Sea urchins are one of the more commonly encountered creatures when snorkeling in our seagrass beds. At times these little pin cushions can be found in great numbers. In some locations there have been too many and community events have been developed to remove some. In the western panhandle they have all but disappeared. But for many parts of the panhandle, they are a noticeable member of the seagrass community.
According to the Florida Department of Health, most sea urchin species are not toxic but some Florida species like the Long Spined Sea Urchin have sharp spines can cause puncture injuries and have venom that can cause some stinging. Swim and step carefully when snorkeling as they usually are attached to rocks, both on the bottom and along jetty ledges. Photo by L Scott Jackson
Sea urchins belong to the phylum Echinodermata. The term echinoderm means “spiny skin” and is a good name for this creature. This group also includes the sand dollars, sea cucumbers, and the most famous member, the sea stars. Echinoderms are considered advanced and primitive at the same time. Advanced in the sense of organ development, sensory perception, and food gathering. Primitive in the sense that they have radial symmetry, like many of the more primitive invertebrate groups; bilateral symmetry is considered more advanced. The entire phylum is marine, they have no freshwater, nor terrestrial members, and they do like the water salty – at least 20 parts per thousand, and some need it higher than that.
As mentioned, the sea stars are the “star” of the group. They usually have five arms that radiate from a central disk region. On top there are usually small knobs or bumps which are the remnants of their “spiny skin”. Some species, though none in our area, have elongated spines. Beneath the arms is a radial canal which houses a series of gelatinous suckers called tube feet. The sea star can fill these with water using a unique system called the water vascular system. The tips of the tube feet or concave and, when full of water, can create a suction cup that is used for pulling themselves along the bottom and for grabbing food. These canals all meet within the central disk in what is called a ring canal and the water that fills them is sucked in by the sea star through a screen-like structure on the top of the central disk called a madreporite (“screen sieve”). The central disk is where the mouth is located, and it is located on the bottom. Food is worked into the mouth, digested, and excreted through an anus on the top of the central disk. Most species have some form of eye at the end of each arm and have a good sense of smell and taste.
Sea stars are predators, collecting small organisms they are fast enough to catch (which is not many really). But they can also take on larger slow prey, like shellfish. When they approach an oyster, which is sessile and cannot run away, they will grab each of the valves (shells) of the oyster with one of their arms. They will draw water into their water vascular system creating suction on the tube feet and “stick” onto the shells. They will then force the oyster open. Once open they will invert their digestive tract out of their body in a process called evisceration, consume the oyster, then retract the digestive tract leaving two empty shells on the seagrass bed floor. These empty shells are often found by snorkelers – though there are other predators of bivalves.
The Florida Orange Sea Star.
Photo: Florida Sea Grant
Sea urchins differ from their sea star cousins in the way their body is laid out. Imagine you had a five-armed sea star laying on the ocean floor. Imagine taking each of the five arms and rolling them upwards so that the tips of each touch above the central disk. Can you imagine this looking like a ball? A sphere? Now cover the now exposed underside of the arms with long spines (quills) and the tube feet extend between the quills. You have a sea urchin.
Another difference would be the mouth. At the terminus of each arm near the mouth is a single tooth. With five arms, there would be five teeth. Scientists call this set of five teeth Aristotle’s lantern and the urchin uses this to scrap algae from rocks, shells, and grass blades. They are herbivores, moving along feeding on a variety of seaweed and seagrasses in the system.
There is concern with many snorkelers that the quills (spines) of the sea urchin are venomous. That is the case with some species around the world, but not in our area. That said, they are sharp, and the purple urchin (more common in our rock jetties and artificial reefs) hurts. Their quills are sharp and often break off in the skin causing discomfort, much like a splinter. You do not want to handle them, but if you do – handle them with care.
Sand dollars are close cousins of the sea urchin and are in the same class (Echinoidea). If you can imagine taking a round sea urchin and squashing it flat like a pancake, you have a sand dollar. There are also echinoderms in this group that are not as round as sea urchins, but not as flat as sand dollars and are called heart urchins, or sea biscuits. These can be found in grassbeds at the eastern end of the panhandle, but are more common in south Florida.
