Marine Life: Life on the Benthos III Evolutionary cladogram of the major marine taxa. The organisms are more complex and/or derived as you move from sponges to chordates. Phylum Mollusca - the shelled (mostly) invertebrates There are about 200,000 spp. and only the Phylum Arthropoda (later) has more representatives. This is very abundant and diverse group which includes the following: the primitive chiton, Class Polyplacophora; snails, Class Gastropoda; slugs and nudibranchs, also Gastropoda; clams, Class Bivalvia; and, squid, octopi, and nautili, Class Cephalopoda These organisms all exhibit BILATERAL SYMMETRY (the Cnidarians have RADIAL Symmetry in comparison). They have a complete gut, with a true coelom. Most representatives have a brain and a complex nervous system. If a shell is present it is secreted by the specialized tissues of the mantle. Reproduction in this group is sexual, and a few are hermaphroditic (both male and female). Many have a planktonic larval stage that looks very different than the adult (and usually) benthic stage. Most representatives have a radula for feeding, either by scraping algae off rocks or feeding on other animals. There are a number of important subgroups, which will be covered below. Examples of typical Mollusks: a) chiton, Class Polyplacophora; b) clam, Class Bivalvia; c) snail, Class Gastropoda; d) tusk shell, Class Scaphopoda; and, e) octopus, Class Cephalopoda. The mollusks in the Class Monoplacophora were thought to be extinct, but were rediscovered in the 1952 during a deep sea sampling trip off the coast of Costa Rica. - Neopolina sp. Looks similar to limpets, but that is where the similarities end. Neopolina is the only genus that remains from a once prominent group. This organisms is neither a chiton or one of the limpets, but one of the ancestral types to these forms. Neopolina is considered a LIVING FOSSIL. Living Fossils are organisms that have remained largely unchanged from the ancestral or fossil types. There are about 5 species in this genus that have been described. All Neopolina have been collected in all major ocean basins from 2000 to 6000 meters (6,000 to over 18,000 feet). Neopolina doesn’t appear to exhibit torsion (twisting of the visceral mass), one of the key characteristics for gastropods. These organisms are considered rare, but this may have more to do with where they live. This is the primary reason why these organisms are also rare in marine biology labs. Neopolina spp., a specimen collected from Costa Rican waters in the 1970's (left) and an artist illustration (right). Used with permission. Class Polyplacophora - these are the chitons Polyplacophora means “many plates”. This is one of the key characteristics for this group. Like the monoplacophorans, the chitons are a primitive group. They do not exhibit torsion, but they do have a muscular foot like the snails. Unlike the snails, they do not have a well defined head region. Chitons are found primarily in the intertidal and subtidal zones, and feed on algae by scraping the plants off the rocks with their radula. Chitons rely on crypsis (blending coloration) and clamping tightly to rocks for protection. Chitons can be difficult to find because they blend well with the environment. A typical chiton, Tonicella lineata from the rocky intertidal. A tidepool sculpin, Oligocottus maculosus, will feed on chitons in the intertidal zone. Even though it may seem that chitons are relatively safe from predation, there are some organisms will regularly prey on chitons. The tidepool sculpin has modified jaws to remove chitons from rocks. Seastars (from this day forward, they are SEASTARS and NOT starfish!) also will prey on chitons. Intertidal seastars like these ochre sea stars, Pisaster ochraceus, feeds on bivalves, snails, chitons, and barnacles, to list a few. Image used with permission. Reproduction is sexual in this group and the larvae are planktonic. The largest chiton, the giant Pacific chiton (or gumboot chiton), Cryptochiton stelleri, may grow to 13 inches and about 2.5 pounds. Elephant seals will consume these on occasion. An impressive gumboot chiton from the Washington intertidal zone. It is the only chiton that does not clamp tightly to rocks. Class Bivalvia (meaning two halves)- the clams One of the key characteristics of this group is two symmetrical shells, connected at a hinge (usually). Many of these are important filter or SUSPENSION feeders. Some mussels can pump over 20 L of seawater across their gills a day. So in addition to filtering the water food plankton and dissolved gases, they help to remove contaminants from the environment. It is one of the reasons that mussels ans some clams are unsafe to consume certain times of the year. They will concentrate toxins and contaminants in their tissues by a process called bio-accumulation. This occurs when a critter like a mussel ingests a toxin or pollutant without negative affects. The problem occurs when another animal consumes the concentrated material in the mussel (more about this later). Part of a