Giving examples from estuaries and mangroves, review how animals manage to thrive in the stressful conditions of intertidal mudflats.
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Introduction
Mangroves are generally found in tropical regions due to them being intolerant to frost therefore do not survive in air temperatures lower than five degrees Celsius. Estuaries are found where rivers meet the sea. An estuary is “a semi-enclosed coastal body of water which has free connections to the open sea and within which sea water is measurably diluted with freshwater derived from land drainage” Pritchard (1967). Mangroves are woody trees and shrubs that thrive at the point where the land meets the sea, generally these are tropical zones. The exact value of mangrove forests is unknown as not only do they provide timber, fish and shrimp but they also provide coastal defenses to stop erosion and nurseries for juvenile fish (Hogarth,1999) furthermore the removal of carbons is invaluable as they remove 23×1012 g C per year which is 15% of the carbon collected in marine sediment each year (Jennerjahn and Ittekkot, 2002). Estuaries and mangroves have extremely variable conditions. These areas have a rich diversity and abundance due to high dissolved organic matter content, this leads to high interspecific and intraspecific competition and therefore some very selective adaptations. These adaptations are used to not only withstand environmental hazards such as high temperatures, variable oxygen concentrations, times of low water availability and predation, yet thrive. This essay will answer how mudskippers and crabs thrive in environments with these threats present.
Mudskippers
Mudskippers are amphibious fish that are well adapted to terrestrial life which is a necessity as intertidal mudflats can be both aquatic and terrestrial for long periods of time. They have peculiar adaptations that allow them to be successful in an amphibious lifestyle. Mudskippers can be found in estuaries and mangroves, they are related to gobies (Hogarth, 1999). Water retention can be an issue when it Is low tide therefore some mudskippers are evolved to be extremely resistant to water loss, Periophthalmus koebeute are known to spend more than 90% of their time out of the water whilst Periophthalmus cantonen can remain outside the water for two and a half days. They can survive these extremes by reducing the amount of water lost due to evaporation and reducing nitrogen waste in their body by 40% out of the water so as not to poison the body, the heart and blood water levels dropped first so that the other organs water levels remained constant (Gordon, 1978).
Periophthalmodon schlosseri have developed a method of gaseous exchange on land without lungs to combat the low tides. This method consists of expanding their bucco-opercular chamber which can hold 16% of the mudskippers body volume as a way of transporting air containing oxygen to their respiratory capillaries which are abundant in the entirety of the bucco-opercular region (Gonzales, 2011). Infants are generally more vulnerable than the mature of the species especially when an embryo has no defense however for Periophthalmodon schlosseri to leave an embryo in their burrow would also be fatal as dissolved oxygen concentrations inside the burrow can fall to almost zero percent of the expected surface water percentage. To deal with this issue P.scholosseri expands its buccopharyngeal cavity bringing in air from the surface of the burrow, it then returns to the burrow where it releases this air. An adult can repeat this for over 30 minutes. This behavioral adaptation is used to supply the embryo with oxygen where it can grow away from the risks of predation (Ishimatsu, 1998).
The drier terrestrial environment affects the eyes of the mudskippers greatly by drying them out. To deal with this they have evolved water filled skin folds behind each eye which allows them to be moistened when necessary (Lee, 2002).
Mobility is a challenge on thick mud, therefore Periophthalmus koelreuteri have developed two main modes of terrestrial movement, crutching and skipping. Crutching entails the pectoral fins acting as a pivot for the body to move around whilst thrusting the body forward simultaneously, this is a slow discontinuous movement. The pelvic fins provide a small amount of force to this movement. Skipping is caused by the axial muscles from the tail to propel the mudskipper through the air (Harris, 1960). Obtaining nutrients throughout the year is always a challenge, this has led to Boleophthalmus pectinirostris developing a unique way of keeping a consistent food supply during the colder season. It forms a walled territory in which it allows shallow pools to form an environment suitable for microalgae to grow which can then be used as a food source (Chen, 2007).
Fiddler crabs
Fiddler crabs (Genus Uca) are crustaceans the males possess one enlarged chela used for attracting females and warding off rival males by waving it, it can also be used as a weapon (Zeil,Jochen et al 2006) whilst the smaller chela used for feeding. They are remarkably colourful and their burrows can be seen up and down the shore primarily in mangroves. The females are less colourful and have two regularly sized claws.
Some fiddler crabs can survive anoxic conditions for up to 40h by increasing their oxygen dept, this results in an increased respiratory period to remove the lactate build up (Hogarth, 1999), this is an ability they have evolved to combat low oxygen levels caused by respiring bacteria in the mud (Hogarth, 2015).
