Factors Affecting the Viability of Bryozoan Statoblasts

by Anna G. Radke
BIOL/WATR 361, Spring 2015

Key taxa: Bryozoa, Phylactolaemata

Temperate fresh waters are harsh ecosystems for aquatic animals. Inland waters shrink and swell in volume, and occasionally dry up entirely. In higher latitudes they commonly freeze in winter. Ecosystem productivity varies seasonally. Toxins and contaminants wash in off the land. Organisms in these environments must be capable of surviving highly variable conditions. To this end, many aquatic invertebrates have adapted to life in freshwater by evolving resting stages, which can lay dormant until conditions improve (Caceres, 1997). Among these resting structures are the statoblasts of freshwater bryozoans.

Bryozoans (Phylum Bryozoa or Ectoprocta) are clonal, colonial filter-feeding animals. There are many interesting aspects to their biology and phylogenic relationships, but the asexual statoblasts — hard capsules of cells capable of surviving harsh environments and regenerating the colony — are among the most interesting.

Three classes of bryozoans are currently recognized. Of these, Stenolaemata is wholly marine, and Gymnolaemata inhabits primarily brackish waters (Carter et al., 2010). The third class, the freshwater Phylactolaemata, will be the focus of this essay. This class alone of the bryozoans forms statoblasts as an adaptation to freshwater environments. The degree to which these statoblasts resist adverse environmental conditions, and what cues trigger them to break dormancy, are areas of sometimes significant disagreement. The effects of various environmental conditions on statoblast germination have been the subject of several studies. While in some cases results are similar, in others they differ wildly. This contribution will review the existing literature, attempt to discover where consensus exists, and point out where more research is necessary.

Many organisms of temperate freshwater environments have some resistance to both freezing and desiccation (Caceres, 1997). It would be reasonable to suspect the statoblasts of phylactolaemate bryozoans (found in temperate freshwater) to be capable of surviving this seasonal variability.

Hengherr & Schill (2011) found evidence of the formation of ice crystals within the statoblast of Cristatella mucedo at relatively high (-5°C) temperatures without causing cell damage, suggesting freezing tolerance (Block, 1991). Oda (1979) found that C. mucedo was both greatly resistant to freezing and slightly more likely to hatch after exposure to freezing temperatures. However, other investigators report significantly reduced germination of statoblasts of Plumatella or Pectinatella species which had been frozen for any length of time (Bushnell & Rao, 1974; Mukai, 1974). Interspecies variation, possibly related to local climate, seems to be the driving factor in freezing resistance.

Statoblasts of certain bryozoans may survive extended periods of dryness, possibly by the production of trehalose sugars as water replacements (Rogick, 1938; Hengherr & Schill, 2011). Some investigators have had better success with storing statoblasts in dry conditions if they are also kept cool (Bushnell & Rao, 1974). Statoblasts may be quicker to germinate if dried temporarily (Oda, 1979). In other cases statoblasts will germinate poorly and fail to establish new bryozoan colonies if dried for any period.

Rogick (1940) ran identical experiments on statoblasts of several species. She found Lophopodella carteri capable of germination and growth after up to 4.25 years of dry storage; Fredericella sultana capable of germination but not growth after approximately 2 years; and Pectinatella magnifica, Hyalinella punctata, and Plumatella repens entirely incapable of germination after drying. Mukai (1974) found that Plumatella gelatinosa statoblasts were capable of surviving extended drying, but became dormant, hatching only after a much longer period than expected. It is possible that Mukai’s (1974) findings on induced dormancy are more widely applicable across genera, and some experiments were not long enough to allow statoblasts to break dormancy and germinate.

Seasonal variation in temperature and light conditions affects ecological productivity and the amount of food available to consumers, such as bryozoans. It might be reasonable to assume there is some ideal time for bryozoan growth, when most statoblasts would hatch. Any combination of time, temperature, or light might be expected to trigger or inhibit statoblast germination.

Seasonal dormancy is a type of temporal inhibition. P. gelatinosa statoblasts appear to undergo semi-obligatory dormant periods — statoblasts collected in summer and fall germinate most prolifically the following spring (Mukai, 1974). However, this species has only one generation a year. When multiple generations arise in a summer, as in P. magnifca, statoblasts may hatch immediately (Oda, 1979). If experiments are conducted during the “dormant season” for a species, fewer statoblasts may germinate than otherwise expected. However, the relative germination rates should be approximately the same for treatments and controls, so seasonality seems unlikely to affect experimental results.

Temperature seems to be an area of relative agreement. Most investigators find that statoblasts survive desiccation better at lower temperatures. In fact, the overall survival rate is increased if they are kept cool, no matter moisture conditions (Smyth & Reynolds, 1995; Rogick, 1938, 1941). Germination requires warmer conditions. Consensus points to 20-25°C as ideal for germination, with a range from 8-30°C (Bushnell & Rao, 1974; Smyth & Reynolds, 1995; Hengherr & Schill, 2011). Temperatures greater than 60°C appear to inhibit germination (Mukai, 1974). The exact temperature ranges vary between authors and may be species-dependent.

