Sunlight has an important role in both terrestrial and marine environments, powering the process of photosynthesis that provides energy either directly or indirectly to nearly all forms of life on earth. The diel, or 24-hour cyclical migra tions of epipelagic species, are at least in part active responses to changing light levels. Epipelagic refers to the upper levels of the ocean that are penetrated by enough sunlight for photosynthesis to occur. Aurelia aurita approaches the surface during the day, at both midday and midnight, or only at night, and becomes scattered throughout the water column at night or during the sunlit days. Diel migrations probably do not occur in the bathypelagic zone (about 3,280-6,562 ft or 1-2 km); migration in the mesopelagic zone (about 656-3,280 ft or 200-1,000 m) depends on the levels of available light in that zone. Sunlight is necessary for vision as well as photosynthesis. Many animals rely on their vision to capture prey, avoid predation, and communicate with one another.
Bioluminescence is a type of visible light produced by marine animals such as scyphozoans, hydrozoans, ctenophores, squids, thaliaceans, and fishes. It may be used for counteril-lumination or as ventral camouflage. Another possibility is that bioluminescence is a useful defense mechanism against potential predators.
Turbidity refers to the cloudiness of sea water caused by the suspension of sediment particles and organic matter. High concentrations of suspended particles in the water over offshore coral reefs are considered a stress factor for coral colonies because they reduce the amount of light for photosynthesis and smother coral tissues. Nevertheless, many reefs with large growths of coral are found in relatively turbid waters, such as the fringe reefs around the inshore continental islands in the Great Barrier Reef lagoon. This finding suggests that turbid water is not necessarily harmful to coral. Fine, suspended particles provide a large surface area for colonization by microorganisms that produce organic nutrients. By limiting light penetration, turbid water also limits the distribution of both benthic algae and phytoplankton, which are at the base of the web.
All lower metazoans are ectothermic (sometimes referred to as "cold-blooded"), which means that they retain the same temperture as their surroundings. Because of this restraint; invertebrate physiology has evolved to operate in a specific temperature range for each species. Most organisms can tolerate only a narrow range of temperatures; changes above or below this critical range disrupt their metabolism, resulting in a lowered rate of reproduction, injury, or even death. Since temperatures change less rapidly in the open sea than in shallow waters, species in shallow waters can tolerate a wider range of temperature than deep water species. Temperature often influences the distribution, reproduction, and morphology (form and structure) of these organisms. Colonies of Obelia geniculata and Silicularia bilabiata living in cold water develop long, branching hydrocauli (stalks), whereas colonies of these species living in warm waters have short stems with few branches. Gametogenesis in the hydrozoan Sertularella mi-uresis begins when the temperature reaches 50°F (10°C) and stops when it reaches 64°F (18°C). Coryne tubulosa reproduces asexually at around 57°F (14°C), but produces medusae when the temperature falls to 35°F (2°C). The acclimation temperature of Chrysaora quinquecirrha polyps is about 51°F (10.50°C), but the upper lethal temperature dose, defined as the temperature at which 50% of the test animals die (LD-50), is 95°F (35°C).
Salinity, or the level of salt content in seawater, can affect invertebrates. Species that have evolved to live in freshwater can rarely live in salt water, and few marine species can tolerate low salinities or freshwater. This becomes quite apparent when one studies species richness (number of species) as one moves down a river into an estuary. Species richness (number of species) is relatively high in freshwater, then decreases considerably as salinity increases to about 5ppt, where most freshwater species cannot exist. Species richness then increases with salinity as more low-salinity-tolerant species are encountered. Species richness is at its greatest at the mouth of the estuary, where fully marine species occur with estuar-ine species.
Salinity can effect the morphology of organisms. For example, the shape, number, and size of tentacles of Cordylophora caspia polyps is affected by salinity. The scyphozoan medusae of Rhopiena esculenta can survive at levels of salt concentration as low as 8 parts per thousand (ppt), the scyphistomae to 10 ppt and the planulae to 12 ppt. The estromatolites of Phyl-loriza peronlesueri, however, form in hypersaline (very salty) waters. A rise in the salt content of the Baltic Sea allowed A. aurita and C. capillata to expand into northern waters, and allowed Rhizostoma pulmo and A. aurita to move from the Azov Sea into the Black Sea. The hydroid Laomedea flexuosa increases its production of gonozooids when the seawater concentration is around 30-40 ppt; at higher concentrations, however, the colonies begin to degenerate. The cephalo-chordate Branchiostoma nigeriense becomes opaque above salinities of 13 ppt.
