How the distribution of colonies of the hydroid Laomedea flexuosa is limited to a narrow belt along the lower littoral zone

Abstract

In the White Sea the colonial hydroid Laomedea flexuosa inhabits a narrow belt of the lower littoral zone. How is so limited a habitat determined? We studied the time of planula release and the behaviour of larvae during the free-swimming stage and settlement in natural and laboratory conditions. Three methods were used to record the tidal-dependent dynamic of planula release: (1) plankton collecting bags around Fucus distichus macrophytes with mature hydroid colonies; (2) active stirring of Fucus with hydroids in a container with water, which is an old way to stimulate planula release; and (3) direct counts of the mature planulae in gonangia. The dynamic intensity of L. flexuosa planula release was investigated according to 3–4 phases of the tidal cycle. A moment of general release of larvae was shown to be correlated with a period of low water. Following the incoming tide keeps planulae in the littoral area. The planulae of L. flexuosa have a short period of swimming (less than 30 minutes), unlike those of Gonothyrae loveni and Dynamena pumila whose swimming period is about 36 hours. Quick settlement and tidal planula release could explain the strong spatial limitation in the distribution of L. flexuosa within the lower level of the intertidal zone. It is hypothesized that distribution over large areas is most likely by drift of colonies attached to detached seaweeds. Our outlook about the biological ways of ecological niche differentiation among marine organisms is broadened by the results of this study.

Effect of air exposure on the growth and distribution system in the colonial hydroid Dynamena pumila (L., 1758)

“…This species tolerates freezing, draining and temperature fluctuations associated with it (Marfenin, 1971). D. pumila has similar adaptations to air exposure as the previously described Laomedea flexuosa Alder, 1857 from the same biotope (Marfenin, Belorustseva, 2008). Relatively short weakly branched stems with short internodes and compactly arranged hydrants make it possible to form dense thickets, which retain moisture, preventing colonies from desiccation.…”

The association of coloniality with parental care of embryos

“…Most of the estimated times range from nearly immediate to about a day, though occasionally longer. Examples include larvae of pterobranch hemichordates (Lester, 1988), and entoprocts (Mariscal, 1965; Wasson, 1998), bryozoans (Keough & Chernoff, 1987; Reed & Cloney, 1982; Wendt, 1996), scleractinian corals (Carlon, 1999; Carlon & Olson, 1993; Hughes et al, 2019; Richmond, 1997; Ritson‐Williams et al, 2009; Sammarco & Andrews, 1989), octocorals (Coelho & Lasker, 2014; McFadden, Donahue, Hadland, & Weston, 2001), stylasterine hydrocorals (Gleason & Hofmann, 2011), other hydroids (Marfenin & Belorustseva, 2008), and sponges (Leys & Degnan, 2001; Maldonado & Young, 1996). Observations of larvae of colonial ascidians in nature have provided convincing evidence that settlement and metamorphosis occur soon after larval release and within short distances.…”

“…For some hydrozoan colonies, a medusa that has been reduced to a reproductive structure receives sperm from another reduced medusa, it broods embryos, and releases planulae. Local recruitment enabled by brooding can aid persistence of a population where a suitable habitat lasts long and is restricted to a small space or narrow zone: planulae of a campanulariid hydroid, Laomedea flexuosa , are released at low tide and can settle within a half‐hour, behaviors that accomplish local recruitment within the low intertidal zone (Marfenin & Belorustseva, 2008). Some hydroids with medusoids do not brood, however; the medusoid is released and embryos dispersed rather than protected on the colony.…”

On a collection of hydroids (Cnidaria, Hydrozoa, Hydroidolina) from the west coast of Sweden, with a checklist of species from the region

“…In Danish waters it is abundant where suitable substrates exist, and penetrates deep into the Baltic Sea (Kramp 1935b). The restricted vertical distribution of this hydroid in the White Sea was attributed by Marfenin & Belorustseva (2008) to intensive release of planula larvae during low tide, and to their subsequent rapid settlement as the water began to rise.…”