Sepiidae are the second largest of the Cephalopoda, contain two genera, and have diverged into over 100 different species. The genus Sepiidae are classified in is the Sepia officinalis, and it has the main distinctions of the Sepiidae including myopside type eyes, short arms, long retractable tentacles, an oval mantle, narrow head, flattened body, and fins on the lateral sides of the mantle. Sepiidae are also in the mollusk family and Decapoda order.
The Sepiidae species are separated by the barrier of distance, ecological isolation, as many live near Asia, many along the coast of Africa and in the Mediterranean, and just 4 species live in European waters. They also live in varying depths in the ocean ranging from deep waters to along the shallow coast on reefs. Those in the Mediterranean area have split into numerous subpopulations as some migrated to areas far enough away that there is no longer mating between previously reproducing populations. This has resulted in restricted gene flow and caused the development of new species due to different environmental pressures on mutations within the different Sepiidae genotypes. Because of this, there are many physical and morphological traits that have been selected upon and deemed beneficial in the different environments they inhabit. Firstly, the composition of Sepiidae with respect to their anatomy should be explained. Sepiidae consist of arms, tentacles, fins, eye porse, a shell, a head, a funnel, and a mantle.
Some of the differences that have been observed within the Sepiidae include cuttlebones strength, mantle width, tentacle length, and rows and density of suckers on the hectocotylus, along with overall size, shape, and color. Sepiidae are able to camouflage with their surroundings by altering the color of their skin and therefore have grown accustomed to mimicking the background of where they live particularly well. So, a specific species would be better at camouflaging itself in the environment it inhabits more so than a different species. Evidence of this is seen in the sepia latimanus that is the most well adapted to camouflage in coral reefs as they are the most common Sepiidae that lives in them.
The varied sizes helps each species survive predation from the many different predators they face in their different environments such as sharks, dolphins, fish, seals, and even seabirds. Trait differences in Sepiidae have also been attributed to their different prey focuses that live at different depths in the oceans and seas such as crustaceans like crabs, shrimps and prawns. These animals have harder shells and are different in size in different regions of the world, so longer tentacle and arm length correlates to the Sepiidae preying on the larger forms. Sepiidae are a perfect example of different species adapting different morphologies in order to eat different food, similar to Darwin’s finches.
The many species under Sepiidae have been classified as new or different species with phylogenies; however, another method of classifying them is by morphology. As previously described, there are many differences in phenotypic traits and they only grow further apart when comparing species. For example, the Sepia apama has been given the nickname the “giant cuttlefish” as it more than doubles the weight of a common cuttlefish and has a much longer mantle length. Another example of speciation can be clearly seen when analyzing the Sepia mestus as it is known as the “the reaper” cuttlefish due to its unique bright red appearance.
The filling of ecological niches by the Sepiidae is extremely impressive and the total number of species topping 100 proves their adaptability to new environments and independent evolution. Allopatry has contributed to the diversity of Sepiidae as the isolation by distance of many species prevents gene flow and reproduction between them. So, the rate of speciation was not very quick as allopatric taxa are slower to develop reproductive isolation, yet with millions of years on their side Sepiidae now have great variance.
References:
http://tolweb.org/Sepiidae/19987
Images sourced from wikapedia
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