Welcome back to the discussion of animal phyla. This article is Part 2 in our animal phyla series. In Part 1, taxonomy, sponges, cnidarians, and worms were discussed. Make sure to check it out here!
Mollusca
Molluscs are a group of invertebrate animals that include snails & slugs (gastropods), clams & mussels (bivalves), octopuses and squids (cephalopods), and chitons (polyplacophora). Molluscs are characterized by being bilateral, triploblastic, being protosomic, a muscular foot for locomotion, a mantle (a group of specialized cells) that produces a calcareous shell (not in cephalopods), and a feeding organ called a radula. Almost all the seashells you find at the beach are made by molluscs!
Cephalopods are interesting because, unlike the rest of the molluscs, they have developed a unique nervous system where they have a more complex brain and several neurons in their limbs. In total, octopuses have about 500 million neurons, which is the amount one would expect to see in a dog! Most of its neurons reside in its limbs, and are highly independent and have the ability to process information without “consulting” the brain. Their nervous system is more sophisticated than some birds, fish, and reptiles!
There are approximately 500,000 species of molluscs that play a role in both terrestrial and marine ecosystems. Mollusc behaviors like filter feeding, grazing, decomposing, and predation contribute to a balanced ecosystem. Filter feeding, done by molluscs such as oysters and mussels helps improve water clarity which in turn allows for algae and other photosynthetic organisms to thrive. Grazing, done by the gastropods and chiton, play an important role in regulating the populations of algae. Cephalopods are predators, and they regulate the population sizes of other molluscs and small fish. Lastly, certain snails are decomposers, which helps keep the ecosystem rich in nutrients by eating dead animals and other organic waste.
California has an abundance of molluscs, including several species of chiton, nudibranchs, terrestrial snails, and mussels. My favorite, however, is the octopus. A native californian octopus is the Two-spot octopus (Octopus bimaculoides).
Echinodermata
The phylum echinodermata contains sea stars (Asteroidea), sea urchins & sand dollars (Echinoidea), sea cucumbers (Holothuroidea), brittle stars (Ophiuroidea), and sea lilies & feather stars (Crinoidea). They are triploblastic (i.e. they have three germ layers: ectoderm, mesoderm, endoderm), have radial or bilateral symmetry, have no cephalization (central brain), and use a water vascular system (consisting of tube feet) to move around.
Unlike the rest of the creatures discussed up until this point, echinoderms are deuterostomes, meaning that their anus develops before their mouths. Humans (and all chordates, as you will learn shortly) are also deuterostomes. This is special because it shows that chordates and echinoderms are more closely related than we are with any other phylum.
All echinoderms are marine animals, and contribute to a balanced ecosystem through grazing, supporting keystone species, nutrient cycling, and decomposing. (A keystone species is a species of organism that has a disproportionately large impact on their environment relative to their population size, so much so that the function of the ecosystem depends on them). Echinoderms like sea stars and urchins are grazers, meaning that they eat things like kelp and algae. This helps control the potential overgrowth of these vegetations. Urchins, specifically in California, are the main food source for sea otters. Sea urchins play an integral role in California kelp forests, and interact closely with sea otters to create a harmonious balance between urchin populations, sea otter populations, and kelp density. Sea cucumbers and other burrowing echinoderms help cycle nutrients in the water, oxygenate it, and move around other organisms. This is called bioturbation. Sea cucumbers are also decomposers, so they are, in essence, the waste control of the ocean.
In California, there are a number of beautiful echinoderms. Some examples of them are the Ochre sea star (Pistaster ochraceus), the Bat star (Patiria miniata), and the Pacific Purple sea urchin (Strongylocentrotus purpuratus).
From left to right: Ochre Sea Star (Pistaster ochraceus), Bat Star (Patiria miniata), Pacific Purple Sea Urchin (Strongylocentrotus purpuratus). We ask that you do not handle wildlife, this photo was through a project and permissions were given by authorized entities. Source: Tabitha Martinez
Arthropoda
Arthropoda, the phylum of insects, crustaceans, spiders, and several other creepy crawlers, is the largest phylum in the animal kingdom containing at least one million species (scientists estimate there could be anywhere between three million and 30 million species)! Moreover, 80%-90% of all animals on earth belong to this phylum. Needless to say, an entire article can be dedicated to this phylum alone. For additional information on arthropods, be sure to check out online resources such as Britannica - Arthropod, Animal Diversity Web - Arthropoda, and Lumen Learning - Subphylums of Arthropoda.
