Hatchling ecology

Temperature may be the most important determinant of developmental time in marine invertebrates.  Interestingly, results from studies on Nucella ostrina at the Bamfield Marine Sciences Centre, British Columbia and a review of literature on developmental times for other muricids and other invertebrate groups show that, relative to the total time required for maturity, muricids take longer to hatch at a given temperature than other invertebrates.  The author remarks on the unusually precise dependence of time to hatch on temperature (see graph), even though factors such as egg size, number of embryos per capsule, ratio of nurse egg to embryo, and egg-capsule morphology vary considerably. The comparatively long time to hatch coupled with this precision suggest that developmental rates in muricids are constrained at some fundamental but unknown level.  Palmer 1994 p. 177 In, Reproduction and development of marine invertebrates (Wilson et al., eds.) The Johns Hopkins U Press, Baltimore. Photograph courtesy Dave Cowles, Walla Walla University, Washington wallawalla.edu.

NOTE  the graph includes data for N. ostrina from Alaska to British Columbia as well as for other direct-developing muricids

Whelks that hatch as crawl-away juveniles do not immediately take up the adult way of life.  For example, studies at the Bamfield Marine Sciences Centre, British Columbia show that newly hatched whelks Nucella ostrina immediately crawl to specific microhabitats, such as beds of green algae Cladophora columbiana, clusters of mussels Mytilus spp., and assemblages of barnacles Semibalanus cariosus.  There is abundant prey in these microhabitats and the graph shows that the hatchlings remain there until they reach 2-3mm shell length. 

Other experiments by the researchers indicate that in these habitats the youngsters are protected from desiccation¹ (upper histogram on Right), and predators² (lower histogram on Right), in comparison with what they might experience on the open rock surface³.  Densities of potential predators may number up to 440 ^. m^-2 in habitats of N. ostrina.  Add to this the risk of possible mortality caused by UV light, dislodgement by waves, and damage from water-borne debris, and the potential for hatchling survival is poor, indeed.  Gosselin & Chia 1995 Mar Ecol Progr Ser 128: 213; Gosselin & Chia 1995 Mar Biol 122: 625.

NOTE¹  the data show mortality over a 4-h period in the intertidal zone for 3 replicates of 20 hatchlings for each microhabitat.  All hatchlings die on the open rock surface, while less than 2% die in moist Cladophora beds.  Field temperatures are not specified by the authors, but they note that the day of the experiment was characterised by sun and wind, and “high" potential desiccation stress

NOTE²  the data show mortality over a 5-h period in the laboratory for 6 replicates of 20 hatchlings for each microhabitat.  Each set of hatchlings is exposed to 2 predatory hermit crabs Pagurus hirsutiusculus and P. granosimanus, and one shore crab Hemigrapsus nudus.  Mortality of hatchlings is less than 36% in the protective microhabitats but close to 100% on the open rock surface.  Of 45 potential predatory species examined in the laboratory, only P. hirsutiusculus and H. nudus cause substantial hatchling mortality

NOTE³  the horizontal lines at the top indicate values that do not differ significantly

To determine if Nucella ostrina hatchlings are being transported to protective microhabitats in water currents, the authors install mesh traps located 15cm above the substratum surface.   Not only are no hatchlings caught in the traps during three 48-h trials during the time of hatching of N. ostrina, but the authors could observe no behavioral responses in the laboratory to currents suggestive of transport capability.  Hatchlings of other gastropod species that use water currents for transport respond to moderate water currents by raising their feet and releasing mucous threads, and to turbulent water currents by detaching and floating off.  It seems that N. ostrina hatchlings crawl to their protective microhabitats at an estimated rate of 4mm ^. min^-1, as measured on smooth rock surface in the laboratory, and they appear to do this immediately upon leaving their capsules.  As a general conclusion, the authors suggest that the use of protective microhabitats may be the key to survival through the vulnerable early juvenile period in many benthic marine organisms.  Gosselin & Chia 1995 Mar Ecol Progr Ser 128: 213.

NOTE  the traps are 2 mesh bags, one  filled with mussels, the other with Cladophora

NOTE information on behaviour of hatchlings of Lacuna vincta, which use mucous threads for transport, can be found elsewhere in the ODYSSEY: LEARN ABOUT LITTORINES: REPRODUCTION & DEVELOPMENT

Although the biggest ontogenetic¹ change in the lives of most marine invertebrates occurs at metamorphosis, other significant but less conspicuous shifts can occur later one.  In studies at the Bamfield Marine Sciences Centre, British Columbia the author of previous Research Studies 2 & 3 adds further detail to the story of “ecological shifts” during the juvenile life of the whelk Nucella ostrina.  For example, survival of juveniles exposed to desiccation on rock plates at 22^oC increases with increasing size (see histograms upper Left: note that all treatments start at Time zero at 100% survival).  Once a shell length of 6.5mm is reached, survival is 100% over an 8-h period.  This is sufficient to withstand a full cycle of tidal exposure. 

