Telir sylw i nodweddion gwahanol fathau o draethau, clogwyni ac aberoedd Cymru, gan dynnu sylw at draethau penodol y gall darllenwyr ymweld nhw. The second title in the Cynefin series, aimed at children and adults alike. This book deals with seaside natural life - animals, birds, insects, reptiles, amphibians, fish, molluscs, crustaceans, lichen, plants, seaweed and shells, with reference to the diverse characteristics of Welsh beaches, cliffs and estuaries.