mussel (Mytilus californianus) bed from the Palos Verdes region of southern California. The gills are used for gas exchange and for feeding. There is great diversity in this group, with more than 15,000 species described thus far. Many clams have a muscular food for burrowing soft sediments. Others live permanently attached to the substrate, like mussels, oysters, and rock scallops. Bivalves vary in size, shape and habitat. The largest clam is the giant clam (Tridacna spp., left) may reach weights to 450 pounds and a width of 4 ft. This one is releasing sperm into the water column. Oysters (Crassostrea spp.) are highly prized as food by many and are now cultured in oyster farms (above right). Scallops are also commercially valuable, with many species highly sought after. Some scallops (Argopecten spp.) are mobile and can swim by snapping their shells together (bottom right) while others are sessile and live permanently attached to the bottom. Bivalves feed on planktonic organisms and may concentrate toxins from organisms like dinoflagellates. Typical gastropod snail with shell. Some gastropods lack shells, like slugs and nudibranchs. Class Gastropoda - snails, slugs and nudibranchs (the last two lack shells). These familiar organism are found in terrestrial and aquatic environments. The name gastropod means ‘stomach-foot’. Because they exhibit a muscular head-food arrangement. Most shell bearing gastropods demonstrate TORSION, which is the twisting of the visceral mass and is unique to the gastropods. Some snails are herbivorous and will scrape algae off rocks with their radula (see illustration above). While others are carnivorous and may use a structure similar to the ribbon worm to capture prey, and they may produce a strong toxin. Conus californicus, the most venomous gastropod off the California coast. Live speciemens should not be handled! Image courtesy www.pt-lobos.com. Some gastropods are carnivorous and may use a structure similar to the ribbon worm to capture prey, and they may produce a strong toxin, an example would be the cone shell (Conus californicus is our local example). They are amazingly fast and can capture and subdue small fish and crustaceans with its venom. While the venom isn’t lethal to humans (unless you happen to be allegic to bee stings), but the injury is painful. Many gastropods and bivalves are HERMAPHRODITIC. Hermaphrodites may produce both sperm and eggs during their life time (these would be serial hermaphrodites). Some do this at the same time, functioning as male and female (synchronous hermaphrodites). Some gastropods from aggregations and will segregate according to gender with males at one level in the pile and females at another. A stack of reproductive slipper shells (Credula spp.), with the larger females on the bottom and the males on top. Occasionally immature juvenile are at the very top, but do not contribute gametes until they mature (and you thought I was making this stuff up). Many gastropods and nudibranchs (marine slugs) lay benthic eggs, but produce planktonic larvae. This is important for dispersal. Gastropods produce larval called VELIGER LARVAE. These larval snails and slugs have wing like structures to move through the water. The trochopohore larval stage is similar among annelids, mollusks, and arthropods, which strongly suggests a common ancestor. Gastropods may spend weeks as part of the planktonic community (they are meroplanktonic). Gastropods are mostly benthic, but there are a few pelagic/planktonic types. Egg case of Lewis’ Moon (Euspira lewisii) snail around the shell. We regularly get these in our otter trawl collections in the nearshore environment. Veliger larva of a snail. Note the thin shell and modified foot for swimming in the water column. This life history strategy allows many organisms to exploit multiple habitats during their lives. Not all gastropods are benthic. There are some that spend their entire life cycle in the plankton. Pteropods are one type of planktonic snail. These are holoplanktonic. The pteropod (Clio pyramidata, left) and the heteropod (Atlanta peronii, right) are occasionally common in the open oceanic waters at depths to 500 meters (mesopelagic zone). Heteropods are another type of planktonic snail, which is also holoplanktonic. Both groups (subclasses) have adaptation for life in the water column, including wing-like projections, and modified feeding structures. We will see more about this later, but both groups also exhibit NEOTENY. Neoteny refers to the retention of the larval form in the adult stage. This adaptation is important in many marine organisms allowing them to survive in the plankton, and may be a key factor in the evolution of vertebrates . Some gastropods are commercially valuable like abalone. There populations are endangered due to habitat destruction, overfishing, and a disease called ‘withering foot syndrome’. The abalone are taken for their meat and shells, but there are strict rules that must be observed Some gastropods have either an internal shell or no shell at all.