Arthropods are characterized by segmentation, being protostomes (mouth develops first, then anus), having cephalization (brain/head), having an exoskeleton (hard shell), and undergoing a molting process (shedding of exoskeleton) called ecdysis (roundworms also undergo ecdysis!). Also, one cannot talk about arthropods without discussing flight. This evolved independently about four times: in birds, bats, pterosaurs, and insects. In insects, it played an integral role in how evolutionarily successful they have been throughout time.
The main groups of arthropods are arachnids (spiders, scorpions, ticks, mites), crustaceans (shrimp, crabs, barnacles), hexapods (insects: bees, butterflies, beetles, wasps, ants, etc.), and chilopods & diplopods (centipedes and millipedes, respectively).
A common misconception is that spiders are insects. In fact, arachnids and insects are distinctly different. For one, arachnids have eight legs instead of six, like insects, and arachnids don’t undergo metamorphosis like insects do. Metamorphosis refers to the development of an insect where there is an egg, larvae, pupa, and adult stage. Another difference is that insects have wings and antennae; arachnids do not. Instead, they molt (i.e. shed their exoskeleton) as they age. Arachnids and insects also differ in the number of eyes that they have, and how they operate: arachnids can have up to 12 eyes while insects tend to have two eyes. Arachnid eyes are simple eyes while insect eyes are called compound eyes. To complicate things, there are some arachnids and insects that have no eyes at all!
Another common point of confusion lies in the difference between butterflies and moths. What makes a moth a moth? What makes a butterfly a butterfly? The differences between butterflies and moths lie in their antenna, wings, and activity. Butterflies have long thin antennae with knobs at the ends, perch with their wings folded vertically over their body, and are usually diurnal. Instead, moths have shorter, hairy antennae, perch with their wings open and flat across their body, and are commonly nocturnal. To learn more about the differences between these creatures, make sure to check out OCH’s Butterfly article by Rebecca Cummings’ here.
Arthropods have an immense ecological role in several different habitats. In terrestrial habitats, arthropods such as bees, butterflies, and moths serve as pollinators and are also a food source to animals such as bats, lizards, skunks, birds, and other spiders. In the ocean, crustaceans are ecologically important because they are a vital food source to whales and seals. Arthropods that reside in or among the soil, like beetles and centipedes, vastly improve the health of the soil by nutrient cycling, decomposing waste, moving things around, and distributing water.
Needless to say, there are a plethora of arthropods in Southern California. Some of these include the gooseneck barnacle (Pollicipes polymerus), monarch butterflies (Danaus plexippus), orbweaver spiders (Araneidae family), and the common desert centipede (Scolopendra polymorpha).
From left to right: gooseneck barnacle (Pollicipes polymerus), monarch butterflies (Danaus plexippus), orbweaver spiders (Araneidae family), common desert centipede (Scolopendra polymorpha) Sources: from left to right - Kelly Fretwell/Biodiversity of the Central Coast, Thomas Bresson/Wikimedia Commons, André Karwath/Wikimedia Commons, Wikimedia Commons
Chordata
You’ve made it to the phylum you belong to, the chordates! All mammals (mammalia), fish (Actinipterygii and Sarcopterygii), birds (aves), amphibians (amphibia), and reptiles (reptilia) belong to this phylum. Additionally, sharks (Chondrichtyes), tunicates (Urochordata), lancelets (Cephalochordata), hagfish (Mixini), and lampreys (Petromyzontida) also belong to this phylum.
In the above phylogeny, you can see that not all chordates are considered vertebrates. More specifically, tunicates and lancets are invertebrates, but are still chordates.
From left to right: Gold-mouth sea squirt (Polycarpa aurata), European Lancelet (Branchiostoma lanceolatum)
Sources: from left to right - Nick Hobgood/Wikimedia Commons, Hans Hillewaert/Wikimedia Commons
As previously mentioned, chordates are the second animal phylum to be characterized as deuterostomes (along with echinodermata). They are also triploblastic, have complete cephalization, have a notochord, breathe via lungs or gills (develop pharyngeal pouches at some point), have an endoskeleton (cartilaginous or bony), and have a tail at some point in development (even humans have this - it's called the coccyx). Cephalization refers to when an animal has a head that contains a culmination of neurons, usually at the front end of their body. A notochord is a rod-like structure that runs along the spine of the animal, and is used for communication via the nervous system. In humans, the notochord is observed only in the embryonic stage of development, and eventually develops into parts of the vertebral column. They have at least a complete digestive and circulatory system.
