Mar 26, 2013

1. Mar 26, 2013 Stabilimentum Pumps, gas exchange, circulation Reproduction

2. Spiders and silk • Spider silks are proteins with tensile strength, extensibility and a great ability to store strain energy. The only ‘poor-performance category’ of silk is resilience: which is really a virtue since one doesn’t want resilience in a trap. Silk’s physical features combine to make an orb web effective in stopping projectiles, i.e., flying insects. “The work [energy imparted] of prey against web has to be dissipated somehow – the device is, after all, a catch-net and not a trampoline!” (Vogel, p. 344) and you don’t want the trapped prey to be flung back and out of the trap. • Orb weavers Araneidae: float a silk line on the wind, anchor it at both ends, drop a line from its middle, create radii as a scaffold to walk upon and finally lay down a viscid (sticky) spiral to trap flying insects. silk spigots: spinnerets Microangela

3. Argiope (garden spider) is an orb-weaver spider. Many orb-weavers add a stabilimentum to their web. • Stabilamenta are thick silk radiated near the web hub and before the viscid spiral zone, quite variable in form (consistent with a hypothesis of tension correction). Muhammed Mahdi Karim Ted McRae

4. Alternative hypotheses of selective advantage: Reinforcement Tension adjustment Camouflage Visual marker Because functioning as an insect trap, web silk is selected to reflect poorly in the UV where the vision of is effective; but this is not so for the stabilimentum silk which reflects UV well (?). Spiders also do push-ups at the advance of a big blundering non-prey item making themselves conspicuous. notice leg allignment Eisner T. 1983. Spider web protection through visual advertisement: role of the stabilimentum. Science 219: 185-187. Eisner showed that the presence of visual markers significantly affected the time a web could last without mechanical damage (from flying birds.

5. Some spiders keep their webs up night & day, and some spin anew each evening, taking their web down again at dawn; this latter group don’t make stabilimenta and their webs were used in Eisner’s experiment, the residents having been captured. Tested preserving capacity of visual web markers: triangular strips of white paper forming an X • 30 natural webs without stabilimenta (control). • 30 comparable webs adorned with artificial equivalents of stabilimenta. • Webs with artificial stabilimenta survived “intact through the early morning period when birds are on the wing”.

6. Pumps for circulation:(Wikkipedia): a positive displacement pump makes a fluid move by trapping a fixed amount and forcing (displacing) that trapped volume into the discharge pipe. This is a pretty good definition of a animal heart. • A theme of our course has been the incompressibility of fluid and so its role in effecting hydraulic skeletons or jetting locomotion. Fluid incompressibility is also the basis of pumps: fluid contained as part of a vessel system or vessel and sinus system can be circulated – and used to transport important chemicals (hormones, respiratory gases etc.) – through the agency of muscular organs that constrict a space and displace the incompressible fluid out of a cavity or along a tube, giving it motion. • Pumps can displace air or blood. An example of an air pump: the abdominal air sacs of insects such as the locust: an example of a blood pump: heart of a locust, a fish, a human. Hearts occur in many different places: it would seem relatively easy to evolve a heart: invest a vessel containing fluid with more muscle in its walls and install a valve to prevent backflow. • First the tracheal system and its role in pumping – ventilation.

7. Tracheae are branching tubes conducting air inward from spiracular openings. They need to be prevented from collapsing against the force of the surrounding haemolymph and other tissue. They are strengthened by spiral cuticular taenidia.

8. Internally tracheae end blindly in fine intracellular tubes known as tracheoles. “These are partly filled with fluid and the extent to which the air penetrates down them varies with the state of activity of the insects. If it is very active, or if oxygen is witheld, the air extends further down the tubes and comes nearer to the tissues, thus shortening the final path of slow diffusion in water (Ramsay J.A. Physiological Approach to the Lower Animals) “The great advantage of the tracheal system is that a high oxygen tension can be maintained in the tissues without energy being wasted in maintaining a rapid flow of blood... But on the other hand, since the rate of diffusion is inversely proportional to the distance, the tracheal system is not readily adaptable to the needs of larger animals.”

