Animals exhibit specialized adaptations for wetlands, as well. Many benthic invertebrates, such as some nematodes, crabs, and clams, have either higher concentrations of oxygen-transporting pigments, or pigments with higher oxygen affinity than usual. Some organisms are effective at conserving oxygen. Fiddler crabs (Uca spp.) can maintain their resting respiration rates at oxygen levels down to 5 to 15% of saturation. At lower oxygen
TABLE 15.19 Physiological Adaptations to Wetlands Conditions
Accumulation of malate
Increased levels of nitrate reductase
Slight increases in metabolic rates
Ability for root growth in low oxygen tensions
Absence of alcohol dehydrogenase (ADH) activity
Nonwetland species concentrate ethanol, a toxic by-product of anaerobic respiration, when growing in anaerobic soil conditions. Under such conditions, many hydrophytic species produce high concentrations of the nontoxic metabolite malate instead, and unchanged concentration of ethanol, thereby avoiding accumulation of toxic materials (e.g., Glyceria maxima, Nyssa sylvatica var. Biflora).
Nitrate reductase is an enzyme involved in conversion of nitrate nitrogen to nitrite nitrogen, an intermediate step in ammonium production. Nitrate ions can accept electrons as a replacement for gaseous oxygen in some species, thereby allowing continued functioning of metabolic processes under low soil oxygen conditions. Species that produce high levels of nitrate reductase include Larix larcina.
Anaerobic soil conditions effect short-term increases in metabolic rates in most species. However, the rate of metabolism often increases only slightly in wetland species. Examples species include Larix laricina and Sencio vulgaris.
Some hydrophytic species (e.g., Nyssa aquatica, Myrica gale) are capable of transferring gaseous oxygen from the root system into soil pores immediately surrounding the roots. This prevents root deterioration and maintains the rates of water and nutrient absorption under anaerobic soil conditions.
Some species (e.g., Typha angustifolia, Juncus effusus) have the ability to maintain root growth under soil oxygen concentrations as low as 0.5%. Although prolonged (> 1 year) exposure to soil oxygen concentrations lower than 0.5% generally results in the death of most individuals, this adaptation enables some species to survive extended periods of anaerobic soil conditions.
ADH is an enzyme associated with increased ethanol production. When the enzyme is not functioning, ethanol production does not increase significantly. Some hydrophytic species (e.g., Potentilla anserina, Polygonum amphibium) show only slight increases in ADH activity under anaerobic soil conditions.
Source: U.S. Army Corps of Engineers (1987).
levels (down to 2% of saturation) they can further reduce their respiration to survive. Clams can respire anaerobically for a time with their shells closed.
The simpler animals are osmoconformers; they adjust their cell osmolarity in response to the external salinity. More complex animals are osmoregulators, which maintain their internal environment in the face of external changes. The difference depends on the ability of the organism to excrete salt. In crabs it also depends on the permeability of the shell to salt. Crabs such as Cancer are always submerged; they have a more permeable shell and are an osmoconformer. Crabs that spend part of their time out of the water, such as Uca spp., are excellent osmoregulators. Their shells are relatively impermeable to salt. Other species fall in between.
The annelids are divided into two groups. The polychaetes are osmoconformers. Eggs and sperm are released into the water, where fertilization occurs. Oligochaetes and
TABLE 15.20 Reproductive Adaptations to Wetland Conditions
Prolonged seed viability
Seed germination under low oxygen concentrations
Some plant species produce seeds that may remain viable for 20 years or more. Exposure of these seeds to atmospheric oxygen usually triggers germination. Thus, species (e.g., Taxodium distichum) that grow in very wet areas may produce seeds that germinate only during infrequent periods when the soil is dewatered. (Note: Many upland species also have prolonged seed viability, but the trigger mechanism for germination is not exposure to atmospheric oxygen.)
Seeds of some hydrophytic species germinate when submerged. This enable germination during periods of early-spring inundation, which may provide resulting seedlings a competitive advantage over species whose seeds germinate only when exposed to atmospheric oxygen.
Seedlings of some hydrophytic species (e.g., Fraxinus pennsylvanica) can survive moderate periods of total or partial inundation.
Source: U.S. Army Corps of Engineers (1987).
leeches, on the other hand, are osmoregulators. Fertilization is internal, and the resulting zygote is then enclosed in a cocoon.
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