Arthropods Are The Largest Animal "Phylum"

Aquatic Clades
Clade Crustacea

  • Branchiopoda: Brine Shrimp & Daphnia
  • Copepoda: Copepods
  • Ostracoda: Seed Shrimp
  • Cirripedia: Barnacles
  • Amphipoda: Amphipods
  • Isopoda: Isopods
  • Decapoda: Crabs, Lobsters, etc.
  • Stomatopoda: Mantis Shrimp

Aquatic Clades Cont'd

Clade Trilobita: Trilobites†

Clade Chelicerata

  • Merostomata: Horseshoe Crabs & Eurypterids†
  • Pycnogonida: Sea Spiders

Non-Arthropod Ecdysozoa
Clade Tardigrada: Water Bears

Clade Cephaloryncha

  • Kinorynchs
  • Priapulans
  • Loriciferans

 

Arthropods & Allies Molt An Exoskeleton

Molting Animals Form Clade Ecdysozoa
Arthropods have a rich fossil record, but their phylogenetic relationships remain unclear. Taxa proposed to be sister groups include Annelida, Onychophora, Tardigrada and others, but hypotheses of phylogenetic relationships have been conflicting. For example, onychophorans, like arthropods, molt periodically, have an open hemocoel, and appear related in morphological and mitochondrial DNA sequence analyses. Like annelids, they possess paired nephridia and similar muscle fibers. Studies show that the 18S ribosomal DNA sequences are similar among molting animals: arthropods, tardigrades, onychophorans, nematodes, nematomorphs, kinorhynchs and priapulids. The results suggest that ecdysis (molting) arose once and support the idea of a new clade, Ecdysozoa. Annelids do share with arthropods a ventral nerve chord, dorsal heart, and segmentation, but there is no DNA support for a clade uniting annelids with arthropods and their allies.

Arthropod Characteristics
Arthropods and Annelids share four features: segmented bodies, paired appendages, a dorsal vessel or heart, and a ventral nerve chord. But their ancestry is remote. Unlike the closed coelom of annelids, arthropods have an open circulatory system with a hemocoel. Arthropods share two synapomorphies: 1) 2) jointed legs and 2) compound eyes.

Exoskeleton Molting Causes Step-like Growth

Metamorphosis And Molting
Arthropod cuticles have chitin fibers in a cross-linked protein matrix. Aquatic species supplement it with calcium salts. Cuticles provide good support and protection, but being rigid they restrict an arthropod’s growth. To grow to adult size, arthropods must molt periodically. Because they are changing from one suit of armor to another, growth is step-like and discontinuous rather than continuous. Out of their armor between molts, they are vulnerable to predation since they are easier to eat – think soft-shelled crab. Changes between molting stages can be modest or large, and result in a metamorphosis or change of form.

The Molt Cycle
1. The multilayered cuticle separates from the epidermis.
2. Molting fluid dissolves the old inner cuticle, recycling its chemicals to build a new, more spacious (and still soft) cuticle beneath the old outer cuticle.
3. The arthropod’s body expands within the soft inner cuticle by taking in water or air, causing the old outer cuticle to fracture and shed.
4. The cuticle hardens when its proteins become cross-linked. Crustaceans deposit calcium salts for additional strength.
5. Space taken up by air or water is gradually replaced by living tissues as the arthropod grows within its exoskeleton, until no space is left and a new cycle of molting occurs.

Describe this process as if you were experiencing it, using two layers of clothes in place of the inner and outer cuticle. Include the changes in the appearance and nature of the clothing material.
Two Side Clades Related To The Arthropoda
Cephalorhyncha Is A Marine Benthic Clade With 3 Members
Kinorynchs and loriciferans are microscopic and live in the spaces between grains of sand and mud. There is little of interest to say about priapulans except that their name comes from Priapus, the Greek god of penile erection. Speaking of which, in the 1800’s French soldiers in North Africa came down with priapism – painful and prolonged erections. This condition can be caused by an overdose of an aphrodisiac called “Spanish Fly” made from beetles. The soldiers had not been proffered this potion but instead had feasted on frog’s legs. Yet it turns out that the frogs themselves had been banqueting on the beetles. The active ingredients in the beetles (cantharidins) became concentrated in the leg muscles of the frogs. What is this phenomenon called?

