Introduction
Cryptic colorations, mimicry, and polymorphism have long fascinated ecologists. Darwin was one of the first to describe organisms hiding in their environment and was one of the primary observations leading him to his theory of evolution via natural selection (Wilbur and Steneck 1999). Mimicry of surroundings had been studied in many organisms, perhaps none more known than the cases of industrial melanism described by H.B.D Kettlewell in 1956. Two different phenotypes of pepper moths (Lepidoptera) were present in pre-industrial England: dark and light. Very few dark moths were found at this time since the light phenotype was cryptic with the trees, thus the dark phenotype experienced high predation and decreased fitness. With the industrial revolution came coal-burning factories that emitted soot making the trees darker.
From http://www.science.siu.edu/plant-biology/PLB117/Nickrent.Lecs/Systematics.html
Many terrestrial organisms use habitat mimicry and camouflage to avoid predation (Stilling). This blending in of an organism with the background color of its habitat is a common method of avoiding detection by predators (Stilling) and has evolved repeatedly among animals making the potential prey difficult to spot against its background (Campbell et al.). Since the trees were darker, the dark phenotype experienced a higher fitness since it was cryptic and the white phenotype was subject to predation. Although there are many similar examples on land, few examples are found in the marine environment (Wilbur and Steneck 1999). Rocky intertidal periwinkles exhibit dramatic morphological variation across environmental gradients on both small and large geographic scales (Trussell 2000). It should be intuitive that cryptic organisms experience a higher fitness than those who are not. Fleeing is a common anti-predator response, though it can be very expensive energetically, so cryptic species need to expend less energy (Campbell et al.). Thus organisms having a greater ability to camouflage themselves experience higher fitness.
Here is a simplified example of Darwinian Evolution
The rocky intertidal or littoral region is an ideal place to conduct marine research due to its accessibility, multitude of specie diversity, and is a crucial part of the marine ecosystem occurring between highest high and lowest low tides (Castro and Huber, Robertson 30 September, 2003). These areas generally occur on steep coasts without large amounts of sediments since they have been recently uplifted or are still rising as a result of geological events (Castro and Huber). The intertidal regions have a diverse array of epifauna, some of which are sessile and others are mobile. Several paradigms exist about intertidal organisms and the areas they inhabit, which are divided into distinct bands called zones. Different organisms have different adaptations that make them ideal for a particular environment. Several biological, physical, and anthropogenic factors influence the distribution of organisms, but the general paradigm for rocky intertidal communities exists that the lower limit of an organism is due to biological stressors and the upper limit is due to physical stressors (Castro and Huber; Campbell et al.; Robertson 2 October 2003). In the upper intertidal, organisms are rarely submerged and need to be able to withstand a large fluctuation in temperature and be resistant to desiccation (Castro and Hubert; Robertson 30 September 2003). These organisms are subject to terrestrial predation as well as severe desiccation pressures. The organisms in the mid intertidal must be somewhat resistant to desiccation and be able to withstand a large fluctuation in salinity and temperatures along with competitive pressures from other organisms (Castro and Hubert; Robertson 30 September 2003). Organisms in the low intertidal are submerged the majority of the time so must be more apt at dealing with competitive pressures from other organisms (Castro and Hubert; Robertson 30 September 2003). The dominant intertidal periwinkle, Littorina littorea is slightly larger than L. obtusata and does not have any different carapace phenotypes. These two species are usually found in the same regions of the intertidal. In an effort to prevent desiccation, the periwinkles clamp themselves to the rocks to seal in moisture (Castro and Hubert). Organisms in the low intertidal are rarely exposed to air and are primarily subject to competitive pressures (Castro and Hubert). Primarily Geoffrey Trussell has observed phenotypic plasticity in several different instances with L. obtusata in shell thickness with an increase in predation and foot length. The shell thickness of L. obtusata increases when subject to high predation of Carcinus maenas and the foot size increases when subject to high wave action (1996, 2000).
L. obtusata resembles the Ascophyllum nodosum air bladders and is often found in association with the fronds so may be camouflaged from visual predators (Burtness). It is primarily found scattered in and around the Ascophyllum nodosum and Fucus vesiculosus canopies (Burtness, Robertson 2 October 2003). The primary visual predator of L. obtusata are C. maenas (Trussell; Robertson 27 August 2004). Geoffrey Trussell noted that rocky intertidal periwinkles exhibit dramatic morphological variation across environmental gradients on both small and large geographic scales (2000). This geographic and historical variation in carapace morphology is thought to have been produced by shell crushing by predators such as C. maenas (Trussell 2000). L. obtusata and L. littorina play vital roles in intertidal communities by limiting the abundance of ephemeral algae and even the success beneath the canopy of the algal sporelings of canopy-forming seaweeds (Burtness). L. obtusata often feed on the fucoid alga they settle on such as Fucus spiralis, Fucus vesiculosus, Fucus serratus, and Ascophyllum nodosum (Trussell 1997).
In this study, we observed L. obtusata on three separate dates at Northeastern’s Marine Biological Research Station in Nahant, Massachusetts USA. We believe there will be a significant relationship between L. obtusata carapace morphology and substrate color and this relationship will be predatory mitigated. Taking samples back into the lab where they will be offered different options for substrates will further test this hypothesis. We believe there will be no statically significant results from this portion of the experiment because the relationship is predator driven.
Life History Methods Results Discussion
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