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Marine Life: Life on the Benthos IV

Phylum Echinodermata- sea stars (NOT starfish), sea urchins, sea cucumbers and relatives

Representative groups for the Phylum Echinodermata. What do you notice that is different about these critters?

The key characteristics for this group include are dramatic change in the embryological development. The developmental series is more similar to the higher organisms than to the lower invertebrates. The echinoderms are usually included with the DEUTEROSTOMES (all vertebrates, including us are deuterostomes). Moreover, the larval stages for all echinoderms is bilaterally symmetrical. The most adult echinoderms exhibit pentamorous radial symmetry (5- sided), but all adult echinoderms are radially symmetrical (some sea stars have 4, 6, 20 or more arms). All echinoderms have a water vascular system and tube feet. The tube feet are used for locomotion, as well as feeding for most echinoderms. The tube feet are manipulated by the changing water pressure in the water vascular system. Some echinoderms, like urchins and sun stars can move quickly. There are a number of important subgroups with specific key characteristics. The larval stage of the echinoderms is called the echinopluteus stage.

The echinopluteus stage of various sea urchins. The larval stage for all echinoderms is bilaterally symmetrical, while the adult stage exhibits radial symmetry.

Sea Stars, Class Asteroidea Sea stars display the echinoderm bauplan or body plan. They usually have 5 arms, but some have 4, 6, or more than 50. Hundreds of tube feet protrude from the oral surface of the arms in the ambulacral groves. Sea stars can move in any directions and will use their tube feet to do so. This is amazing because sea stars lack a complex nervous system and eyes. How do they know where or which way they are going? Sea stars have a calcareous exoskeleton consisting of plates, allowing for flexibility.

Sea stars (NOT star fish from this day forward!) are also covered by spines, and small claw-like structures called PEDICELLARIAE. Pedicellariae are critical structures and they used to keep the aboral surface of the sea stars free from debris. These echinoderms do not have gills, but gases are exchanged through the skin. Sea stars covered by debris would suffocate.

Most sea stars are predators and will feed on slow moving or attached organisms like bivalves, snails, barnacles, etc. Some are scavengers, and a few feed on unwary fish (like I stated, some sea stars are very quick).

General sea star anatomy. Note that the top or dorsal surface is called the aboral surface (the side without the mouth). The bottom or ventral surface is called the oral surface. Adult sea stars go through life with their

mouths pressed against the benthos (pleasant thought right?).

Reproduction in sea stars may be sexual or asexual, and the larvae are planktonic. Sea stars, like all echinoderms, have remarkable powers of regeneration and may regrow lost parts. If there is enough of the internal anatomy, that lost part will grow into a new sea star. This is extremely unusual for an advanced organism.

The common ochre sea star (Pisaster ochraeceus) may occur in a variety of colors. The coloration reflects the genetic variation within populations. These sea stars are commonly seen at low tide along rocky shores.

The ochre sea star is they ‘keystone species’ of our California rocky shores. The sea star has a profound effect on the Mid Tide Zone due to its presence there at high tide. The most common sea star species in southern California tidepools is one of the knobby sea stars, Pisaster ochraceus, also called the ochre sea star. Each ochre sea star can eat up to 80 adult mussels each year and thousands of barnacles. This is the 'keystone species' in our rocky intertidal. Without its presence the mussels would dominate and species, like the aggregating anemone, would be crowded out. Furthermore, there would not be the great diversity of species encountered in the Low Tide Zone (below sea level) that I like to refer to as 'treasures.' Everything would be overgrown by mussels. Technically, a keystone or cornerstone species is one the regulates the densities of prey organisms in an area. If this predator is removed, prey organism stocks would dramatically increase.

Ochre sea star in feeding position on a mussel. When a sea star eats, it takes time!

