EXAMPLE ESSAY: BODY SIZE
N.B. This is the question I chose because I felt you as a class would be less successful compared to the other questions. While you may not have been able to do this answer, you are perfectly capable of answering the other questions in a similar fashion. Also, this is the maximum answer; you would need most but not all examples to get full points. Most importantly: a) you need to hit all the major contrasts implicit in your question, b) you need to ensure you support those contrasts with real examples, and c) you need to write clearly enough that I can understand what you aer talking about.
What adaptations typically correlate with being small (interstitial) body size versus larger? Use examples from at least three taxa to support your answer, and explain WHY these adaptations are more appropriate for one lifestyle versus the other.
- complexity: smaller organisms tend to lose (or not have originated) systems seen in larger organisms. Gas exchange, and circulation can often be handled by diffusion only (5mm limit) in smaller organisms.
- Rotifers, tardigrades lack specialized gas exchange, circulation, structures. If diffusion can supply/drain tissues, there is no sense wasting space on specialized structures.
- all large-bodied animals with complex gas exchange structures (e.g. gills in molluscs, gills or tracheae in arthropods) while smaller members of these same phyla often lose these structures (e.g. many amphipods, small nudibranchs without gills)
- digestive tract without extensive musculature: many smaller organisms move food particles through tract via cilia (e.g. Bryozoans, rotifers) or by packing more food in at the mouth (e.g. Nematodes) in order to save space and investment. Larger animals (e.g. Annelid worms, molluscs) have digestive tracts with musculature in lining to move food by peristalsis. This is more efficient for large boluses of food than is ciliary motion.
- Excretion handled in smaller organisms by protonephridia if present, not metanephridia; the latter can handle larger volumes, but are "leakier".
- Polychaete annelid worms: smaller members with protonephridia, larger with metanephridia
- Turbellarians (Platyhelminthes) with protonephridia, but cells are always within 5mm of surface
- larger bodied simple phyla (sponges, cnidaria) have thin tissue layers where all cells close to external medium; overcomes lack of waste removal system
- very small or thin organisms (eg. Porifera, Cnidaria with thin tissue layers) lack specialized excretory structures
- Body cavities in smaller organisms tend to be pseudocoels or acoels; larger organisms coelomate. Due to reduced need for development and expansion and support of new structures requiring all tissue layers? Space-saving by eliminating mesenteries, allows better flow for circulation medium if circulatory system lacking. A pseudocoel provides a storage space for body products (e.g. gametes), serves as a hydrostatic skeleton
- eg. all acoel small or limited in body dimensions (flaworms can be long but not thick; Entoprocts all tiny polyps); blastocoels (nematoda, nematomorpha) are all small; tardigrades pseudocoel, even though other panarthropoda are coelomate.
- largest bodied phyla (mollusca, arthropoda, chordata) are all coelomate, at least in large-bodied representatives (e.g. coelom may be lost in some urochordates, but kept in cephalochordates and vertebrates)
- reproduction: tendency for brooding and internal fertilization at smallest sizes (e.g. Rotifers, turbellarians) though not clear cut: both also appear amongst largest animals (e.g. crustaceans that mate and brood, vertebrates). Smallest do not broadcast spawn, though this option seen in larger organisms (e.g. echinoids, asteroids).
- feeding: Large size restricts options
- smallest organisms show all types of feeding: e.g. small crustaceans can suspension feed (Anostracans), parasitize (Rhizocephalans), deposit feed or macro feed (Isopods) and some taxa can do both, such as Ophioroidea (Echinodermata) that switch among macro, deposit, suspension feeding.
- Parasites are typically small-bodied or thin (Platyhelminthes: Cestoda; Arthropoda:Insecta),
- Few suspension feeders are large-bodied except certain specialized habitats (e.g. Crustacea: Thecosomata; Echinodermata: Ophiuroidea and Crinoidea), and except those feeding on macroplankton (e.g. Cnidaria: Scyphozoa, Chordata: Vertebrata: Mammalia: Cetacea).
- Largest-bodied are mostly restricted to macrofeeding; some deposit feeding in worm-like forms (e.g. Annelida, Holothuroidea), which are not as massive. e.g. giant squid (Mollusca: Cephalopoda), 1m sea stars (Echinodermata: Asteroidea), king or spider crabs (Arthropoda)