Note: This article is the fourth in a 10-part series that delves into the natural history of marine animals you are likely to encounter when diving. The pieces are presented in order according to the taxonomic system scientists use when they describe and classify plants and animals. An overview of the articles and the taxonomic system is provided in the February 2001 issue.
In the interest of space, this article covers three phyla: Platyhelminthes (plat-ee-hell-MIN-theez); Annelida (ann-uh-LID-ah) and Ectoprocta (eck-tô-PROC-ta).
Imagine being in an alien environment, a place where you know you cannot survive very long without special breathing apparatus and a supply of an acceptable breathing gas, and everywhere you look you see worms sticking their heads out of holes, tubes and burrows. Red ones, purple ones, pink ones, beige ones, striped ones, spotted ones, big ones, little ones, puffed ones, flat ones, solitary ones and clusters of them. Move toward some and in the blink of an eye many disappear. Others flare and bristle, advertising small tufts of spine-like projections. Still others appear unaware or unfazed by your presence.
When you aren’t focused on the worms, you see colonies of creatures that look like moss. They might be plants, might be animals. Hard to tell.
This description sounds like a scene in a Hollywood sci-fi thriller that sends chills up and down your spine. But you had to figure there was a catch, and there is. As sport divers we are often thrilled when we encounter spectacularly colorful marine worms and colonial animals known as bryozoans (bry-ô-ZÔ-ans). Sometimes we fail to realize that these worms and bryozoans are what they are, but there is little question that many species are visually striking and getting a good, close look can make a dive a memorable one.
In this column we examine the natural history of two phyla of marine worms, Platyhelminthes (plat-ee-hell-MIN-theez) and Annelida (ann-uh-LID-ah) and the phylum Ectoprocta (eck-tô-PROC-ta) which consists of the bryozoans. All of these animals are relatively simple invertebrates. The primary reason for including them in one piece instead of separate pieces is the interest of magazine space, and I do not suggest that the species described in these phyla have an unusual relationship that dictates they be handled collectively.
In general, worms seem relatively simple creatures. It is easy to understand why one might assume that all worms would be described in the same phylum. However, that is not the case. “Worms” is a common name applied to animals that display a variety of characteristics. There are actually four phyla that include worms, but only two are prominent in Neptune’s realm.
While it is easy to think of worms as small and insignificant, it’s a mistake to underestimate their importance in many marine ecosystems. Scientists are quick to point out that many species of worms are important sources of food for several marine creatures.
The open sea and reefs in tropical and temperate seas are rife with worms. For example, a study conducted at Heron Island, part of Australia’s Great Barrier Reef, discovered that on a single coral head there were more than 1,400 worms representing 103 species.
Phylum: Platyhelminthes (from Greek for “many worms”) The Flatworms
Worldwide, 30,000 species of worms are described in the phylum Platyhelminthes. The overwhelming majority, 25,000, are relatively unattractive, parasitic organisms such as tapeworms. But before deciding to clean your attic instead of continuing to read, you will probably want to know that many of their 4,000 marine relatives are very beautiful creatures indeed. These species are collectively known as flatworms (also polyclad flatworms), and many are so visually striking that novice divers mistake them for nudibranchs. However, flatworms are just that, flat. Nudibranchs, which are types of mollusks, have much thicker bodies and are members of a different phylum.
The oval-shaped bodies of many polyclad flatworms are as colorful as they are flat. It is believed that their bright coloration warns predators such as fish-eating worms that these worms are noxious. Many specialists conclude that if that were not the case, these exposed, slow movers would be such easy prey that their species could not survive.
Most polyclad worms are less than 3 inches (7.6 centimeters) long, but some reach a length of close to 6 inches (15.24 cm). They crawl along the sea floor by gliding along a self-secreted mat of mucus. They power the movement using numerous, microscopic bristles. As divers, we usually spot flatworms as they move along the bottom, but occasionally some species use an undulating motion of their body to swim through the water column.
The bodies of flatworms have a distinct front and rear and top and bottom. If split down the middle, their right and left sides would mirror each another. This type of body plan is known as bilateral symmetry, and it represents a significant evolutionary development.
Several other features found in flatworms are worth noting. While flatworms lack sophisticated organ systems, they do possess numerous bundles of light-sensitive nerves which can be considered primitive eyes. Not surprisingly, flatworms are very sensitive to light, and many species avoid bright light and being exposed during daylight hours.
Flatworms display a variety of methods of feeding. Many are carnivores and scavengers, and most species are highly cannibalistic. Some species use another method to acquire food by taking advantage of a mutually beneficial relationship with algae that live in their tissues. The algae, which gain a place to live, produce food for the worms as a natural byproduct of their own photosynthesis. Mother Nature never ceases to amaze.