Like several of the previously mentioned phyla, the chordates are extremely diverse and thus take on a wide range of ecological roles, both in terrestrial and aquatic/marine environments. Tunicates reside in the deep sea and act as filters in the water. Most reptiles are predators, and they play an important role in balancing the food webs they participate in. Amphibians, due to their close relationship with aquatic environments, often serve as indicator species. Similarly, fish can be both herbivores and carnivores, so a lot like other phyla that we’ve talked about, they help balance ecosystems.
Birds, for example hummingbirds and quail, pollinate plants and disperse seeds (respectively). Mammals, for example squirrels and bats, also disperse seeds and pollinate. Rodents in particular are important chordates, because they serve as seed dispersers while also being a prey item for several mammals and birds. Finally, when whales die, they sink to the bottom of the ocean. This site is called a whale fall, and it is a feasting site for a plethora of species.
Regarding native (or popular) Californian chordates - this will be left as food for thought to the reader. What animals did you see on your drive to work, or outside your window at home? Were any of them chordates? How could you tell?
Conclusion
The hope of this series, Part 1 and 2, is that it was not only informative, but that it allowed for a broadened perspective regarding life on earth. Hopefully, when learning about the ecology of each phyla, the connectedness between us humans and our environment was evident and when learning about the anatomical and physical differences between animals, you were able to discover a new normal - a normal where we are but one evolutionary design of millions.
While scientists have chosen the job of extracting this often dense information and giving it to the public, our responsibility as citizens of this earth– along with the scientists’ – is understanding the diversity and fragility of life and how we are bound to it. This article, along with its counterpart, displayed how all life forms rely on each other to thrive. For example, plants rely on worms for soil fertility, and worms rely on other animals and their remains to survive, and other animals also rely on worms as a food source. Another example would be the ecology of sea urchins, where the regulation of their population size is essential for the health of other lifeforms, like otters and kelp.
Any contribution, large or small, to the preservation and restoration of our ecosystems is valuable and needed. Oftentimes, the biggest changes in history were started at the local level. Here in Orange County, you can contribute to the future of our ecosystems by voting for environmentally conscious policies, teaching your friends and family about what you learned in this article, signing petitions to stop harmful environmental practices, and staying informed about local environmental issues. Additionally, you can also donate to rehabilitation or conservation organizations/facilities, or volunteer your time at them. Here at OCH, you can help preserve biodiversity by volunteering at our monthly restoration programs! To learn more about this, click here.
Literature Cited
Mollusca
https://mcb.berkeley.edu/courses/bio1a/downloads/Animal_Phylum_poster.pdf
https://manoa.hawaii.edu/exploringourfluidearth/biological/invertebrates/phylum-mollusca
Holland, P. W. H. (2011). The animal kingdom : a very short introduction. Oxford University Press.
Carls-Diamante S. (2022). Where Is It Like to Be an Octopus?. Frontiers in systems neuroscience, 16, 840022. https://doi.org/10.3389/fnsys.2022.840022
https://www.whoi.edu/know-your-ocean/did-you-know/how-are-seashells-made/
https://www.montereybayaquarium.org/animals/animals-a-to-z/two-spot-octopus
https://www.marinebio.org/creatures/marine-invertebrates/mollusks/
Echinodermata
Arthropoda
de Oliveira, A. L., Calcino, A., & Wanninger, A. (2019). Ancient origins of arthropod moulting pathway components. eLife, 8, e46113. https://doi.org/10.7554/eLife.46113
Barnes, R. D. (2024, March 26). arthropod. Encyclopedia Britannica. https://www.britannica.com/animal/arthropod
Holland, P. W. H. (2011). The animal kingdom : a very short introduction. Oxford University Press.
https://www.brandywine.org/conservancy/blog/butterflies-vs-moths-what-are-differences
https://le.kloofconservancy.org.za/what-eats-butterflies-and-moths/
https://www.earthday.org/wp-content/uploads/species/crustaceans.pdf
https://mdc.mo.gov/discover-nature/field-guide/crane-fly-larvae
Menta, C., & Remelli, S. (2020). Soil Health and Arthropods: From Complex System to Worthwhile Investigation. Insects, 11(1), 54. https://doi.org/10.3390/insects11010054
Chordata
Myers, P. 2001. "Chordata" (On-line), Animal Diversity Web. Accessed April 01, 2024 at https://animaldiversity.org/accounts/Chordata/
https://mcb.berkeley.edu/courses/bio1a/downloads/Animal_Phylum_poster.pdf
https://manoa.hawaii.edu/exploringourfluidearth/biological/invertebrates/phylum-chordata
https://shapeoflife.org/phyla-pages/chordates/role-ecosystem
https://www.nhm.ac.uk/discover/what-happens-when-whales-die.html
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