The sea cucumber is an echinoderm more often found in rocky or coral reef communities, but there are some found in the seagrass beds. To see the relationship between them and their cousins, imagine taking a round sea urchin, lie it on its side, and extended the body so that it is no longer a round ball but an elongated worm-looking creature… sea cucumber. These are primarily scavengers and deposit feeders within the community.
Sea Cucumber
Photo by: Amy Leath
Being a resident of the western panhandle, we have noticed a mass decline of echinoderms in our grassbeds. As a kid in the 60s and 70s we never saw large numbers, as they do in the eastern panhandle, but we did see them. Now they are gone. One suggestion as to why has been salinity. Our bay system over here has more river discharge than those further east and the lower salinity may not support larger populations. The increased development of the years, and the methods of dealing with stormwater, may have created a system that echinoderms do not like. Whatever the reason, finding sea stars and sea urchins in our grassbeds is rare.
The eastern panhandle still has them. And, at times, too many. In recent years there has been an increase in sea urchin populations in St. Joe Bay that has led to overgrazing of the turtle grass. This could lead to a decline in suitable habitat for bay scallops, which the community depends on economically. The state currently sponsors a “Sea Urchin Round Up” event using humans to help control the overabundance of sea urchins.
Echinoderms are a visible, and interesting, part of our seagrass community.
In terms of diversity and abundance, the Phylum Arthropoda is the most successful in the Animal Kingdom. Between them all, there are over one million species. They can be found in all habitats, from the deepest part of the ocean to the highest places in the mountains, from the polar region to the most extreme deserts. Most are insects, but there are also arachnids, centipedes, millipedes, and the ones most common to the marine environment – the crustaceans. With the numerous species within this group, and new ones being discovered all the time, the classification of arthropods is constantly changing. Currently Crustacea is considered a subphylum and there are about 30,000 species within.
Insects are one of the most abundant forms of life on the planet. Photo: Princeton University
There are several keys to the success of arthropods. Number one, their shell. It was seen with the mollusk that having a hardshell to protect your soft body was a winner. However, mollusk make their shells from heavy calcium carbonate. Though this provides excellent protection against most predators, it did slow them down considerably making it much easier for predators to catch them. It is understood that in the world of defense, speed is important. The arthropods make their shells from a strong, but much lighter material called chitin. This material is strong but serves as the creatures’ exoskeleton and must be shed periodically as the animal grows.
Number two, their legs. The name “arthropod” means jointed foot and one glance at the legs of any of these, you will see why scientists call them this. To increase speed animals, need to break contact with the surface of the ground. Birds are the best, lifting off and flying – the fastest form of location there is. The slug-like mollusks have their entire bodies in contact with the sediment, as the “slug” along the bottom of the sea. Many creatures have developed legs and walk, this is the case of the arthropods. Some hop great distances, like the flea. Others can actually swim, like blue crabs. And many of the insects have wings and can fly. But these jointed legs, along with a lighter shell, have been very effective defense for these creatures.
Number three, their sense organs and brain. Though not as intelligent as octopus and squid, arthropods are very aware of their environment and very quick to respond to trouble or a food source. Drop a piece of cheese during a picnic and see just how fast the ants find it. Heck don’t drop the piece of cheese and see how quickly they find it! These animals have a series of hairs, bristles, and setae connected to their shell that can detect movement and pressure changes in the environment. There are canals, slits, pits, or other openings in the shell that can detect odors. And then they have their compound eyes. Compound in the sense there are more than one lens. Each lens does provide an image of the target (in other words, they do not see 100 images of you) but rather each provides a level of light intensity sort of like individual pixels in a computer image, or the image we see when the camera displays “squares” of light so that you cannot read someone’s license plate, or the logo on their t-shirt – we see this on TV news and shows often. Compound eyes do not produce as clear an image as our eyes, but they are MUCH better at detecting motion, and there is an advantage to this. Try stomping on a cockroach, or swatting a fly, and you will see what I mean.