Moving water is essential to lower metazoans because it supplies food and dissolved gases; prevents the accumulation of sediment; and disperses waste products, medusae, and larvae. Aglaophenia picardi resorbs the tissues of its hydrocauli into the hydrorhiza when the surrounding water is relatively stagnant, but regenerates them when the water begins to move more rapidly. The speed and direction of current flow affect
A sponge feeding. (Illustration by Barbara Duperron)
the form and size of some lower metazoans. The size of hy-droids is usually inversely related to the speed of water movement; large specimens are found in calm water and smaller specimens in rougher water. Aglaophenia pluma develops un-branched hydrocauli about 0.6 in (1.5 cm) tall in shallow, turbulent water, but produces branched hydrocauli as high as 19.6 in (50 cm) in deeper water with bidirectional currents. Planar (flat) forms such as A. pluma, Plumularia setacea, and Eudendrium rameum are most abundant where the current tends to flow in one direction, while radial or arborescent (treelike) forms such as Lytocarpia myriophyllum, Nemertesia antenna, and E. racemosum flourish in bi- or multidirectional currents. The distribution of species that inhabit coral reefs and display highly specific patterns of tolerance is greatly affected by water movement. Morphological differences in hydroids and anthozoans are also regarded as indicators of distinct patterns in water movement.
The majority of organisms are not able to survive in great depths (below 3,281 ft or 1,000 m). In general, the number of invertebrates is highest in shallow water communities and decreases as water depth increases. However, species diversity may be quite high at great depths on the abyssal plain where the environment has been extremely stable for millennia. Though diversity can be high, biomass may be low in these deep benthic habitats, because the lack of light prevents any primary production. Therefore, these habitats are usually limited by food and depend on organic input from sunlit seas above.
The lack of mixing and primary production result in oxygen-minimum layers in the ocean, and many species are either adapted to lower oxygen concentrations or avoid these areas. The scyphozoans Periphylla periphylla and Nausithoe rubra show high levels of the anaerobic enzyme lactate dehydrogenase, probably as an adaptation to moving at depths between 1,312 and 4,921 ft (400-1,500 m), which has minimal levels of oxygen.
Pollution may result from contamination by sewage, hydrocarbons, polyvinyl biphenals (i.e., PCBs), pesticides (e.g., DDT), and heavy metals such as cadmium, copper, lead, mercury, and zinc. Experiments have revealed that exposure to pollutants can lead to sublethal effects in hydroids, including changes in the curvature or branching of the hydrorhiza; loss of hydranths; stimulation of gonozooid production; or changes in the rate of growth. Low concentrations of metal ions such as copper and mercury may inhibit growth regulation in hydroids while increasing the growth rate in Laomedea flexuosa and Clavopsella michaeli. In Elefsis Bay, a polluted area of Greece, populations of Aurelia aurita have multiplied to rates of more than 1,500 medusae per 10 m3. Certain species of Rhizostoma have survived in parts of Madras Harbor that have been polluted by diesel oil; however, the presence of crude petroleum in the waters of Alaska has caused a reduction or cessation in the strobilation in the polyps of A. aurita, and the production of ephyra and polyps with both morphological and behavioral abnormalities. Pelagia noctiluca, a scyphozoan from the Mediterranean Sea, acquires high concentrations of cadmium, lead, mercury and zinc. Individuals of the species Chrysaora quinquecirrha have been found to have highly concentrated levels of the herbicide pendimethalin in their tentacles; they show no change in behavior at concen trations of the pesticide that are lethal to fishes such as perch. The dumping of raw sewage in may tropical areas of the world destroys coral reefs by increasing turbidity that prevents light penetration, increasing sediment loads that smother corals, and increasing nutrient loads that encourage algae growth that can out-compete corals.
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