Adaptations to sessility
Sessile organisms are those that spend their lives attached to the substrate. The most familiar examples to humans are vascular plants. Although sessile animals are exclusively aquatic, the basic issues confronting land plants will help us orient to their biology by analogy. Plants:
- exploit a diffuse, widely distributed food source (sunlight)
- can change their posture to optimize food capture, but are unable to move about
- must be able to exploit food and respond to challenges coming from any direction
- must compete with others for access to food primarily by means of growth strategies
- use passive and chemical defenses against those that would eat them
- employ a variety of strategies to attach to the substrate.
The issues confronting sessile animals are fundamentally similar, except that:
- they live in water
- food typically consists of particulate matter or small prey items
- mobility can be either limited or impossible.
Caveat: No animal is completely, absolutely sessile. Even those that are permanently attached to the substrate as adults have some sort of free-swimming or planktonic (drifting) larval stage. For many, the shades of gray increase.
Thus, for our purposes, a sessile animal is one that spends the majority of its adult life attached to or rooted in the substrate and unable to move around. This definition excludes creatures like crinoids and most sea anemones, but includes:
- Some adults are capable of moving in emergencies but typically don't.
- Some motile suspension-feeders animals don't move much if they have found a good place to feed, but may crawl or swim to good feeding cites on a daily basis.
This life-style has one distinct advantages:
- Low energy requirements. A sessile creature can subsist on relatively small amounts of food, especially if its diet is supplemented by zooxanthellae (photosynthesizing symbionts). Nevertheless, sessile organisms tend to have:
- Low metabolic rates
- Simple body plans (either ancestrally, as in corals, or secondarily, and in bryozoans)
Surmounting the general challenges listed above has stimulated the repeated evolution of a surprisingly uniform suite of adaptations:
Radial symmetry: Sessile organisms need to be able to capture food and respond to stimuli, regardless of the direction from which they come. Thus, they tend to display radial symmetry. In the case of cnidarians, this is a plesiomorphic (ancestral) trait. In many others, it is secondarily imposed over a fundamentally bilaterally symmetrical body plan, as in the suspension feeding "Christmas-tree worm" annelids at left. In most cases with bilaterians, the feeding organ is superficially radial, even when the rest of the critter isn't. (E.G. bryozoans and tunicates.) The fossil record, however, contains sessile bilaterians whose entire bodies were superficially radial. E.G. Edrioasteroids - Paleozoic sessile echinoderms.
U-shaped guts: Bilaterians must approximate radial symmetry by having a U-shaped gut (see the bryozoan schematic above) or secondarily evolving a blind-ended gut (as many brachiopods have.)
Exceptions: Not all sessile bilaterians are secondarily radial, however. Brachiopods retain strong bilateral symmetry, even in their feeding organs. In phoronids, their soft-bodied relatives, the feeding organ, the lophophore is exposed. It is covered with cilia. Food particles intercepted by the lophophore tentacles are moved to the mouth by ciliary action. In brachiopods, the lophophore is enclosed between upper and lower valves. These control the flow of the ciliary current across the lophophore, eliminating the need to be able to capture food coming from any direction.
- Gas exchange: Most sessile organisms are small enough to depend strictly on simple diffusion to exchange gasses with the environment. Even in relatively large (cantaloupe-sized) cnidarians, most of the body's living tissue is concentrated in thin layers of endoderm and ectoderm. Scaling these creatures isometrically quickly brings them up against the limits of simple diffusion.
- Feeding: As a sessile organism scales up, its volume scales faster than its food collection surfaces, again imposing an upper size limit. In cnidarians, the gut is infolded into a series of mesentaries, to increase its surface area, an adaptation frequently reflected in their skeletons.
Coloniality: For most, the solution to scaling issues is coloniality, in which an original zooid arises from parental gametes then buds off a series of clones. In colonial corals, bryozoans, and tunicates, individuals are connected by strands or sheets of living material, through which they share nutrients and nerve impulses. The colony's shape is independent of that of the individuals, and is relatively free of their scaling constraints. Additionally, it can vary in shape ecophenotypically, with different morphs for high and low energy regimes.
Cone shells: We've previously addressed the convergent evolution of cone-shaped armored skeletons. In addition to what's shown below, many sponges, including archeocyathids also conform to this pattern.
It's worth noting that these creatures have converged on this shell shape despite great differences in their feeding appendages.
One probable advantage of this growth habit: Individuals can gain a foothold in a small piece of substrate when they are young, then expand to occupy more of it.
- The coral is an ambush predator/suspension feeder using tentacles armed with nematocyst (stinging cells) (and probably zooxanthellae)
- The brachiopod uses ciliary action of a lophophore
- The bivalve engulfs particles through a fleshy siphon and uses small labial palps (tentacles) to remove edible bits.
Plicae: In many bivalved organisms, most notably brachiopods, the commisure at which the valves meet forms a series of zig-zag plicae. Functions include:
Positioning and attachment: Position has two crucial aspects:
Typically, sessile organisms have to attach to rigid substrates. This limits their feeding to within a few cm. of the bottom, and excludes them from soft (sandy or muddy) substrates. E.G.: brachiopods typically attach to the substrate using a very short fleshy stalk, the pedicle.
Orientation: Using the pedicle, it can adjust its orientation to optimize suspension feeding. Its feeding, however, is limited to within 2 - 3 cm of the substrate, and the substrate must be rigid.
- Location: Some brachiopods overcame this limitation by attaching to larger suspension-feeding organisms like stalked echinoderms, either with their pedicle (in which case they could still control their orientation somewhat) or using spines on their valves (in which case they depended on the substrate-host to orient them properly).
- Others pioneered soft substrates, rafting on them by means of deep globular morphologies or the support of elongate spines.
The utility of such strategies depended on the energy of the environment.
- Moderate energy: E.G. a reef environment below low tide line. A direct pedicle or spiny attachment to a rigid substrate or host.
- Low energy only: E.G. a sandy bottom below wave base. Rafting on soft sediment. (Even moderate energy here would bury the rafter.)
- High energy: E.G. Intertidal zone. Here, sessile creatures must be firmly attached to a hard substrate, as in brachiopods cemented to scallop.
These adaptations are not mutually exclusive.
NOTE: The creature in this example is not a brachiopod, but a bivalve mollusk close to the ancestry of oysters. Interestingly, oysters and their kin have convergently evolved many of the adaptations of rafting brachiopods, but from a radically different origin. Whereas the rafting brachiopods derived from sessile, attached forms, oysters are descended from fully motile, infaunal burrowers.
Finally, sessile organisms are among the great shapers of the marine realm, acting collective as makers of reefs - interlocking frameworks of the remains of marine organisms that stand out topographically from their surroundings. Many different organisms contribute, and over the Phanerozoic, the cast of reef builders has changed, but always, the frame builders are sessile.