And number four, a high reproductive rate. You see this in many of the “prey” type species. Most arthropods are dioecious (males and females) and can produce millions of offspring at rates that you could never consume them all. So, they survive and can quickly support gene flow and adaptation. These are well designed animals.
Blue crabs are one of the few crabs with swimming appendages.
Photo: Molly O’Connor
In the crustacean world we find several groups. They differ from their arthropod cousins in that they have 10 jointed legs, and two sets of antenna (one set long, the other short). They include at least 30,000 species including the shrimplike cephalocarids, the shrimplike branchipods, the shrimplike ostracods (common in the deep sea), the roachlike copepod (part of the plankton we spoke about in another article in this series), the mystacocards, branchiurans, the familiar barnacles, krill, the roachlike isopods, flea like amphipods, and the most familiar of the group – the decapods – which includes the crabs, shrimps, and lobsters. It is this last group we will focus on.
There are three basic body parts to an arthropod. The head, thorax, and abdomen. In crustaceans the head and thorax are fused into one segment called the cephalothorax (the head of a shrimp or crawfish). The abdomen is what most call the shrimp and crawfish tail (the part we usually eat).
Insect body parts.
Crabs are the ones we most often encounter when exploring the seagrass beds. Not that the others are not abundant, they are, they are just not seen. They differ in that their abdomen is curled beneath their cephalothorax. The most commonly encountered is the famous blue crab (Callinectes sapidus). Most crabs have modified two of their 10 legs into chelipeds (claws) and most folks seeking crabs for dinner are aware of these claws. The blue crab belongs to a group called the protunid crabs which have modified two additional legs into swimming paddles – they can swim. They are often found crawling around the edges, and within, the seagrass searching for food. When detected over sand, they quickly bury themselves and sometimes people step on them not knowing they are there. When spooked they often will emerge with chelipeds extended and when the time is right, will scurry off running sideways with one cheliped pointed at you. They can get quite large and are a popular fishing target for both recreational and commercial fishermen. The males (the ones with the long then telson on their curled tailed) are more common in the upper estuaries. The females (the ones with the more round telson) frequent the lower bay. During breeding season, the males will move to the lower estuary to find a female. Once found he will crawl on her back and “ride” for a couple of days in what commercial fishermen call “doublers”. At some point the male will provide a tube filled with sperm called a spermatophore to the female. He then moves on. The female will store the spermatophore until she feels it is time to fertilize the eggs, then does so. The eggs begin to develop beneath her abdomen in a spongy looking mass. Early in development the mass is an orange color. Closure to hatching it is brown. Females carrying this spongy mass are called gravid and are illegal to harvest in Florida. The larva will be released in the millions as tiny plankton and go through several life stages before becoming young crabs and starting the whole story again. These popular crabs live for about five years.
Male and female blue crabs.
Photo:
Another crab found in the grassbeds is the spider crab (Libinia dubia). This crab does resemble a spider, is slow moving, and very hard to see. It has small chelipeds and feeds on debris and organic material collected by the grasses. They too can get quite large and resemble the king crabs harvested in Alaska.
Stone crabs (Menippe mercenaria) are more often associated with rocky bottoms, or artificial reefs, but they have been found in burrows and crevices within grassbeds. The have wide-stocky chelipeds, which is a favorite with some seafood lovers. Those in the grassbeds do not get as large as those found around the reefs of south Florida, where they support a large commercial and recreational fishery.
The stone crab has been a popular seafood target in Florida for decades.
The hermit crab is a common resident of grassbeds. The most frequently encountered is the striped hermit (Clibanarius vittatus). Like all hermit crabs, they lack an external shell covering their abdomen and must cover their tail with an empty mollusk shell. Their curled abdomen can grab and wrap around the columella within the mollusk shell and carry around their new home. These hermits have been found in a variety of mollusk shells and are found roaming the beaches at low tide feeding on organic debris and cleaning the grassbeds.