Tardigrada Are The Caterpillar-like “Water Bears”
“Slow walking” water bears are well-known for their cryptobiotic abilities. They can dry out and remain dormant for years, to reawaken when they are hydrated. They feed upon plant juices with a piercing mouth. Claw-tipped legs and a chitinous molted exoskeleton reflect their very close relationship to arthropods. Females will lay their eggs inside their molted exoskeleton – why would they do that?
Despite Spiffy Compound Eyes, Trilobites Are Extinct
Clade Trilobita Lasted 300 Million Years
These “three lobed” bottom-dwellers were the most abundant arthropod in the seas until 245mya. Trilobites could curl up into a ball (like pillbugs) as a defensive posture, and some had spines as extra protection. Note their compound eyes under a dissecting scope – they’re calcite.

Only Arthropods Have Compound Eyes
Compound eyes have multiple individual light sensors called ommatidia. Each is at a slightly different angle and gathers light from that portion of the animal's visual field - hence they form a mosaic image. Eyes with more ommatidia form finer images (dragonfly eyes each have 30,000 of them). Our camera eye has 6 million light-sensing cones, so its images are far superior. But compound eyes are better at detecting motion.

Chelicerate Subclades Have Chelicera: Front Legs Used For Feeding

Subclade Merostomata Are Horseshoe Crabs and Extinct Sea Scorpions
Horseshoe crabs are the most ancient extant arthropod: their larvae are called "trilobite larvae" due to similarities in appearance to them. Their extinct cousins, Eurypterids (sea scorpions), reflect in appearance a link to Arachnids. Merostomata are the only chelicerates with compound eyes: what appear to be nostrils in the front of the head are simple ocelli. They respire with book gills –spot them underneath.

Subclade Pycnogonida Are Sea Spiders
The marine sea spiders are strange for a number of reasons. Their walking legs contain their gonads. Like terrestrial spiders, sea spiders have external digestion: they inject digestive fluids into Sponge or Cnidarian prey and suck up the resulting “soup”. A gastrovascular cavity extends into each leg. Their legs and body are very thin, meaning a very high surface/volume ratio. They are also very sluggish. What does this suggest about their respiratory and excretory systems?

Crustacean Subclades Are Named For Leg Specializations

All Except The “Shell-like” Ostracoda
Seed shrimp have a bivalve-like shell – a feature so unusual the clade is named for it. They live in salt and fresh water, and can be collected in ponds on campus. Ostracods and some other crustaceans (e.g., Branchiopods, Copepods, and Cirripedians) transform from an initial nauplius larval stage through other stages until they become an adult. Since they share a nauplius stage, what does that tell you?

Specialized Legs Serve Many Roles
Some use legs for respiration (Branchiopods), some have wispy feet for feeding (Cirripedians), and some have oar-like feet for swimming (Copepods). They may be used for reproduction, ventilation, walking, swimming, safe-cracking, and other activities. Amphipods have two types of legs, and lobsters have many. In Crustacea, legs are diverse.

Subclade Branchiopoda Respires With “Gill Feet”

Brine Shrimp Eggs Enter Diapause
Sea Monkeys from childhood are the eggs of brine shrimp. Brine shrimp live in ephemeral lakes in desert areas. Since they dry up quickly, brine shrimp have to breed fast and leave behind eggs that are highly resistant to desiccation but hatch out when water returns again – maybe next year, maybe in 300 years. Entering such a long resting state is called diapause, and occurs in other branchiopods and Arthropods as well. Brine shrimp are commonly sold in pet stores as live food for tropical fish. What do you predict about their rates of development, and why?

Daphnia Evolve Into Pollution Fighters
Germany's Lake Constance has suffered from phosphorus pollution since the 1970’s. As a result, Daphnia found more and more toxic cyanobacteria mixed in with the non-toxic cyanobacteria that composed their main diet. Scientists documented the crustacean's rapid adaptation to the more toxic food by hatching a series of dormant Daphnia eggs that were found, level by level, in lake-bottom sediments in a state of diapause. The crustaceans were reared to adulthood in the laboratory and offered cyanobacteria from the lake. Daphnia with 1960’s genes (before the lake was polluted) could not eat the cyanobacteria. However, Daphnia from the late 1970s forward could. DNA tests further showed that Daphnia evolved from a species that could not cope with the toxic bacteria to a species that could.