Susceptibility to predators² (mostly decapod crustaceans) is also markedly less once a hatchling reaches a size of about 6mm (see graph on Right).

Shell coloration also shows ontogenetic shifts (see graph³ lower Left). The hatchlings are 100% white at first and the frequency of this colour decreases to 0% by 6-7mm shell length.  “Intermediate” colours of grey, brown, and orange increase in frequency to about 5-6mm shell length, then black begins to predominate.  These colours increasingly appear as bands until they begin to merge at about 5mm.  Hatchlings of 14mm shell length are mostly black or orange, and banding is almost absent. The author notes that the function of coloration in N. ostrina hatchlings is unclear, but may be camouflaging or, with respect to the lighter hues, a means to reduce temperature stress.  When the hatchlings reach a size of 6-8mm shell length, they tend to leave their protective microhabitats and move out onto open rock surfaces.  Note that by 8mm shell length, or about 4mo of age based on laboratory studies, ontogenetic colour changes are mostly complete.  The author concludes by noting that postmetamorphic life, at least in N. ostrina, should not be termed “adult” because it implies that life through this period is regulated by a consistent set of processes, which it clearly is not.  Gosselin 1997 Mar Ecol Progr Ser 157: 185.

NOTE¹  lit. “self origin” G., referring to an organism’s development

NOTE²  susceptibility to predators is expressed as an index representing the density (# indiv ^. m^-2 ) of all predators of hatchlings in a given area.  Data for 3 areas (Ross Islets, Wizard Islet, and Dixon Island) are combined in the graph presented here

NOTE³  the graph is complex, and requires a bit of noodling. The curves show % frequency of different colour morphs with increasing size of hatchling. The hatched bars show % frequency of banding on the shells, that is, whether the colour is solid or banded

A useful method of marking hatchling whelks Nucella spp. for field study involves the use of calcein, a fluorescent dye.  The snails are immersed in a solution of 100 ppm calcein in filtered seawater for 24h.  The long-duration exposure is important because the dye is incorporated into new shell growing at the aperture of the shell.  The dye fluoresces under visible blue wavelengths at 460nm, and thus requires only a simple filter set and dissecting microscope for its detection.  Tests on hatchlings of Nucella ostrina collected in Oregon show that the dye has no significant effect on growth or survival at the 100ppm concentration, and the mark is long-lasting.  The author cautions that when using calcein for the first time on a selected taxon, investigators should perform pilot experiments to determine the lowest concentration that will result in an effective mark with no deleterious effects.  Moran 2000 Mar Biol 137: 893.

NOTE  another method of marking hatchling whelks as small as 0.9mm shell length is with dabs of nail polish.  The marks are retained for more than 60d in the field and different colours can be used to designate different test groups of snails.  Gosselin 1993 J Mar Biol Assn UK 73: 963.

NOTE  calcein is 2,4-bis-[N,N’-di(carbomethyl)-aminomethyl]-fluorescein: SIGMA #C0875

Based on the premise that Nucella ostrina hatchlings would be larger in habitats with more severe environmental conditions, researchers at the Bamfield Marine Sciences Centre, British Columbia examine hatchling size in 10 closely sited habitats differing in 2 components of environmental stress: wave exposure and predation.  Variation in wave exposure at the 10 sites is shown in the histogram on Left. The sites are arranged in order of wave exposure in the histogram, with size of hatchlings shown for each site. The authors collect 3yr of data but, for visual clarity, only one set is shown here. The data support the authors' hypothesis regarding size and wave exposure, but not their prediction about predation, suggesting that predation may not be as important in determining hatching size as wave exposure.  Whether the factor is the waves themselves, or something associated with them, is unclear. The authors conjecture that waves or water-borne debris may dislodge or crush smaller snails more easily than larger ones, but acknowledge that the exact mechanism is unknown.   Gosselin & Rehak 2007 Mar Ecol Progr Ser 339: 143.

NOTE  the 10 sites are in Barkley Sound, British Columbia, and are within 10km of one another. See map on Right showing location and degree of wave exposure

NOTE  known predators at the various sites include 3 species of hermit crabs Pagurus hirsutiusculus, P. granosimanus, and P. samuelis, and 2 Hemigrapsus species H. nudus and H. oregonensis and density of these at each site represents “predation pressure