9. Metrioptera sphagnorum, bog katydid The large acoustic spiracle is not closable: it conducts sound down to the eardrums • Insects ventilate their tracheal system using primarily the abdomen (tagma). Muscles make the segments of the abdomen of a stridulating katydid telescope in and out; the abdomen pumps like a bellows, supplying oxygen as the insect is singing (rubbing its forewings together). • Air flow is made unidirectional through timing the opening and closing of metameric spiracles. • Front to back flow in the locust: anterior spiracles are opened, all others closed and inspiration by expanding abdomen; then spiracle 10 opens while all others close, abdomen volume reduced giving expiration. one pair per segment: abdominal spiracle acoustic spiracle Dita Klimas

10. Pumps that move water or blood rather than air: respirationfish and insect circulation, closed and open • Circulatory systems are closed high pressure or open low pressure: or some combination of these. • An example of a closed, relatively high pressure, circulatory system is that of a fish: a two-chambered heart pumps in two stages to make high pressure and relatively high flow rate within vessels, arteries and veins, through which blood circulates. • A contrasting example -- an open circulatory system -- is that of an insect: a muscular dorsally situated tube (heart) pumps fluid anteriorly and sits in the blood it pumps, which circulates at relataively low pressure within large body spaces called sinuses. • Open systems may or may not function to transport gases. In the case of the insect, gas exchange is accomplished by a tracheal system working separately. What is the difference between gills and lungs? Gills are evaginations; lungs are invaginations. Gills project out from the body (even if recessed in a branchial cavity); lungs are an inpocketed chamber within a body.

11. Because in insects the tracheal system does the job of moving respiratory gases to and from the tissues, the circulatory system proper can be low pressure, low flow. • There is a heart, a long muscular tube, situated within a pericardial sinus, a space defined by a dorsal diaphragm. The dorsal longitudinal vessel (heart posteriorly, aorta anteriorly) draws in haemolymph from this sinus through ostia (with valves) and pumps blood peristaltically anterior, leaving the vessel in the head to circulate rearward through sinuses. Aliform [wing-shaped] muscles are the antagonists of the circular heart muscles.

12. Gill ventilation in fishes involves pumping both water and blood • Unidirectional flow of water in fishes is generated by a buccal force pump combined with an opercular suction pump. These two pumping actions operate out of phase, to achieve a near continuous flow of water. (Like the phase differences in the cycles of the three muscular fans of Chaetopterusthat pump within its burrow.) The water enters the fish’s mouth, passes through the buccal cavity, then is forced out through gill slits in the gut wall into a branchial cavity containing gills. The branchial cavity is covered with an operculum and the water exits posteriorly between the edges of the operculum and the body. Oxford Illustrated Science Encyclopedia

15. The gills of bony fishes are located in the pharyngeal cavity in a branchial chamber, covered on the outside by the operculum. Inside the branchial chamber thin epidermis is folded into plates called lamellae grouped on filaments. The filaments are arranged along a gill arch (four arches on each side).

16. There are two ways of passing water by blood: same direction parallel current or in opposite directions; the latter is counter-current . It is far more efficient to use counter-current because diffusion gradients are maintained over the length of the passage. Parallel-flow diffusion gradients decrease steadily and so soon diffusion will stop. The same principle applies to heat exchange (porpoises warming their flippers).

17. Structural designs for gas exchange • Keep diffusion paths short. • Circulate the fluid [or move relative to the fluid] [more important if the fluid is water than air, because oxygen has a relatively low solubility in water compared to air]: this is to maintain the steepest possible diffusion gradient. • Maximize surface area (trade-off with water loss: important negative in terrestrial animals). • Utilize counter-current flow between the external oxygen source (Carbon Dioxide sink) and the internal oxygen (Carbon Dioxide) transport.

18. The heart of vertebrates also begins as a muscular tube formed from mesoderm; very early in the embryo’s development the heart begins to beat and circulate blood.

19. Two muscular chambers are arranged in series, ventricle and atrium: why not just one chamber? Why is the atrium less powerful than the ventricle? The same serial arrangement occurs in a fish but atrium is bent to overlie the ventricle Problem of filling a too muscular chamber: need to bring up low pressure of venous blood in 2 stages.

20. Function of the pericardial cavity • The vertebrate heart occupies its own space, the pericardial cavity, a space of the coelom. Makes sense to have an organ like a heart that changes its shape to function situated in its own inviolate space. Don’t want other viscera ‘crowding in’ as the heart contracts and so having to be pushed back out of the way when the heart fills again with blood. • There is an adaptive interplay between the atrium and the ventricle: as each of these chambers contracts, the volume of the pericardial cavity is slightly increased. This means reduced external pressure of the pericardial fluid within the pericardial cavity. • This reduced pressure makes it easier to fill the expanding heart chamber. So the action of contraction of one chamber (e.g, atrium) is coupled to the distension of the other (e.g., ventricle).

21. The form of a sperm and an egg reflects their different functions: sperm with flagellum adapted for locomotion, egg with yolk for food provision World Science

22. Reproduction: the process of generating offspring Parthenogenesisoccurs in winged aphid females: development of their eggs occurs without fertilization by a male. This allows for rapid buildup of offspring upon a newly attained food plant: it is adaptive in properly exploiting a resource after dispersal. Some animals reproduce asexually. Asexual reproduction is making offspring without syngamy (fusion of sperm and egg): there are two forms: budding and parthenogenesis.