A room with a view: a stripped hermit crab sizes up a potential residence
One crab that is often associated with seagrasses is not actually a crab at all. The horseshoe crab (Limulus polyphemus) lacks antenna and is more closely related to the arachnids. This ancient mariner has been plowing the bottoms of estuaries for over 400 million years. They resemble stingrays with their elongated telson and feed on a variety of small invertebrates both in the grassbeds, and in other estuarine habitats. They are quite common in the eastern panhandle and seem to be making a recovery in the western end.
A large horseshoe crab found in Little Sabine.
Photo: Amanda Mattair
Though rarely seen, shrimp are very prolific in our seagrass beds. Pulling a seine or dip net through the grass will expose their presence, usually in high numbers. The most commonly collected species are those known as grass shrimp (Palaemonetes sp.). There are a few species, and all are small and mostly translucent, though one is a brilliant green. Feeding on organic debris within the grassbed these little guys are an important food source for the larger members of the community.
The more famous of the shrimp group are the brown and white shrimp. These are the species we find on our dinner plates and are one of the most popular commercial species in the country. Brown shrimp (Farfantepenaeus aztectus) are also known as bay shrimp and “brownie”. They are a darker brown than the white shrimp and their uropod (the fan on the tail of the shrimp) is lined in a red color. They do not get as large as the whites and are very popular for fried and steamed dishes. The white shrimp (Litopenaeus setiferus) is a larger shrimp, is lighter in color (“white”), and their uropod is lined with a neon green color. Both of these commercially important species spend their juvenile and young adult days in the grassbeds of our estuaries. Later in the fall the adults move into the nearshore waters of the Gulf where they spawn and die. The planktonic larva drift back into the estuary with the incoming tide, finding the grassbeds and the cycle begins again.
The famous Gulf Coast shrimp.
Photo: Mississippi State University
The large diversity of crustaceans within the grassbeds speaks to the importance of this habitat to all marine life. Many are commercially important to the local economy and depend on a healthy ecosystem to survive. All the more reason to protect our grassbeds.
The Emerald Coast Open Lionfish Tournament May 20-21, 2023, at HarborWalk Village in Destin, FL, is gearing up to tackle a pressing ecological challenge while showcasing the power of sport to make a positive impact. This unique tournament, held along the picturesque shores of the Emerald Coast, focuses on combating the invasive lionfish population in the region’s waters.
Lionfish, native to the Indo-Pacific region, have become a significant threat to the delicate balance of marine ecosystems in the Gulf of Mexico. With their voracious appetite and rapid reproduction, these invasive species pose a grave danger to native marine life. The Emerald Coast Open Lionfish Tournament aims to address this issue by encouraging divers and fishermen to actively hunt and remove lionfish from the waters.
Participants in the tournament will compete to catch the most lionfish, utilizing their skills in underwater navigation, spearfishing, and conservation. Sponsors provide cash and prizes for multiple categories including most caught, largest and smallest lionfish. The event provides an exciting platform for experienced divers and newcomers alike to contribute to the preservation of the marine environment.
Beyond the ecological significance, the tournament also offers a thrilling experience for both participants and spectators. Divers equipped with their spears dive into the depths, searching for lionfish while showcasing their prowess and bravery. The tournament fosters a sense of camaraderie and shared purpose among the participants, creating a community dedicated to the cause of protecting marine ecosystems.
In addition to the competitive aspect, the Emerald Coast Open Lionfish Tournament promotes education and awareness about the invasive species. Participants and attendees have the opportunity to learn about the impact of lionfish on local marine life and explore sustainable solutions to combat the issue at the free Lionfish Awareness Festival from 10:00-5:00 each day. Sign up to volunteer at the event if you want to join the fun. The week prior to the tournament is dedicated to Lionfish restaurant week where local restaurants practice the “eat ‘um to beat ‘um” philosophy and cook up the tasty fish using a variety of innovative recipes.
The Emerald Coast Open Lionfish Tournament 2023 represents a unique fusion of sport, environmental conservation, and community engagement. By bringing together individuals passionate about marine conservation, this event serves as a powerful catalyst for change and a shining example of how sport can contribute to the preservation of our natural world. Learn more at https://emeraldcoastopen.com.