Daphnia Dodge Predators And More

Daphnia Migrate To Avoid UV Radiation
The vertical migration of zooplankton into lower, darker water strata by day is explained by avoiding visually orienting fish; however, it is unclear why it has been maintained in fishless areas. In addition to predation, UV or ultraviolet radiation—a hazardous factor for zooplankton in the surface layers of marine and freshwater environments—may cause daytime downward migration. This hypothesis was tested by studying several Daphnia species, both in a controlled laboratory system and under natural sunlight in an outdoor system. Used were Daphnia species that differed in their pigmentation, as both melanin and carotenoids have been shown to protect Daphnia from UV. The extent to which the Daphnia species responded to UV radiation was inversely linked to their pigmentation. These results suggest that ultraviolet avoidance is an additional factor in explaining daytime downward migration. Where would one find Daphnia who didn’t face predation?

Predation Upon Daphnia Causes Polymorphism
Daphnia are also called water fleas. For them, springtime is predator-free while summer is predator rich. In summer, the generations of Daphnia that are born have an alternative morphology with spines projecting from their bodies. Why aren’t they spiny all the time - what tradeoff is made? Daphnia can reproduce parthogenetically. See that the embryos themselves have eggs!

Subclade Cirripedia Filter-feed With Wispy “Curled Feet”

Barnacles AND Their Naughty Bits Are Stuck In Many Places
Barnacles occur on rocks, wood, and even whale skin (when do you think they reproduce)? Barnacles have evolved the longest penises relative to their body size of any animal (9 X body length). There’s no penis envy, as they are hermaphrodites. Why’s it so long? Sessile barnacles must reach out and touch someone. Animals vary in the relative sizes of body parts. A barnacle (black) with a minimum penis size, let’s say 6”, gets six (orange) mates. Those twice as long, 12”, get twice as many mates (orange + blue) and leave twice as many offspring. Genes for larger size increase in the population. Now with 12” as average, those with a penis only 50% longer (18”) leave even more descendents. Fill in the blanks! Why stop at 9 X?

Sacculina is a Parasite
Sacculina starts out as a nauplius then become a cyprid larva (that is unique to cirripedians), before attaching to a crab. They not only grow throughout the host's body, protruding from the host’s abdomen, but they also hormonally control it so that energy resources are directed away from reproduction by the host into growth and reproduction of the parasite itself. Host male crabs loose their male hormones, becoming feminized. Female crabs fail to produce eggs. How do we know that they are cirripedians at all?

Subclade Copepoda Are “Oar Footed” Plankton

Copepods Are The Ocean’s 1° Consumers
At the base of the food web are plants. Phytoplankton are the ocean’s most important 1° producer organisms, fixing CO2 as carbohydrates. Carbohydrates are energy. Copepods consume most of this biomass. Since they are at the next feeding level or trophic level, they are called 1° consumers. But 100 gms of phytoplankton don’t make 100 gms of copepod, since animals can’t completely assimilate all the energy they ingest. (Why do animals defecate?) Also, energy is used in metabolic activities (swimming, digestion, etc.). This energy loss also ensues when carnivores (2° consumers) eat herbivores (1° consumers), bigger carnivores (3° consumers) eat 2° consumers, and so on. Hence, there is on average only 10% transfer of energy from one trophic level to the next. How will this loss of energy influence the amount of biomass as you go from lower to higher trophic levels? Draw a bar diagram of this effect. Is this the way things are?

Vertical Migrations
Sensing light with a simple eyespot, copepods migrate to the surface at night to feed and retreat to deep dark cold water during the day. Other zooplankton join in this migration, which detected with sonar is called the deep scattering layer or DSL. It took the inventors of sonar quite awhile to figure out why it seemed that the ocean floor depth shifted between night and day! Three hypotheses for why copepod migrate are 1) the cold depths lower their metabolic rates to save them energy (what’s a flaw in this hypothesis?), 2) deep water currents carry copepods in a different direction than surface currents are moving, so that when they rise up the next night it is into a fresh new patch of phytoplankton, and 3) dark depths allow avoidance of visual predators during the day. These copepods don’t migrate – they parasitize fish. Looking at them, what has changed about them as a result of this new lifestyle?

