Protostomes are one of the most diverse and abundant groups in the animal kingdom. Their distribution, variety, and abundance are largely the result of evolutionary adaptations to climatic changes in their respective environments. At present, they inhabit a wide range of terrestrial and marine environments. Many protostomes are familiar to most people, including spiders, earthworms, snails, mussels, and squid, to mention just a few. Their lifestyles, origins and diversity all have underlying structures and functions that depend on the interactions between abiotic (nonliving) and biotic (living) components of the environment both past and present. From the fossil record, protostomes first appeared about 600 million years ago, although researchers believe that many of the early members of this group became extinct. Those few that did survive, however, evolved and radiated, or diversified, into the variety of protostomes that biologists recognize today.
Protostomes are presently classified into annelids, arthropods, mollusks, brachiopods and bryozoans. The annelids are thought to include about 9,000 species known to be living in marine, freshwater, or moist soil environments. Perhaps the best-known annelid genera are Hirudo (leeches), Nereis (clam-worms) and Lumbricus (earthworms).
By contrast, the largest group in the animal kingdom is the arthropods, which account for almost three-quarters of all living animal species, and have adapted successfully to most terrestrial and aquatic habitats around the world. Early arthropods known as trilobites are an extinct group that have been extensively described from the fossil record. With regard to present-day species, some arthropods are free-living while others are parasitic. This extremely large group of protostomes includes the crustaceans (e.g., Astacus, crayfish; Carcinus, shore crab); the myriapods (e.g., Lulus, millipedes); the insects (e.g., Periplan-eta, cockroaches; Apis, bees); and the arachnids (e.g., Scorpio, scorpions; Epeira, web-spinning spiders).
Mollusks are the second largest group in the animal kingdom, comprising around 100,000 known living species. Most are marine (e.g., Mytilus, mussels; Loligo, squid; and Octopus), although some are such well-known terrestrial animals as Helix, the land snail, and Limax, the garden slug.
Brachiopods (lampshells) and bryozoans are marine organisms that are distinguished by a feeding structure called a lophophore. Bryozoans are colony-forming animals attached at the base to the substrates on which they live. The most common bryozoans include such encrusting species as Bower-bankia and gelatinous colonies like Alcyonidium, some of which provide food and shelter to many small benthic organisms. Understanding how protostomes interact with one another and their physical environment, yet continue to survive from generation to generation is a central theme in their ecology.
Ecology is a broad topic, yet it has a number of themes that apply to all living organisms. At its most basic level, ecology is the study of interactions between animals and the abiotic and biotic factors in their environment through the acquisition and reallocation of energy and nutrients. Ecologists also examine the cyclic transfer of these elements to sustain life processes. The major themes in ecology include limiting factors; ecosystems; population issues; ecological niches; species interactions; competition; predator and prey dynamics; feeding strategies; reproductive strategies; and biodiversity. Research related to these themes has produced some of the most complex and diverse findings in the animal kingdom when it is focused on protostomes.
The concept of limiting factors in ecology is related to the control or regulation of population growth. These factors include abiotic as well as biotic aspects of the environment. For protostomes, the availability and consumption of food is an important biotic limiting factor. During the planktonic stages of many benthic (ocean bottom) protostomes, for example, the seasonal abundance of phytoplankton in the water column will directly affect the mortality of protostomes, and hence the number or success of individuals during recruitment.
Other limiting factors are abiotic, particularly temperature, salinity and light. These factors affect the type and number
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