Flatworms, like many marine animals, are hermaphrodites — individual adults possessing male and female reproductive capabilities. However, cross-fertilization with different members of their species is the rule as opposed to self-fertilization. Self-fertilization could help flatworms avoid the risk of meeting a cannibalistic mate during a romantic interlude. However, the genetic recombination achieved through cross-fertilization makes it far more difficult for a single disease, predator, or environmental factor to wipe out an entire population.
When flatworms mate, each partner deposits sperm in the copulatory sac of its mate. It is not yet well understood how flatworms distinguish between potential mates and animals with cannibalistic designs. After mating, the fertilized eggs are soon released into the water column where they become part of the plankton before eventually settling and becoming part of the benthic, or bottom-dwelling, community.
Also, some flatworms can reproduce asexually. In these species an individual that loses part of its body not only displays remarkable regenerative powers by remaking the lost part, but the lost body part often grows into a new, complete flatworm. This regenerative ability is especially valuable to species that inhabit relatively shallow rubble zones where debris that can damage their bodies is often moved around in surf, rough seas and tidal flow. For many species such dangerous conditions would be life threatening, but some flatworms are capable of turning the tables and using the situation to their advantage.
Phylum: Annelida (from Latin for “ringed ones”)
The Segmented Worms
Tubeworms, Christmas Tree Worms, Feather Dusters and Bristle Worms
About 8,000 species of worms are described in the class Polychaeta (poly-KEY-ta) that is part of the phylum Annelida. These worms are referred to as polychaete (poly-KEET) worms, annelid worms and segmented worms. As the word segmented implies, the bodies of these worms are divided into obvious sections. The segmented worms possess separate digestive, excretory, respiratory and circulatory systems and a variety of sensory organs.
No doubt, “worms” and “beautiful” are words not usually associated with each other, especially when the worms in question are the terrestrial species most familiar to us. However, the worm turns when it comes to many marine species. Mother Nature was especially generous with the blessings of beauty she bestowed upon many marine worms. Those species known as Christmas tree, feather duster and bristle worms are especially handsome.
In most cases we only see about one-tenth of the body of these worms. We see the gill plumes which look like a whorl of colorful feathers. Also known by the term radioles, the gill plumes help trap food particles and serve an important role in respiration. These worms burrow into the substrate and the rest of their bodies are found inside of the tube they live in. Hence, the name tubeworms. Built of sand and other inorganic matter that is combined with secretions from the worms, the tubes are self-made and semipermanent.
As many divers know, Christmas tree worms, feather dusters and other tubeworms are quick to draw their gills inside of their tubes when threatened. Surprise and sometimes frustration are often the result when you try to move in for a closer look. These worms are especially sensitive to changes in light intensity and to movement, and they are quick to protect themselves by withdrawing their gill plumes. Some species then close their tube with a hard trap door known as an operculum, some of which are armed with a protective spine. If you remain still and wait, they often open like a bouquet within a minute or so.
Sexes are typically separate in polychaete worms. However, many males lack gonads. Sperm is produced in cells that line the body cavity, a trait not typical of members of other classes in the phylum. Fertilization of eggs occurs outside of the body of the female as sperm are spread over egg masses floating in the water column or attached to the substrate.
The larval animals that spring forth look a bit like miniature, hairy missiles and become part of the community of plankton. These creatures are known as trochophores (troc-Ô-fours). Those that avoid predation grow and add body segments. Some of the surviving larval animals eventually settle to the sea floor where they will live as adults, while adults in other species live in the water column.
Some species that live in tubes attached to the substrate as adults depend on currents to supply them with a constant source of food, which consists primarily of organic matter and tiny planktonic creatures. Food particles are filtered out of the water by the intricate sieve created by the gill plumes. Other tube dwellers such as terebellid (tera-BELL-id) worms use long tentacles that stretch over the sea floor to capture their food. Cilia on the tentacles passes food to the mouth the way a conveyor belt moves items. The “tentacles of worms” is the answer to the frequently posed question, “What are those long, stringy things that we see stretched out over the reef on night dives?” And now you know.
In turn, polychaete worms are heavily preyed upon by a variety of animals, especially rays and other fishes. In southern California, bat rays feed heavily on polychaete worms. Often the empty tubes of tube-dwelling species can be found in depressions in the sand where bat rays were feeding, and partially digested worms can be found in the stomachs of bat rays. In addition, in many parts of the world it is common to witness a variety of fishes grubbing through the sand as they feed on worms and other prey.
While many species of segmented worms are comparatively sedentary, a variety of species known as bristle worms are not. The bright colors and delicate appearance of these free-living worms often catch the attention of divers. Their ornate bodies, which typically vary in color including red, orange and green, are bordered with tufts of bristle-like fibers. Occurring in a range of tropical and subtropical seas, bristle worms are known to exceed 12 inches (30.48 cm) in length but many are 3-5 inches (7.62-12.7 cm) long. They are observed crawling on, under and around coral rubble and other debris in rubble zones, but can also be found on sea fans and other gorgonians as well as the sea floor.