The tube feet can pull two pieces of shell apart for hours (or days, if needed) until their prey tires. Sea stars never tire of pulling open a shellfish because they have hundreds of tube feet - always resting a few. Once there is the tiniest crack (a tenth of a millimeter is all that is needed) the stomach of the sea star can emerge, ooze into the crack and digest the prey. In general, it takes more than six hours to consume a mussel (no fast food here). The upper limit of where sea stars prey on their favorite food, the mussel, is 2.5 feet above sea level because the rocky shore dries out every six hours when you get higher than this (the High Tide Zone) and sea stars do not tolerate that much dryness.

Thousands of tube feet are located in the ambulacral grooves on the oral surface of the tube feet. The tube feet are moved via the water vascular system. Damaged or lost tube feet are quickly replaced.

The aboral surface of the sea star (left). The key to the water vascular system is called the madrepore (right). The sieve plate regulates the amount of water that moves through the water vascular system and

moves the tube feet.

The sea star is one of the top predators in the ocean - few things prey on sea stars. It is the desperate shark and a few sea otters, and elephant seals that are the main sea star predators. Even then, if the predator just bites off an arm or two, the sea star has amazing regenerative abilities, and can often regrow missing arms.

The bat star, Patiria miniata, is another common resident of the southern California subtidal zone.

Like ochre sea stars, bat stars come in a variety of colors. Many bat stars have a commensal polychaete worm that lives among the tube feet in the ambulacral grooves.

The oral surface of the bat star. If you look closely, you can see the commensal polychaete worms in both images. The image on the right illustrates how sea stars can evert their stomachs for feeding (gross

and cool).

I couldn’t resist! How often do you get to see an everted sea star gut! You can also catch a glimpse of the polychaete worms too. Image courtesy Genny Anderson.

The crown of thorns sea star is an important predator in coral reef communities. If their populations are unregulated and dramatically increase, they can decimate coral polyps. There was a time when the crown of thorns sea star threatened corals of the Great Barrier Reef, and was hunted by Australian divers. When the sea stars were found, they were cut to pieces. Instead of killing the sea stars, the divers created a population boom because of the sea stars ability to regenerate lost parts and asexually reproduce (true story). These sea stars are not affected by the stinging cells of the corals.

Crown-of-thorns sea stars (Acanthaster spp.) May be found in tropical seas all over the world. They are a major threat to coral reefs when their populations dramatically increase.

Brittle Stars, Class Ophiuroidea Brittle stars are similar to sea stars, but their arms very long and flexible. This is the most diverse echinoderm group, with over 2,000 described species. Brittle stars are found from the intertidal zone to the hadal zones (over 11,000 meters). They feed on particulate organic material and small animals they collect with their tube feet (they would be deposit feeders). Unlike their asteroid cousins, their tube feet lack suckers.

Like their asteroid cousins, brittle stars occur in a variety of colors.

Some brittle stars appear to exhibit negative photo-taxis, so they move away from light. They are often found under rocks, attempting to avoid the organisms that feed on brittle stars. They may detach an arm or two to confuse predators. This is called AUTOSTYLY. The arms move on their own while the rest of the animal gets away. They can regrow them in a few weeks time. Like the sea stars, reproduction may be sexual or asexual, and the larvae are planktonic.

Feather Stars, Basket Stars and Crinoids, Class Crinoidea Theses echinoderms are found primarily in deep sea habitats. Sea lilies are another representative of this group, but these are benthic and restricted to deep water. Crinoids have dichotomously branched arms, and there may be several branches. They use their branched arms to feed on drifting plankton. Like brittle stars, their tube feet lack suckers and secrete mucous to trap planktonic organisms. Crinoids will often perch on hard substrate and extend their arms downstream of the current to feed.

Various feather stars (center right and left) and basket stars (far right and left). Most crinoids are found in deeper waters.

Most crinoid sea lilies (left) are sessile and live attached to the substrate, but there are a few mobile species (right) that use the multiple branched arms to crawl along the bottom or swim in the water column.

As with the other echinoderms we have covered thus far crinoids have remarkable powers of regeneration, reproduce both sexually and asexually, and are only found in marine environments.

