Many marine species synchronize the different stages of their life cycles with opportunities to colonize environments by means of migration or passive transport associated with the movement of water. Physical and biological coupling is necessary to keep the life cycle from being interrupted and needs to be extremely precise, particularly when a life-cycle stage occurs in a habitat that is crucial to the ecological success of a species but which exists only ephemerally.Pederson and Peterson [1xBryozoans as ephemeral estuarine habitat and a larval transport mechanism for mobile benthos and young fishes in the north-central Gulf of Mexico. Pederson, E.J. and Peterson, M.S. Mar. Biol. 2002; 140: 935–947Crossref | Scopus (18)See all References][1] have presented a new, very elegant and accurate study of one such habitat. When studying the transitory appearance of certain benthic bryozoans in estuaries in The Gulf of Mexico, they noted that the larval stages of many invertebrate and fish species used this transitory habitat as an area for shelter and feeding. This type of habitat sometimes replaces another ephemeral habitat of great strategic importance, that of macroalgal and sea-grass populations. During periods of high growth of foliage and thalli, meadows of macroalgae provide shelter and protection for stages of many marine species that are most vulnerable to predation. Such habitats play a key role in explaining the life cycles of species and also the interannual variations in populations that have to synchronize their larval stages with the factors affecting opportunities to colonize ephemeral habitats. The authors concluded that habitats of this kind not only play a role as nursery areas, but are also involved in transport, as, for instance, in the case of sargassum drift algae.In their role as mechanisms for augmenting concentration levels, hydrographic fronts can also be regarded as transitory key habitats for many species in that they are highly variable as to their location and intensity. This concept of transitory key habitat has yet to receive the attention that it deserves, and further development of this concept might well give rise to different views or explanations of physical and biological coupling of life cycles. Other transitory habitats, for example coral reefs, permanent meadows of macroalgae, and deep coral reefs, are better understood. However, colonization of permanent habitats of this kind requires less precise spatio-temporal synchronization than does the colonization of ephemeral habitats.