Bristle worms do not look threatening, but looks can be misleading. A bit of “pay heed” advice is warranted. Bristle worms are armed with numerous fine, fiberglass-like, venomous spines. Even with light handling, the spines in the tufts can easily puncture thick calluses. And that is not all. If you handle a bristle worm while wearing gloves you might think you got away without any harm. But beware here, as well. Your gloves will likely be covered with loose spines, and they can easily puncture and irritate your skin. Brush your face with your gloved hands and you will likely zap your face. Ouch!
More active at night, bristle worms are voracious feeders and they can often be seen crawling on the animals they eat. Their favorite prey include fire corals, some hard corals, some soft corals and sea anemones. Bristle worms are also known to scavenge upon dead animals.
A species known as the sponge worm is another free-living polychaete. Sponge worms are found on the toxic species of Caribbean sponge known as dread red or touch-me-not. It is wise to avoid touching any Caribbean sponge covered by numerous small worms.
A variety of species polychaete worms known as Palolo (pa-LOW-LOW) worms are noteworthy for their sex lives. It has been well-documented that in a reef-dwelling South Pacific species, eggs and sperm become concentrated in the tail segment of sexually mature adults. At dawn one week after the November full moon, the tails of these worms detach and float to the surface. As they surface, the tails break apart and fertilization of the eggs takes place. In many places, the density of tails, sperm and eggs is considerable and the surface becomes a thick soup. Many marine animals are drawn to the event to feed while South Pacific islanders collect the tails and surface soup for a feast.
Phylum: Ectoprocta (from Latin for “outside anus”)
Bryozoans: The Moss Animals
If you don’t believe you have ever heard the word “bryozoans,” you are not alone. Bryozoans are small, colonial invertebrates that often occur in small clumps attached to the substrate in reef communities, on the blades of many kelp plants and on sea grasses. The colonies sometimes catch our eyes because of their beautiful lace-like design and pastel colors, but they are not the attention-grabbing creatures that make it into the “dive-here-you-gotta-see-this” brochures.
But being comparatively small and obscure does not make bryozoans irrelevant. One species, Schizoporella floridana, literally built all of the land in south Florida with their calcium-based skeletal deposits.
Worldwide there are 4,000-5,000 spe-cies of bryozoans, but in most cases distinguishing between the species is a task best left to specialists. Many species have yet to be described or even given common names. In fact, I can recall only a few common names given to any species other than the general term, bryozoans.
Many species bear at least some resemblance to the plants we know as mosses. This should not come as much of a surprise given that the name bryozoans is derived from the Greek words for “moss” and “animals.”
Most bryozoans are lightly hued in shades of pink, tan, beige, brown, orange and white. The size of typical colonies ranges from only a few inches in diameter to several feet, but even comparatively large colonies are usually low profile. Many species bear strong visual resemblance to fine lace-like corals and some colonial worms which live inside of whitish self-made calcium-based tubes, and it is typical even for experienced divers to become confused when trying to identify them.
A single colony of bryozoans typically consists of thousands of individual animals that share a common skeletal network. Each animal in the colony is known as a zooid (ZOO-id). Despite their tiny size, zooids are well-developed and each individual has several intricate organ systems that accommodate respiration, circulation, digestion, elimination of wastes, muscle movement and input from their nervous system. Individual zooids in a colony are assigned tasks for the good of the whole. For example, some zooids are primarily responsible for defense. They are equipped with jaws used to bite intruding organisms.
Bryozoans are primarily filter feeders. They are equipped with a small, feather-like organ known as a lophophore (LOF-o-for). This organ traps tiny food particles as the animals filter the water.
Adult bryozoans are hermaphrodites. In some species sperm and eggs are produced at the same time, and in other species the cycles alternate. Some species guard fertilized eggs within a brooding area in the colony, while in other species free-swimming larvae are left to fend for themselves as part of the community of plankton.
The Importance of Bilateral Symmetry
The emergence of bilateral symmetry is significant to those who adhere to the Darwinian Theory of Evolution. Bilateral symmetry allows for the specialization of different body parts with regard to different body functions. As a result, bilaterally symmetrical animals can move from place to place more efficiently than animals such as anemones and coral polyps whose bodies display radial symmetry. Animals that display radial symmetry are generally more sedentary, moving only when they must for the sake of survival, or in the case of radially symmetrical animals like jellyfishes, these creatures simply end up where outside forces such as current and swell take them. Animals whose bodies display bilateral symmetry are far better at locomotion, seeking food, locating mates and avoiding predators.
Over the eons the number and complexity of specialized organs in animals that are bilaterally symmetrical continued to develop and increase. The central nervous system of these species became concentrated, large, longitudinal nerve cords. Nerve cells also became grouped, an evolutionary process that led to the development of the head and brain found in specimens described in the more advanced phyla. The list of bilaterally symmetrical animals includes some invertebrates and all vertebrates such as fishes, marine mammals, sea snakes and turtles.