Sea Urchins and Sand Dollars, Class Echinoidea There are about 900 species of urchin in the oceans world wide. In the sea urchins, the endoskeleton forms a round rigid test with moveable spines and pedicellaria (which we have seen before in the asteroid sea stars). Locomotion is achieved by moveable spines, and tube feet. The mouth is located on bottom (oral surface) of the organism, and has an intricate system of jaws and muscles called ARISTOTLE’S LANTERN. This complex structure is used to bite of bits of algae. Most urchins are herbivorous, but a few are carnivorous.

Sea urchins can be prolific reproducers (sexual only). If urchin populations are not controlled by their predators (like sea otters), their populations can quickly grow. The urchins will then feed on all of the available algae in the area, and the result is an “urchin barren”

The internal anatomy of the urchin is the water vascular system, digestive system, but is mostly gonad. Reproduction is this group is sexual, the sexes are separate, and the gametes are released into the environment. Urchins may be induced to spawn in the laboratory by injecting them with a solution of potassium chloride (KCl).

Purple (Strongylocentrotus purpuratus, the smaller purple urchins) and red (S. franciscanus, the larger dark urchins) are found in nearshore environments along the California coast. They are important prey for a

variety of organisms.

Above is an image of the mouth (oral surface) of the sea urchin. This is also called Aristotle’s lantern. Herbivorous urchins use these jaws to graze on algae.

Two species of common sea urchins, Strongylocentrotus spp., are annoying to careless tidepoolers and scuba divers who may get their spines lodged under their skin. The spines are not poisonous but should be removed. They can cause serious problems if they lodge near a joint, and infections are common. The smaller and more lavender-colored species are S.purpuratus, and the larger and usually darker-colored species is S. franciscanus. It is mostly the red urchins that are collected by local sea urchin divers, who harvest them for the five reproductive organs inside. When ripe, these organs are a delicacy in local and Asian markets and served in sushi bars … they are called uni (and no, I haven’t tried it...yet). The urchin is prey for several fish and some sea otters. In fact, some sea otters feed so exclusively on sea urchins that the purple pigment of the urchin is incorporated into their bones and, when dead, they have lavender-colored skeletons. The same thing happens to a local shark,Heterodontus francisci, or the California horn shark, will consume so many purple urchins that the spine at the base of the dorsal fin will turn purple.

When urchin populations explode (may be due to the loss of a predator, left), they consume all of the plant material in that area. When the food supply for the urchins runs out, they will starve. The scene on the right

is what happens as a result. It can take several years for an area to recover after an urchin invasion. Lastly, some urchins are used in biomedical and cancer research. The embryological development of the urchin is very similar to ours. Moreover, the cell adhesion properties are similar to. Investigators like our own Dr. Steve Oppenheimer have been working to unravel why cancer cells behave the way they do. He and his students use urchins as a model system to address these questions.

Larger purple urchin on the left and two smaller white urchin (Lytechinus pictus, right). The white urchin is one of the few carnivorous species off our coast.

Not all sea urchins rely on spines for their survival. Despite the spines, there are a number of critters that eat these spiny treats. The slate pencil urchin (left) from Hawaii and the unusual flower urchin (right) from the

Philippines rely on warning coloration and toxins for protection. The pedicellaria for the flower urchins contain the toxins.

Last bit of information about sea urchins. The spiny types are visited by a number of other organism that find safety from their predator their. Small shrimp, fish, and even a few species of snail hide within the spine of urchins. This is a type of symbiosis called commensalism.

Sand Dollars are flattened versions of sea urchins. Most sand dollars are deposit feeders and are found in the soft sediments of nearshore environments. Both sea urchins and sand dollars produce pelagic larvae.

Sand dollars are found in sand or mud substrate habitats. They have very short spines and where they occur, they occur in stacks. Images courtesy Mary Jo Adams 2004.

Sea Cucumbers, Class Holothuroidea Sea cucumbers represent yet another modification of the echinoderm body plan. These worm like echinoderms lack obvious pentamorous radial symmetry, but have 5 rows of tube feet. Unlike almost all of the other echinoderms, sea cucumbers have an endoskeleton comprised of calcareous spicules scattered over the wart skin.