A Lionfish Removal and Awareness Day festival volunteer sorts lionfish for weighing. (L. Tiu)
In Part 5 of this series, we looked at a group of invertebrates that few people see, and no one is looking for – worms. But in this article, we will be looking at a group that seagrass explorers see frequently and some, like the bay scallop, we are actually looking for – these are the mollusks.
With over 80,000 species, mollusk are one of the more successful groups of animals on the planet. Most fall into the group we call “seashells” and shell collection has been popular for centuries. There is an amazing diversity of shapes, sizes, and colors with the snail and clam shells found in coastal areas worldwide. As snorkelers explore the seagrass beds it is hard to miss the many varieties that exist there.
Seashells have been collected by humans for centuries.
Photo: Florida Sea Grant
One group are the snails. These typically have a single shell that is coiled either to the right or left around a columella. Some are long and thin with a extended shell covering their siphon (a tube used by the animal to draw water into the body for breathing). Others are more round and ball-shaped. Each has an opening known as the aperture where the animal can extend its large fleshy foot and crawl across the bottom of the bay. They can also extend their head which has an active brain and eyes. Snails lack teeth as we know them, but many do have a single tooth-like structure called a radula embedded in their tongue. They can use this radula to scrape algae off of rocks, shells, and even grass blades. Others will use it as a drill and literally drill into other mollusk shells to feed on the soft flesh beneath.
In the Pensacola area, the crown conch (XXX) is one of the more common snails found in the grasses. This is a predator moving throughout the meadow seeking prey they can capture and consume. Lighting whelks, tulip shells, and horse conchs are other large snails that can be found here. You can often find their egg cases wrapped around grass blades. These look like long chains, or clusters, of disks, or tubes, that feel like plastic but are filled with hundreds of developing offspring.
The white spines along the whorl give this snail its common name – crown conch.
Photo: Rick O’Connor
A close cousin of the snail are the sea slugs and there is one that frequent our grassed called the “sea hare”. This large (6-7 inch) blob colored a mottled green/gray color, moves throughout the grass seeking vegetation to feed on. When approached, or handled, by a snorkeler, they will release a purple dye as a “smoke screen” to avoid detection. Snails secrete a calcium carbonate shell from a thin piece of tissue covering their skin called a mantle. The genetics of the species determines what this shell will look like, but they are serve as a very effective against most predators. Most… some fish and others have developed ways to get past this defense. But the slugs lack this shell and have had to develop other means of defense – such as toxins and ink.
This green blob is actually a sea slug known as a sea hare. It was returned to the water.
Photo: Rick O’Connor
A separate class of mollusk are the bivalves. These do not move as well as their snail cousins but there are NO access points to the soft body when the shell is completely closed – other than drilling through. One creature who is good at opening them are starfish. Seabirds are known to drop these on roads and buildings trying to crack them open. But for the most part, it is a pretty good defense.
Bivalves possess two siphons, one drawing water in, the other expelling it, and use this not only for breathing but for collecting food – all bivalves are filter feeders. They will, at times, inhale sand particles that they cannot expel. The tend to secrete nacre (mother of pearl – shell material) over these sand grains forming pearls. Most of these are not round and are of little value to humans. But occasionally…
The pen clam is a common bivalve found in grassbeds.
Photo: Victoria College.
Oysters may be one of the more famous of the bivalves, but they are not as common in seagrass beds as other species. Most of our seagrass species require higher salinities which support both oyster predators and disease, thus we do not see as many in the grasses. Clams are different. They do quite well here, though we do not see them often because they bury within the substrate. We more often see the remaining shells after they have been consumed, or otherwise died. The southern quahog, pen shell, and razor clam are clams common to our grassbeds.
The one group sought after are the bay scallops. Scallops differ from their bivalve cousins in that they have small blue eyes at the end of each ridge on the shell that can detect predators and have the ability to swim to get away. They usually sit on top of the grasses and require them for their young (spat) to settle out. They are a very popular recreational fishery in the Big Bend area where thousands come very year to get their quota of this sweet tasting seafood product.