23. Budding Carolina biological supply Coelenterates (Cnidaria) reproduce sexually in the medusa form (most are dioecious); but coelenterates also reproduce by budding and by fission

24. Portuguese Man-of-war Recall the colonial hydrozoans: budding is the basis of the colony: asexual reproduction without detachment; but they also reproduce sexually with gonophores.

25. Hermaphrodite: producing both eggs and sperm • Gonads: testes and ovaries, can occur in the same individual or in separate individuals. The latter situation is term dioecious (each gonad ‘in its own house’). If both gonads occur in the same individual that individual is hermaphroditic. • Incidence of hermaphroditismin organisms is widespread: in many invertebrate animals hermaphroditism is the normal condition, enabling a form of sexual reproduction in which both partners can act simultaneously as "female" and "male". • Molluscs: most snails and slugs are hermaphrodites. • Nereis and other polychaetes have separate sexes; but earthworms are hermaphrodites.

26. Epitoky: (Barnes 7th edition p. 303)epitokeis a special reproductive morph of a polychaete worm: “adapted for leaving bottom burrows, tubes, and other habitations” and swimming about in the water column. Body can be divided into two different regions packed with gametes. Enhanced eyes and improved paddle-like setae for better swimming (spatulate setae)Species swarm, coming into close proximity and releasing the gametes into the sea where fertilization occurs.Commonly marine animals may reproduce by releasing gametes into the sea: external fertilization and very different from the adaptations required to deal with fertilization in dry terrestrial habitats

27. Simultaneous hermaphrodites: earthworms are an example of an animal that has both male female gametes functional at the same time. Neverthess the worms mate by exchanging sperm within a mucus cocoon (a terrestrial adaptation).

28. Slipper limpets (Phylum Mollusca) Crepidulafornicata: sequential hermaphrodites Protandroushermaphroditism: first a male then a female. Male gonads mature first and the individual passes through the early part of its life as a male with embryonic ovaries. For a time it is a transitional hermaphrodite and then its male gonads become nonfunctional and it ends its life as a female with functioning ovaries producing eggs. Stacks of slipper limpets have 8 to 12 individuals, all oriented in the same direction; all cling by their ‘sucker-like’ foot to the shell of the individual underneath. The basal individual in each stack is usually attached to the shell of dead limpet that is bonded into the substratum. The most basal living individual is a female with mature ovaries that broods (aerates) eggs within the mantle cavity. The youngest and smallest individuals are at the top of the stackare immatures gradulally becoming adult (functional) males; they arrived atop the stack as dispersing larvae.

29. Slipper limpets contin. • Functional male has a testis where spermatozoa are produced and a long intromittent organ (penis) that can reach down the stack to fertilize the functional females down below. • Timing of these sex changes is influenced by the environment in an interesting way. When isolated a small male will develop into a transitional (hermaphrodite) and then a female at a much smaller size than usual. This is apparently a device – an adaptation for most hermaphrodites – of self fertilization in the absence of any partners. • A male kept in the presence of several functional females will grow to a much larger than normal size. This developmental effect is governed by release of pheromones into the water: the female releases this chemical which controls the development of the males in the stack above. In effect the mature female regulates the masculinity of the other individuals in the queue. • Adaptive value of protandry: protandry involves sexual dimorphism: it separates the role of a female from that of being male,deferring the larger energetic demands of being female (eggs with nutrients) to later, when body size is greater, so allowing more accumulation of stored energy. • Protandry also avoids brother-sister matings since animals that start life at the same time are not going to be of different sexes at the same time

30. Robertson, D.R. 1972. Social control of sex reversal in a coral-reef fish. Science 177: 1007-1009 • Many fishes exhibit protogynoushermaphroditism where the individual develops ovaries first and is a functional female and then subsequently changes into a male with functional testes. • Examples: Labridae are common fishes on coral reefs: their common name is wrasse. So too are Scaridae(parrot fishes) and Seranidae (groupers, sea basses). • Along with this may go a female biased sex ratio: many more females than males. • As an example Labroidesdimidiatus blue-headed wrasse. [This fish cleans ectoparasites off other fishes at a ‘cleaner station’.] There are social units: one male with a harem of 3-6 mature females and a number of immatures. The females are yellow with a white underbelly and some black spots. The immatures are yellow (see photo on right).

31. These fish are territorial on the reef. Removal of the harem-holding male through mischance or old age, permits the alpha female in the dominance hierarchy to initiate rapid developmental changes. She develops male colouration and her female gonads degenerate while her male gonads initate development to maturity. This happens very quickly, in a matter of a few hours, and so the harem acquires a new male

32. Pseudoscorpions: spermatophores • Males and females never meet. The males place spermatophores (sperm packages) at various locations in their habitat and the females are attracted and settle over the trigger. Pheromones attract the female to the spermatophore.