Sea cucumbers come in all shapes, sizes and colors. The one on the far right, Amphgymnas bahamensis, may be found at depths of 1500 meters or more and can grow to over 1.5 meters (better than 4 feet) in length. The warty sea cucumber (Parastichopus californiensis) in the ‘alarmed’ state (far left) and as it is normally seen.

Most sea cucumbers are deposit feeders and will actively burrow into soft sediments. Others will extend their feeding tentacles into the water column to feed. Because they are soft bodied, some rely on toxic secretions to inhibit predation. Others will violently eject their internal organs into the environment. This process is called EVISCERATION (I told you marine biology is cool. You should check the internet to see if there is a video clip of this ‘process’). Like other echinoderms, they have remarkable powers of regeneration and will regrow the parts displaced by evisceration and it takes them from 10-14 days to regrow these lost parts.

A variety of animals feed on sea cucumbers, and there is a commercial fishery for this organism (yes, there are some among us who consider these a delicacy). Like other echinoderms, they produce benthic eggs and pelagic larvae. There are a few bathypelagic sea cucumbers that were discovered and video taped back in the late 1980's. These species have adaptations for life in the water column including flattened tube feet, an oral swimming hood, and a transparent body.

A nice representation of some of the echinoderms including a sea cucumber (top left), brittle stars (top right), and crinoid (bottom center).

Another one of the ways sea cucumbers minimize the chances of predation is by having a head region that is difficult for predators to recognize. Go back and look at some of the images. Which end is the posterior end and which is the anterior end? If the predator grabs the wrong end (as it were) the sea cucumber will eviscerate and get away.

More echinoderm diversity: a knobby sea star (top left), a purple urchin (top right), a gree sea urchin (bottom left), and a sand dollar (bottom right). Be sure you can identify the subgroup (class) and key

characteristics for each.

The Invertebrate-Vertebrates (What? Isn’ this a contradiction?) Well to answer my question, this isn’t a contradiction because these organisms are an important transitional group from invertebrates to vertebrates. While they have many of the characteristics of the invertebrate groups we have just covered, these groups have some of the chordate characteristics at some time during their life cycle. The vertebrates are in the Phylum Chordata, and these are their key characteristics: - All possess at flexible NOTOCHORD. - All vertebrates have a DORSAL-HOLLOW NERVE CHORD. - All vertebrates have PHARYNGEAL GILL SLITS at some point during the life cycle. - Finally, all vertebrates have a POST ANAL TAIL (yes, this includes humans too). The invertebrate-vertebrates have some of these characteristics during their life cycle. Which places them somewhere between invertebrates and vertebrates.

There are 3 major subgroups: Subphylum Hemichordata-acorn worms Subphylum Urochordata- the tunnicates and the salps Subphylum Cephalochordata- the lancelets, like Amphioxus spp. Subphylum Hemichordata Small , but important evolutionary group of marine worm like organisms. The larval stage exhibits bilateral symmetry, and as NOTOCHORD and post anal tail. The adult stage is worm like, and lacks these features. There is debate about where this group occurs in evolutionary relationships of other organisms.

Images of acorn worms (left and right) and a poster with their anatomy illustrated. The larvae of most acorn worms is planktonic, and the embryological development is similar to the echinoderms.

Subphylum Urochordata These organisms are also one of the minor, but important aquatic groups. Most tunicates are marine. The larval forms has a NOTOCHORD, PHARYNGEAL GILL SLITS, and a POST ANAL TAIL. The adult stage has the gill slits, which are used primarily for feeding, but also for gas exchange. Some of the tunicates have a pelagic adult stage. The salp and their relatives are a great example of organisms with this life history. These organisms are mostly transparent, and are important filter feeders. Some salps may get very large, and others may have COMMENSAL organisms living in their body cavities.