Bay Scallop.
Photo: FWC
There is another group of mollusk that are – at times – encountered in the seagrass beds… the cephalopods. These are mollusk that have lost their external calcium carbonate shells and use other means to defend themselves. This includes speed (they are very fast), color change (they have cells called chromatophores that allow them to do this), literally changing the texture of their skin to look and feel like the environment they are in at the moment, and expelling ink like some of the slugs. This includes the octopus and squid. Both are more active at night but have been seen during daylight hours.
The chromatophores allow the cephalopods to change colors and patterns to blend in.
Photo: California Sea Grant
As mentioned, shell collecting is very popular and finding mollusk shells in the grassbeds is something many explorers get excited about. You should understand that taking a shell with a living organism still within is not good. Some areas, including state parks, do not allow the removal of empty ones either. You should check before removing.
When most hear the word worm unpleasant things come to mind. “Gross”, “dirty”, “decaying”, “disease”, “rotten”, are a few. And then there is the whole parasite thing. But then there are those who like them. Gardeners, kids, and fishermen find earthworms in particular pretty cool. They are not the typical creature we look for on a hike, or search for on a TV documentary, and they are certain not at the top of creatures you would be looking for while exploring a seagrass bed – but they are there.
According to the 4th Edition of Robert Barnes’s Invertebrate Zoology (1980) there are at least 11 phyla of worms, and he mentions no fewer than six classes. It is an extremely diverse group of creatures, and many are the bridge between the simple animals and the more complex. In this article we will focus on four phyla of the more common worms, or least the ones most commonly known.
These phyla are divided by body shape and internal complexity. The simplest are the flatworms (flukes and tapeworms), the more complex are the annelids (earthworms and leeches). The vast majority of these animals are very small (less than one centimeter) and not seen by the casual snorkeler. Some of the nemerteans (a phyla of flatworm) and the segmented polychaetes (related to earthworms) are quitter large and are easily seen by us.
The polychaetes may be the most familiar to us. These are segmented worms in the phylum Annelida. They are segmented like their cousins the earthworm but differ in that (a) each segment has a small leg-like structure (parapodia) and (b) they like marine conditions.
Neredia are one of the more common polychaete worms.
Photo: University of California Berkley
They resemble centipedes crawling in and amongst the seagrass blades and are often found within seashells we pick up and explore. They are basically harmless, and many species are the “garbage feeders” doing an excellent job keeping the system clean. We often see their “volcano” like burrows in the sandy areas within a seagrass meadows and many species produce glob-like gelatinous egg sacs that seagrass explorers confuse with jellyfish. There are also those that produce tubes. Some of these tubes are paper-like with bits of shell and other debris embedded in them. They produce these using mucous from their bodies to cement them together, place the tube within the sediment in a vertical position, and then live in them. Other tubeworms will produce their tubes out of shell material (calcium carbonate) forming snake-like structures on the surface of shells and discarded beer cans. And then there are a few called bristle worms. These are large and crawl across the surface of the substrate but have thin spines that extend off their bodies like cactus. Many of these do have venom and can be quite painful.
Polychaetes are the most advanced of the marine worms in the seagrass beds. Possessing a brain that is connected to sensory organs that can detect light and chemicals in the water, they can both find prey, and avoid predators. Prey varies between species. Most polychaetes can invert their pharynx (rather quickly) to grab prey using tooth like jaws. The carnivorous ones feed on small invertebrates (including other polychaetes). Others will use these jaws to scrap algae from shells and grass blades and are scavengers. Most reproduce sexually where there are both males and females and they possess a complete digestive tract (including a mouth and an anus) – as you will see… not worms do. In 1980 there were about 5300 species of them worldwide.
Nemertean worms are another large marine worm, but one few people have seen. This is because they are more nocturnal by habit. They are not segmented but rather are flat and gelatinous. They possess a proboscis that can be “launched” by the worm, that has a stylet (spear) at the tip – like a pole-spear gun. They use this to hunt invertebrates and find them by using their sense of smell. They also reproduce sexually, and there are separate males and females, but many will fragment into smaller worms when irritated. There are two classes and four orders of these worms worldwide.