Subclade Isopoda Has “Equal Feet”

Marine Isopods Are Herbivores And Parasites
Many isopods are unremarkable herbivores. But some are fish parasites convergent with ticks, while others parasitize fish gills. My favorite eats the tongue of its fish host, then hangs onto the remaining nub and actually becomes a prosthetic tongue – how would you like to French kiss this!

Pillbugs and Sowbugs Are The Only Terrestrial Crustaceans
Pill bugs (Armadillidium) can roll up into a ball, whereas the similar-looking Sow Bugs (Porcellio) are more flattened and cannot. Sometimes their exoskeleton is purple-blue, indicating infection by a lethal viral infection. They breath with gills.

Subclade Amphipoda Has “Two Feet” Types: Normal And Specialized

Sand Fleas Have Special Hopping Feet
Like a biting flea, sand fleas are laterally compressed and have back legs designed for hopping. Indeed, such saltatory locomotion is best accomplished with this type of body form, and hence it’s an example of convergent evolution. But unlike an insect flea, sand fleas are mostly herbivorous and don’t bite you on the beach. Sand fleas and their relatives are found on sandy beaches, in the intertidal, and even the subtidal.

Skeleton Shrimp And Whale Lice Are Specialized
Skeleton shrimp (a.k.a. caprellids) are generally microscopic, cryptically colored and/or shaped organisms that typically sit in algae or colonial hydroids. They ambush with grasping or raptorial appendages to capture prey that wander by. Whale lice live among the barnacles on a whale’s skin, where they eat flaking skin and raw sores. This lifestyle has produced the evolution of convergence in body form with their cousins, the insect body lice.

Subclade Decapoda Has Ten Legs, Including Pincers

Crabs Are Lobsters With Their Tails Tucked Between Their Legs
It’s not that they’re cowards. In fact, in many crabs, males will have enlarged claws for fighting with one another or displaying to the ladies. But back to the tale of the tail. The long, segmented abdomen commonly known as lobster tail on restaurant menus is shortened and curled under in crabs. In females it is broad to protect eggs. In males it doesn’t have to be. Hence, its width allows one to sex a crab - examine them for this feature. So why do East Coast lobsters have big claws while West Coast species are tiny-clawed? It’s their diet – they need big pincer with big muscles to eat the muscles of big mussels.

Whales In The Antarctic Eat Tiny Shrimp-like Krill
Overhunting of blue whales has resulted in their global decline by 99%. They and other whales feed on krill (decapod kin) in the Antarctic Ocean. With the virtual elimination of blue whales, populations of the much smaller Minke whale have expanded - why? What do you think happened to krill populations during this period? Draw a graph showing population trends over time of blue whales, Minke whales, and krill. Remember that krill are r-type species while whales are K-type, and hence their populations grow faster. There are too damn many of us - now we are have started to not only harvest the smaller, formerly less economical Minke whales, but we are also harvesting the krill and turning them into “fish cakes”. Extend your graph to show these impacts on their populations. What impact will krill harvests have on the chances of recovery for blue whale populations?

Subclade Stomatopoda Has Two Alternative Foreleg Modifications

Mantis Shrimp Are Easily Confused With Decapod Shrimp
Mantis shrimp are aggressive predators. Their forelegs are modified as either spears to capture or punchers to pulverize their prey. So while in body form they resemble shrimp, they can be distinguished easily from them if you look at those front legs. Compare the foreleg of a shrimp to that of a Stomatopod. Also, their eyes are quite different – whose look more developed, and why do you suppose that is so?

Mantis Shrimp Show Sexual Selection
Many mantis shrimp males are highly territorial. In those with punching forelegs, it is their rivals that get pulverized. Males will display bright colors to advertise their territories and to attract the ladies. Sometimes bright colors will be hidden on the inner foreleg surface, then revealed during a threat display. Why would they want to keep such bright, obvious colors hidden when not in use against a rival?