Salps are the pelagic, free living form of the tunicate. Though they look simple, they are derived organisms. Some have pelagic amphipods that live in their cavities. Phronima spp. is seen in salps (center and right). These are females and they will lay their eggs in the living salps. When the young amphipods hatch, they

consume their ‘home’. After maturing and mating, the female amphipods look for salps to rear their young (Think you have it bad? You could be a slap, er salp).

Salps may be found from the surface (epipelagic zone), to the deep sea (bathypelagic). There is high diversity in this group, and there may be many unnamed salps in marine environments world- wide.

Some of the tunicates have sessile adult stages (but planktonic larval stages). The sea squirts have a pelagic larval stage (with development similar to the echinoderms), but a benthic or sessile adult stage. Like the salps, they retain the pharyngeal gill slits for feeding and gas exchange. While some of the sea squirts resemble sponges, but they are advanced invertebrates, with previously mentioned chordate characteristics.

These sea squirts may resemble simple cnidarians, but they are our distant cousins. Like the salps, other organisms may be found living inside these tunicates.

Sea squirts may found in colonies (far left) or individually (left). Sea squirts and salps are filter or suspension feeding organisms. Sea squirts may be found from the intertidal zone to the continental shelf.

Subphylum Cephalochordata The most advanced of the invertebrate-vertebrates are the cephalochordates. This is another minor group that is entirely marine. These are fish-like organisms that have the NOTOCHORD, PHARYNGEAL GILL SLITS, POST ANAL TAIL, as well as a simple DORSAL HOLLOW NERVE CHORD during the larval stage, and the dorsal nerve chord disappears in the adult stage. So, because the dorsal hollow nerve chord is not the same has the vertebrates, and because these organisms lack a backbone, they are grouped with the invertebrates.

These organisms have cirri around there mouths, and they have long medial fins. Most lancelets are burrowing organisms and are found in soft-substrate marine environments. These organisms feed on INTERSTITIAL (these are organisms that live between the sediment grains) and burrowing invertebrates. Another term for interstitial is INFAUNA. Believe it or not, there is a fishery for these organisms and they are used a meat supplement, especially in some Asian markets. These organisms are important forage (prey) for many nearshore predators, so they may be important in the transfer of energy in marine ecosystems

. Lancelets or amphioxus are fish like critters found on soft substrates in nearshore environments. Where they

occur, they are very abundant. The far right image shows a close-up of the head region. We previously defined NEOTONY in an earlier discussion. A very good working definition of neotony is the retention of the larval stage to the adult stage. This is important because it may help to explain (from an evolutionary perspective) the origin of fishes and other vertebrates. The Garstang Hypothesis suggests that tunicates were the ancestor for the vertebrates. Other groups have been suggested as vertebrate ancestors, but there are major problems. Cephalopods, arthropods, and other invertebrate groups have been suggested as ancestors, but there are development issues. The lower invertebrates are Protostomes, so it would be unlikely that any of these primitive groups gave rise to the Dueterestomes.

As we have moved from simple organisms, to more derived critters, this is an important consideration. My goal is not to contradict your cultural or religious beliefs, but to present this information from the scientific perspective. Let’s continue our discussion of even more complex (and dare I say) interesting forms of Life in the Sea.

LIFE IN THE NEKTON Introduction

There are a few organisms that can be classified as fast or continuous swimmers. NEKTON describes the organisms that can swim independent or against the major ocean currents. We have already covered the squids, and that was one of the invertebrate groups that qualifies as nektonic. The other group in is in the Subphylum Crustacea an includes the mysids, euphausids, and natant (which means free swimming) decapod shrimp. The fast moving cephalopods and crustaceans together make-up the MICRONEKTON. These are smaller organisms, but can swim a speeds greater than the major currents for brief periods of time.

Major crustacean micronektonic groups (with the correct spelling...).

Here are some images of the above. How can you tell the difference between these groups? See below.

There is one more distinguishing characteristic for the above. Most of the decapods have a sharp, almost ninety degree bend in the abdomen (above, bottom critter). This is one reason why these are such fast

swimmers.