Most of the remaining worms are either so tiny you will not find them or are endoparasites living within some vertebrate or invertebrate host within the seagrass community. The more famous group are the flatworms. These include the parasitic flukes and tapeworms. However, there is a class of free-swimming flatworms that crawl (or swim) around the “seagrass forest”.
The swimming ones are known as tubellarians. Most are clear or opaque, but in the coral reef community some are very colorful. Flatworms are the more basic members of the worm group. They lack a complete digestive system and must both eat and expel waste through the same opening (the mouth). Some tubellarians feed on small invertebrates which they capture and engulf without using teeth. Others feed on dead and decaying creatures, again – cleaning the environment. There are some that live commensally with mollusk and crustaceans. There is one that is rather large and can be seen on the gills of horseshoe crabs. They do have a simple brain and sense the world by detecting light, a sense of touch, and smell. In 1980 they reported about 3000 species worldwide.
This colorful worm is a marine turbellarian.
Photo: University of Alberta
The parasitic flatworms include the famous flukes and tapeworms. These live within the bodies of their hosts feeding on mucous, cells, tissue, and blood (yep – here is the “gross” “disgusting” thoughts we mentioned at the beginning). They possess tough skin to protect them from the digestive enzymes of their host. Most reproduce sexually but are hermaphroditic (each worm being able to produce both sperm and egg). Most require secondary hosts to complete their life cycle. For example, they may live in the gut of a spotted seatrout but will need to complete their larval stage in the gut of a mollusk. So, the eggs are released with the feces of the trout, the larva find a mollusk and enter, develop, and then expelled again via feces to once again find a trout. It’s a wonder they can do this – but they do.
The human liver fluke. One of the trematode flatworms that are parasitic.
Photo: University of Pennsylvania
The tapeworms cycle differs in that their head is round and has a series of hooks they use to embed into the intestinal tract of their host. The rest of their body is flat and segmented (each segment called a proglottid). These proglottids are released into the environment to find another host.
You may now be afraid of becoming infested with parasitic worms if snorkeling in seagrass beds. Fear not… these animals are VERY specific about the host they can survive with – and you are not one of them. Most seagrass explorers do not think of worms when they visit this community, and probably will not see any, but they are there and play an important role in keeping the system healthy.
The word “jellyfish” tends to initiate a similar response in most people – “scream”, “run”, “this is going to hurt”. Being stung by a jellyfish is not pleasant and is something most would prefer to avoid. Our beaches warn us when they are out by flying a purple flag.
When exploring the seagrasses, this is not the first animal people thing they will encounter. Few associate jellyfish with the seagrass community. But within any community there are those we call residents (they reside here) and those we call transients (just passing through). It is the second group that we can place most jellyfish, at least the ones we are concerned about.
The sea nettle.
Photo: University of California at Berkley.
Jellyfish are animals, but not your typical ones. They are obviously invertebrates but differ from most others by having radial symmetry (having a distinct top and bottom, but no head nor tail). They possess ectoderm and endoderm (so, they have a skin layer and some internal organs) but they lack the mesoderm that generates systems such as the skeletal, circulatory, and endocrine. Though they do not have a brain, they do have a simple nervous system made up of basic neurons and some packets of nerve cells called ganglia. They seem to know when they are not in the upright position and know when they have stung something – which initiates the feeding behavior. But they are pretty basic creatures.
When you view a jellyfish the first thing you see is the “bell” and the tentacles – we always see the tentacles. The bell is usually round (radial), could be bell-shaped, or could be flat. It is made of a flexible plastic-like jelly material called mesoglea. Most of the mesoglea is actually water. When you place most jellyfish on the dock and come back in a few hours there may be nothing but a “stain” of where it was. It completely evaporated. There are some exceptions to this, like the moon jelly and the cannonball jelly, who leave thick masses of mesoglea for long periods of time.
Image: Wikipedia.