Temperature is a major issue for organisms in a mangrove as temperatures at the mud surface can rise to 45°C. Uca lactea use evaporation to remove excess heat, they also alter their behavior by spending more time and returning more frequently to their burrows where temperatures are cooler. Burrowing of fiddler crabs can increase the surface are of the mud by 12% however a study in a salt marsh showed that the effects of burrowing increased the surface area by 59%. This burrowing helps bring fresh mud to the surface mixing nutrients so other organisms can benefit.
Retaining water and maintaining a safe body temperature is a fine balance as during low tide water may be difficult to acquire and pools of water are not always present. To retain water Fiddler crabs only release 3mg cm2 h-1 of water through their carapace this however reduces evaporative heat loss.
Fiddler crabs take advantage of their environment by feeding of organic matter during low tide in the rich mud. They are specially adapted to sort the sediment in their buccal cavities using their first and second maxillipeds to separate the sand and undesired organic matter towards the third maxilliped to be discarded whilst the valuable nutrition is moved to the mouth. Bacteria is extremely well digested with >98% of it being assimilated shown by a study using radioactive bacteria as a food source and analysing the faeces. This adaptation allows fiddler crabs to have a plentiful food source as the sediment is vastly rich in bacteria. The richness of soil sustains many organisms. This also then provides plentiful prey for predators, for fiddler crabs this reduces the chances of their offspring surviving to adulthood. To combat the predation of their larvae females may synchronize their release moreover it was recorded that in a mangrove creek in Costa Rica 7.5×1010 Fiddler crab larvae were released annually from merely 3.25 square kilometers of Mangal this method is used to overwhelm predators with excess food supplies so that some of the larvae can survive (Hogarth, 1999).
Grapsid crabs
Grapsid crabs (Genus Sesarma) are found in tropical regions particularly in mangroves. They have a mud-like colored carapace which is trapezoidal in shape, with chelae that can be brightly colored (Hogarth, 1999).Sesarma crabs play a crucial part in the ecosystem of mangroves as they increase the nitrogen content in the soil and decrease the carbon content which aids in mangrove vegetation growth (Lee, 1997). They do this by eating the leaves fallen from mangroves and algae growing on the mangrove's roots, the crab faeces is richer in nitrogen than uneaten leaves this differs from Uca as discussed earlier. These crabs release double the water through their carapace than that of Uca. To compensate with the larger water loss Sesarma have hydrophilic setae at the bottom of their legs that absorb water from the moist mud (Hogarth 1999), this could be considered a superior adaptation than that of Uca as it doesn’t restrict evaporative heat loss as much.
Conclusion
As shown, there are many adaptations that allow these organisms to thrive in intertidal mudflats, regardless of the stressful conditions in which they live. Balancing the variable conditions is often a tradeoff to find the optimum point for the individual. The transition from water to land is one with evolutionary significance and therefore species that show amphibious behavior like mudskippers are of great scientific importance. From the research done it can be concluded that due to their high abundance and biodiversity, estuaries and mangroves are valuable areas that must be correctly managed and used.
References
Chen, Shixi; Hong, Wanshu; Zhang, Qiyong; Su, Yongquan,(2007), ‘Why does the mudskipper Boleophthalmus pectinirostris form territories in farming ponds?’ Journal of the Marine Biological Association of the United Kingdom, Vol.87(2), pp.615-619
Gonzales, Tt ; Katoh, M ; Ghaffar, MA ; Ishimatsu,(2011) Gross and fine anatomy of the respiratory vasculature of the mudskipper, Periophthalmodon schlosseri (Gobiidae: Oxudercinae)
A Journal Of Morphology, Vol.272(5), pp.629-640
Gordon, Malcolm S.(1978), ‘Aspects of the physiology of terrestrial life in amphibious fishes’, journal of Experimental Biology
Harris, V. A. (1960). On the locomotion of the mud-skipper Periophthalmus koelreuteri (Pallas): (Gobiidae). The Zoological Society of London
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Hogarth,Peter (2015) The biology of mangroves and seagrasses UK: Oxford University Press, p8
Ishimatsu, Atushi ; Hishida, Yasuhiro ; Takit, Toru ; Kanda, Takeshi ; Oikawa, Shin ; Takeda, Tatsusuke ; Khay Huat, Khoo (1998), ‘Mudskippers store air in their burrows, 'Nature’.
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Lee, Heather J. ; Graham, Jeffrey B. (2002) ‘Their Game Is Mud', Natural History.
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Prichard,D. W. (1967) ‘What is an estuary: Physical Viewpoint’, American Association for the Advancement of Science.
Zeil,Jochen; Hemmi, Jan; Backwell, Patricia (2006), ‘Fiddler crabs’ Current Biology, Volume 16 issue 2