Light conditions are an area of greater disagreement, with arguments ranging from the necessity of light to the specific day-night cycle which best triggers germination. Some workers insist light has no effect. Hengherr & Schill (2011) found statistically similar germination rates at 20°C regardless of light exposure, and Oda (1979) had the same results if the statoblasts had first been subjected to a period of very low temperatures. Other investigators (Mukai, 1974; Rogick, 1938) insisted that some exposure to light is necessary. The photoperiod, or length of light versus dark periods, has also been investigated. Bergen (1964) found that 8 or 12 hour alternations, or exposure to natural day lengths, worked best. Mukai (1974) suggested that photoperiod is irrelevant; some alternation of light and dark, no matter if brief or unrepeated, is sufficient to induce germination.

Temperature manipulations in several of these studies add a confounding factor to photoperiod (Hengherr & Schill, 2011; Oda, 1979). The most stringent experimental conditions were those of Bergen (1964), who found a regular day-night cycle to be conducive to greatest germination. No investigator seems to have applied identical conditions to multiple species of bryozoan statoblasts, so the possibility remains that this is yet another area of inter-species variation.

Natural waters are sometimes subject to periods of low dissolved oxygen. Statoblasts do not seem to survive these periods well, with germination greatly reduced following storage in low-DO water (Brown, 1933). This low tolerance for hypoxic environments might impose a limit on bryozoan colonization of eutrophic waters. Conversely, their preference for shallow areas may negate the need for tolerance of anoxic conditions, as the upper portions of lakes are generally well-mixed and thus aerated, at least in the absence of ice cover.

A variety of chemicals occur in freshwater ecosystems. These might be expected to affect statoblast germination, but almost all research into the effects of chemicals on statoblast germination was conducted by the same investigators (Bushnell & Rao, 1974). This means their findings must stand unchallenged, but further research would be useful to verify their results.

Bushnell & Rao (1974) examined a number of heavy metals and pesticides for their effects on statoblast germinability. Copper sulfate (CuSO₄) had no impact on germination versus a control. Mercuric chloride (HgCl₂) had minimal impacts at low levels, but mortality increased with concentration. Cadmium sulfate (CdSO₄) was lethal at concentrations above 45ppm. Sodium salts of arsenic and molybdenum (AsNa₃O₃; MoNa₂O₄), while keeping germination below the level of the control, actually increased germination rates at higher concentrations.

Copper and molybdenum concentrations in natural waters have been relatively unaffected by human activity, but mercury, cadmium, and arsenic have increased dramatically since the Industrial Revolution (Hothem et al., 1995; Mast et al., 2010). It would make sense that bryozoans would be more tolerant to “natural” heavy metals. It is possible sodium may have some mitigating effect. Statoblasts were also found to be highly resistant to both organo-chlorine pesticides and 2,4-D (a dioxin pesticide), which are highly toxic to most life-forms (Bushnell & Rao, 1974). The authors had no suggested explanation for this result.

Shimada & Egami (1985) exposed the resting stages of various freshwater organisms to gamma radiation, and found bryozoan statoblasts less resistant than sponge gemmules or the resting eggs of rotifers and brine shrimp. Gamma radiation is effectively blocked from the Earth’s surface by the atmosphere and is not likely to have significant impacts on survival. The lower relative tolerance of statoblasts to this radiation may or may not be indicative of a lower overall tolerance to adverse conditions.

Other investigators have found that some statoblasts could pass unharmed through the digestive tracts of amphibians and turtles, with slightly lower survival rates for passage through duck guts (Bushnell & Rao, 1974; Figuerola & Green, 2002). The survival of statoblasts in vertebrate guts may be sufficient to make this a means of dispersal between water bodies. Genetic investigations to determine the interrelatedness of populations may shed light on this.

Spatial distribution has not been investigated for its role in statoblast tolerances. It may be that species tolerances vary by latitude or elevation, with each population adapted to the adverse conditions it most frequently encounters. A thorough study of how a single species of bryozoan responds to all the parameters mentioned above — individually or in combination — has not, to the author’s knowledge, been carried out. Nor have there been many comparative studies of multiple species under identical conditions (with the exception of Rogick, 1940). This is an area very deserving of further investigation.

There exist a few areas of broad consensus regarding the conditions suitable for the survival and germination of bryozoan statoblasts, and many of the areas of disagreement may be related to inter-species variation. Freezing tolerance and desiccation resistance are found in all species, but the degree varies. Light and photoperiod needed for germination may be species-specific, as may seasonal dormancy. The ideal temperature range for germination is fairly consistent between studies. Overall, there seem to be as many areas of disagreement as of consensus, and only further research will clear up the confusion. What is clear is that bryozoan statoblasts are an elegant adaptation to the problem of survival in freshwater environments.

References Cited

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• Block, W. 1991. To freeze or not to freeze? Invertebrate survival of sub-zero temperatures. Functional Ecology 5: 284-290.
• Brown, C.J.D. 1933. A limnological study of certain freshwater Polyzoa with special reference to their statoblasts. Transactions of the American Microscopy Society 52: 271-316.
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