If you look closer at the “bell” you will see shapes within the mesoglea. Some are stripes, and may have color to them, others look like a clover leaf. These are the gonads of the animal. Jellyfish are hermaphroditic (the gonads can produce both sperm and egg), and they reproduce by releasing their gametes into the water column when triggered by some environmental clue to do so.
Around the edge of the “bell” many have a thin piece of tissue called the velum that can undulate back and forth and allow the jellyfish to swim. Swimming can involve moving up or down in the water column, or turning around, but the swimming action is not very strong and the tide and current actually plays a larger role in where the animals go – like pushing them through a seagrass bed.
Under the “bell” is a single opening, the mouth, that leads into a simple gut (the gastrovascular cavity). This serves as the stomach of the creature. But there is no anus, when the jellyfish has digested its food, the waste is expelled through the same opening – the mouth. This is called an incomplete digestive system.
Jellyfish are predators and hunt small creatures such as baitfish. Though they know whether they are upside down or not, and may be able to detect light, most have no true eyes and cannot see their prey. Some species may be able to detect scent in the water and undulate their velum to try and move towards potential food, but most drift in the water and hope the tide carries them to dinner. To kill their prey, they extend tentacles into the water. These tentacles are armed with stinging cells known as nematocysts. Each nematocyst holds a coiled harpoon with a drop of venom at the tip. They are encased in a cell membrane and are triggered when an object, hopefully food, bumps an external trigger hair that will fire the harpoon. This will then trigger the release of many nematocysts and the potential prey will be “stung” by many drops of venom. The venom can either kill or paralyze the prey at which time the tentacles bring it to the mouth. Many jellyfish have venom that is painful to humans, like the sea nettle and moon jelly, others have a mild venom that we do not even notice. Some have a very strong venom and can be quite painful, like the Portuguese man-of-war which has put some in the hospital. The famous box jelly of Australia has actually killed humans. We do have box jellies in the Gulf of Mexico, but they are not the same species.
This box jellyfish was found near NAS Pensacola in November of 2015.
Photo: Brad Peterman
As the tide pushes these transients through the seagrass meadows, their tentacles are extended and small baitfish like juvenile pinfish, croakers, and snapper become prey. But there are resident jellyfish as well.
With the Phylum Cnidaria (the stinging jellyfish) there are three classes. Class Scyphozoa includes the bell-like jellyfish that drift in the water column with extended tentacles – what are referred to as medusa jellyfish. But there are two other classes that include benthic (bottom dwelling) jellyfish called polyps.
Polyp jellyfish resemble flowers. The “bell” part is a stalk that is stuck to a rock, pier, or seagrass blade. Their tentacles extend upwards into the water column giving the creature the look of a flower. Instead of drifting and dragging their tentacles, they hope to attract prey by looking like a hiding place or other habitat. The sea anemone is a famous one, and a good example of the polyp form. But it also includes corals and small polyps known as Hydra. Hydra are tiny polyps that are usually colorless and can easily attach to a blade of turtle grass. Here they extend their tentacles into the water column trying to paralyze small invertebrates that are swimming by or grazing on the epiphytes found on the grass blades.
The polyp known as Hydra.
Photo: Harvard University.
Another jellyfish that drifts in the current is Beroe, what some call the “football jellyfish” or “sea walnut”. This a relatively small blob of jelly that lacks tentacles but rather has eight rows of cilia/hair (ctenes) along its side that move quickly and move this animal through the water. But like their medusa cousins, not against the tide or current. These jellyfish do not sting, they lack nematocysts, and hence are in a different phylum known as Ctenophora. Kids often find and play with them when they are present, and they are luminescent at night. These stingless jellyfish feed on small plankton and each other and are another transient in the seagrass community.
The non stinging comb jelly.
Florida Sea Grant
There are certainly species of jellyfish to be aware of and avoid. But as you look deeper into this group there are harmless and fascinating members as well. Most of these Hydra are very small and hard to see while snorkeling, but they are there. Another creature to try and find while you are exploring and play “seagrass species bingo”. Have fun and stay safe.