One of our local mysids is also one of the larger species. It is found in the mesopelagic (deths from 200 - 500 meters in large numbers.

There are about 780 species of mysid or possum shrimp world wide. They usually occur

neashore over continental shelves, but may also be found in the bathypelagic realm (likeGnathophausia above). About 65% of mysid species occur in shallow coastal waters, while 35% are found much deeper. Keep in mind that new species are being described every year.

Euphausia suberba, one of the most critical organisms in the polar seas. The largest animals on this planet (baleen whales), feeds almost exclusively on one of the smallest. The imge is a close-up of the feeding basket.

This is just one of many species that we would encounter in the deep sea. The shallow water or nearshore shrimp that you might enjoy at dinner or in a shrimp cocktail is in this group too.

Pelagic red crabs are cool! They are seasonally abundant, but only offshore anglers and scientist get to see them regularly.

The semalparous market squid, Loligo opalescens, reproduces only once, then dies. This is the squid that is usually made into calamari in resturants (snackety!). One of the nektonic cephalopods.

The ‘little giant’ or devilfish is one of the larger cephalopods that occurs off our coast. These cold water squids are a minor, but important part of the sport and commercial fishery off our coast. They may grow to 2 meters in length and about 90 lbs. If you happen to get a rather large calamari steak locally or in Mexico,

chances are it is this species. They are a hoot to catch on hook and line.

Now that’s a squid! No trick photography here the smaller of the giant squid gets huge. This one just spawned is about to die.

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Even small giant squid are impressive. Until recently, this species was only known form the beaks found in the stomach of sperm whales, dying specimens that washed up on beaches, or the very rare individual that

was captured in a commercial net. Only one species (recently discovered called the Goliath Squid) gets heavier. This species is longer.

There are a few nektonic octopus, and Japetella heathi occurs off of our coast. Note the squid like morphology, but it is an octopus!

Marine Birds, Reptiles, and Mammals are all included in the nektonic group. So are the fishes. The fishes comprise the most diverse vertebrate group on earth. There are about 28,000 species of fish, and most of these are marine (about 14,000 species). Some of the fishes may live in both freshwater and saltwater during their life times, and the rest are freshwater species.

Fish are the most successful vertebrate group on earth (and they are my favorite too)! These images represent only a fraction of fish diversity: Pacific hagfish, one of the jawless fishes (Agnatha, top left); White

shark, one of the most misunderstood organisms in the sea for reasons that will be made clear in the next lecture series (Chondrichthyes, bottom left); the china rockfish, one of the most striking species of rockfish off our coast. We have 67 species of rockfish, and this is one of them (Actinopterygii, top right); finally the

seahorse, which happens to be my wife’s favorite group. Male pipefish and seahorses give birth to the young (Actinopterygii, bottom right)

Fishes may be found from alpine mountain streams, to hot springs, to the deep sea. They

may be found anywhere there is water. There is a great range in size for fishes. The Indian goby is the smallest (less than an inch) to the whale shark (50-60 ft.). Their life histories, reproductive modes, and feeding strategies, are very diverse. Some fishes produce planktonic larvae, others build and guard nests, and still others give birth to live young. Most fish are predators, but a few feed on algae.

The following are the key characteristics for most fishes: 1) Most fishes have paired fins. 2) Most fishes have gills 3) Most fishes have a two chambered heart. 4) Most fishes have scales. 5) Most fishes have a lateral line, as part of the acoustico-lateralis system. The fishes may be divided into 3 major groups. These are the jawless fishes, cartilaginous fishes, and the bony fishes (which is what was demonstrated above).

We will discuss these groups in more detail during the next lecture series. We could easily spend the entire year discussing all things ‘fish’ in a class that covers the entire year and not cover it all. In fact, I teach Biology 530 (Ichthyology) and Biology 531 (Advanced Ichthyology), both semester long course, and we cannot cover all of the topic for the diversity of fish (but my students will tell you I try). We will go into much more detail about nektonic organisms in the next lecture series.