What is the difference between arthropods and mollusks

Our results are consistent with Feuda et al. Lophotrochozoan xenopsins are a well-supported monophyletic clade, suggesting that xenopsins were present in the lophotrochozoan ancestor. Interestingly, xenopsins are absent from publicly available Platynereis opsins and the Capitella and Helobdella genomes. However, because our sampling from annelids is so sparse given the large number of species in the phylum, it seems likely that annelid xenopsins could be uncovered after broader sampling. Xenopsins are also absent from both the ecdysozoan and deuterostome taxa included in our analysis.

Given that arthropods, chordates, and echinoderms are now well-sampled for opsin diversity, we hypothesize that the absence of xenopsins these groups reflects true losses of xenopsins from ecdysozoan and deuterostome lineages.

Given this hypothesis, we infer that xenopsins were lost at least 3 times in bilaterians: from ancestors of the annelids, Panarthropoda, and the deuterostomes. Increased taxon sampling also allows us to hypothesize the bathyopsins and chaopsins as paralogs present in the last common ancestor of most bilaterians.

These opsins are unusual because of their extreme phylogenetic sparseness, suggesting that if our gene tree inference is correct, these opsin paralogs were lost in the majority of bilaterians. However, we interpret this sparseness as an indication that even our inclusive dataset may still be under-sampling true opsin diversity in animal phyla.

Bathyopsins are found in only two phyla so far, Echinodermata and Brachiopoda, forming a well supported, monophyletic clade in our tree. Given that bathyopsins are represented by one deuterostome and one protostome, we infer that bathyopsins were present in the last common bilaterian ancestor. We have not found chordate or hemichordate representatives. Because opsins from chordates and arthropods are well sampled, it is unlikely that these phyla possess bathyopsins.

We have not uncovered annelid, mollusc or rotifer bathyopsins, yet sparse sampling in these taxa makes it possible that opsin surveys from lophotrochozoans will reveal more bathyopsins in these phyla. We found chaopsins in only two phyla so far, echinoderms and cnidarians, and their monophyly is supported by both high UFboots and single branch tests. Given our dataset and analysis, we hypothesize that chaopsins were lost at least 3 times in bilaterians: twice from deuterostomes chordates and hemichordates and once in the ancestor of all protostomes including both ecdysozoans and lophotrochozoans.

We also find that anthozoans are the only cnidarians that have chaopsins, which suggests another potential loss of chaopsins from the ancestor of hydrozoans and cubozoans. As with the other new opsin types we have described, we are more confident that chaopsins are truly lost from chordates and arthropods compared with the undersampled lophotrochozoans.

Our second major finding is the eumetazoan ancestor likely had at least four opsin paralogs, based on the distribution of cnidarian opsins in our analysis. Instead of inferring eumetazoan c-, r-, and tetraopsins as previously reported Feuda et al.

Along with these three eumetazoan opsins, we infer that the last common eumetazoan ancestor also had a tetraopsin, but this paralog is not present in the cnidarians we surveyed. However, because cnidarians are not well sampled, it is possible that a cnidarian ortholog of the bilaterian tetraopsins may be uncovered.

Overall, we successfully identified well-supported bilaterian orthologs of at least two cnidarian opsins—cnidops as xenopsins, and Anthozoa II opsins as chaopsins, and infer that the last common ancestor of eumetazoans must have had at least 4 different opsins. Adding more opsins from Cnidaria, Ctenophora, and Xenacoelomorpha may help solidify deeper relationships between well-documented opsin paralogs like the canonical c- and r-opsins and the opsin paralogs we have identified in this analysis.

There seem to be two opsin paralogs in ctenophores, but the relationship between ctenophore opsins and those from other animals is contentious, particularly the placement of Mnemiopsis 3 Schnitzler et al. Although Mnemiopsis 3 does have the conserved lysine that aligns at bovine rhodopsin position , it was excluded from Hering and Mayer because it contains an insertion not found in any other opsin.

Its placement in the metazoan opsin phylogeny is also highly sensitive to outgroups as seen in Schnitzler et al. For these reasons, we did not include Mnemiopsis 3 in our analysis. Overall, our estimates of opsin repertoires in the last common eumetazoan ancestor and early bilaterians are likely not greatly impacted by the current controversy about the relationship between ctenophores and other animals Borowiec et al.

At present our results do not distinguish between either ctenophore hypothesis, as the ctenophore opsins we included were not placed in the animal opsin phylogeny with high support.

Opsin evolution is surprisingly complex, hinting at just how much we have yet to learn about how animals use opsins, how these functions shaped the evolution of the gene family, and the physiology and behaviors that require opsins.

Besides spatial vision, opsins are used for myriad purposes, for example as depth-gauges or for circadian rhythms Bennett ; Lythgoe ; Bybee et al. Further, opsins are not only expressed in eyes, but also across the bodies of animals reviewed in Ramirez et al. It is not yet clear to what extent the loss of an opsin paralog within an animal lineage suggests the concomitant loss of the organismal function, or whether other opsin paralogs can take over that function.

For example, r-opsins likely mediate vision in many protostome eyes, but the orthologous melanopsins in vertebrate retinal ganglion cells only have roles in nonvisual tasks.

While opsins are canonical light detectors, two recent studies have shown roles for opsins in both heat sensing and detecting mechanical stimuli in Drosophila Shen et al. These studies provide a tantalizing glimpse into opsin functions in sensory modalities besides light detection. Without understanding the true extent of opsin diversity, we cannot understand opsin evolution, the evolution of eyes and other light sensors, or even how a complex trait like eyes can evolve.

Supplementary data are available at Genome Biology and Evolution online. We would like to thank Davide Pisani and Roberto Feuda for comments on an early version of this article, and the anonymous reviewers on our first submission.

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Overview and Key Difference 2. What are Mollusks 3. What are Arthropods 4. Similarities Between Mollusks and Arthropods 5. Phylum Mollusca is one of the largest phyla in the Kingdom Animalia. It is second only to Phylum Arthropoda. Phylum Mollusca consists of more than , identified species, and they inhabit both terrestrial and aquatic environments on earth.

Mollusks are soft-bodied invertebrates with one or two shells. Moreover, they show bilateral symmetry. The most common examples for mollusks include snails, clams and squids. Generally, all mollusks have a thin outer layer called the mantle, which surrounds the body organs located insides the visceral mass. Mantle secretes the protective shell of the body.

Moreover, gas exchange takes place in gills. Mollusks have an open circulatory system in which the heart pumps blood into the open space around the body organs. Besides, they have a prominent head with a mouth and sensory organs. Mollusks like snails have a well-developed muscular foot for movement and adhesion. Squids have tentacles to catch prey and for movement.

Three major classes of Phylum Mollusca include Gastropoda snails and conchs , Bivalvia clams, oysters, and scallops , and Cephalopoda squids, octopuses, cuttlefish, and chambered. Phylum Arthropoda is the largest group of animals with more than a million different species.

Apart from the jointed legs, arthropods have jointed appendages like antennae, claws, and pincers. These appendages help arthropods to carry out different activities, including feeding, capturing prey, mating, and sensory actions depending on the environment they live in.

These creatures are cosmopolitan, and their body size varies from microscopic mites to large Japanese spider crabs. Arthropods exhibit bilateral symmetry, a segmented body, a body cavity, a nervous system, a digestive system and an exoskeleton. Chitin is the main compound of the hard exoskeleton. The exoskeleton provides protection, support, and covering for the internal body organs and also provides muscle attachment sites.