Sand dollars are flat and burrowing marine animals that belong to the radially symmetrical class of echinoderms (spiny-skinned creatures) along with sea stars, basket stars, sea cucumbers and sea urchins.
They mostly eat plankton, algae, diatoms and kelp as well as a mix of dead particulate organic material called detritus. Their cilia enable many different motions, including feeding, swimming on their side to catch organisms carried by the ocean current and eating sediments to help weight them down.
Their mouths are named Aristotle’s lantern because they have five rows of “teeth” that they use to catch and chomp their food before swallowing it.
Their body is round and they have tube feet that help them move and feed. On their aboral side they have a symmetrical pattern that looks like a flower with five petals.
Crustacean Larvae
Sand dollars are echinoderms (a group of sea shells and worms) that are specialized for living as benthic (bottom-dwelling) adults. They reproduce by releasing sperm and eggs into the water at the same time during spawning periods. This strategy helps sand dollars survive since it increases the number of embryos produced and decreases predation risk.
Sand dollar larvae have spiny skin and tough bristles on their body to protect them from predators. They also use cilia to filter plankton from the ocean, which is how they get food.
Most crustaceans have a number of larval stages that differ from their adult form. These stages can be either “direct developers” that emerge from their egg fully formed and only grow as they mature, or they can exhibit “anamorphic growth,” which means that the individual grows in different ways each time it molts.
The first stage of development in all crustaceans is the nauplius, which appears inside the egg. In this stage, the body is oval in shape and divisible into three regions: head, trunk and anal region. A median frontal eye is present in this phase.
This nauplius stage is similar to the adult in appearance but differs in its number of pairs of jointed appendages. The first pair of appendages is uniramous and is present in the head region, while the second and third pairs are biramous and present at the posterior end of the nauplius.
This stage is present in Cladocera, Ostracoda and Cirripedia. It differs from zoea in the presence of a median frontal eye, the presence of a pair of bivalved shells and the absence of an alimentary canal. The nauplii is the earliest stage of nauplius in all crustaceans and is thought to represent the ancestral form of Crustacea.
Plankton
Plankton are marine organisms that float or drift passively in the water, often carried by currents. They include bacteria, archaea, algae and other microscopic animals that are not able to swim. They are also a key component of the ocean’s food web and are important for the health of ocean ecosystems.
Phytoplankton (also called phytos) are the primary source of nutrients for other marine organisms, including fish and whales. They provide essential fatty acids, vitamins and amino acids. These nutrients are vital to their metabolic functions, allowing them to grow and thrive in the sea.
They are composed of a variety of organisms that live on the surface of the ocean, as well as in the water column. The most common type of phytoplankton is a photosynthetic algae, like cyanobacteria and diatoms.
These photosynthetic plants help the ocean stay clean by filtering the excess chemicals out of the water. They also help control the amount of sunlight that reaches the ocean’s surface, which in turn helps to keep it cool.
However, too much phytoplankton in an area can cause a bloom, which can harm fish and other marine organisms. This can be caused by a number of factors, including climate change and pollution.
Sand dollars are flat invertebrates that belong to the phylum Echinodermata, or spiny-skinned creatures. They are closely related to sea urchins, sea cucumbers and starfish.
They are covered with a curved spine with a projection of “hairs” called cilia that aid their feeding by moving food particles along their body surfaces to a central mouth under the sand dollar. They are omnivorous, eating crustacean larvae, tiny copepods like plankton, diatoms, algae and kelp. They can take up to 48 hours to digest their food, and they expel the byproducts back into the ocean.
Diatoms
Sand dollars have a unique way of eating – they use spines, and cilia on their body surfaces, to move food particles along the bottom side of their bodies to their central mouth. This allows them to eat a variety of different items, including crustacean larvae, tiny copepods like plankton, diatoms, algae, and detritus (dead particulate organic matter).
The diet of sand dollars varies depending on where they are located in the ocean. They primarily eat the larvae of other bottom-dwellers, but also consume deposited and suspended food particles as they travel along the surface of the water.
When conditions are favourable, diatom cells thrive, growing as they receive nutrients from the surface. However, as nutrients begin to decline along nutrient gradients, the majority of cells sink and exit the upper mixed layer of plankton (“bust”). This sinking occurs in response to several factors including loss of buoyancy control, synthesis of mucilage that sticks diatoms together, or production of heavy resting spores.
After sinking, diatoms can either remain on the seafloor as refuge populations or return to the upper mixed layer when conditions become favourable again. When they do, they often form a colony in which they live and grow with other organisms, including other algae.
These colonies of diatoms are called frustules, and they can be highly patterned with pores, ribs, minute spines, marginal ridges and elevations. They are able to be preserved for long periods of time in sediments and can help us to track changes in lake systems over time.
Diatoms are incredibly diverse and complex, with many species having specific tolerances for various environmental variables including pH, nutrient concentration, dissolved oxygen, suspended sediment, flow regime and elevation. This makes them vital to the assessment and monitoring of biotic condition in waters.
Algae
Algae are a diverse group of simple plants that can be found in freshwater and marine environments throughout British Columbia. They grow in association with plants, fungi (as lichens) and animals such as corals and provide important ecosystem functions, including supporting fisheries as the base of the aquatic food chain and supplying much of the oxygen we breathe.
Many algae are photoautotrophs. This means they rely on sunlight and other nutrients for their own nourishment. These include things like omega-3 fatty acids, iron and vitamin K.
Other algae, however, are heterotrophs. They must obtain their nutrients from outside sources, which includes things like bacteria, dissolved chemicals and organic matter. They do this through a variety of strategies, including osmotrophy and phagotrophy.
Sand dollars are filter feeders that use their tube feet and spines to sift for phytoplankton, algae and diatoms to eat. They also catch plankton-like crustacean larvae and small copepods with cilia on their spines.
They eat these organisms through their mouth on the bottom side of their bodies. They have five teethlike sections on their jaws that help them grind up food to be swallowed.
Their skeletons are made of calcium carbonate, which helps them protect themselves from predators. The exoskeleton is often encased in a skin of keratinous spines that look and feel like velvet.
These velvety spines can help sand dollars stay attached to the sea floor and move across it. They can also be used to protect them from ocean waves.
They can also help sand dollars survive the ocean acidification that is occurring due to the increased amount of carbon dioxide dissolved in the water. Fortunately, they are not endangered.
Kelp
Sand dollars are one of the most intriguing sea creatures. Their bodies are very different from those of other echinoderms, including sea urchins, sea lilies and sea cucumbers.
Sand dollar bodies have five paired rows of pores, which are perforations in the endoskeleton through which podia for gas exchange project from the body. The podia are used to move sand dollars from place to place, as well as to breathe on the sea floor.
Another interesting characteristic of the bodies of sand dollars is their petal-like pattern, which consists of five paired rows of holes in the rigid flat disc body that look like small spines. This is called a petaloid and it acts as an ambulacrum, allowing sand dollars to move and feed from place to place.
In addition to eating plankton and microalgae, sand dollars are also able to eat dead organic matter that has been deposited on the ocean bottom, such as detritus from previous marine life forms. The sand dollars would reposition themselves to access this food as the ocean current moved it from point to point.
If they find a location with plenty of food, they may stick together in groups of hundreds. This is thought to be a way for them to make sure they get the most nutritious food before moving on to a better spot.
Kelp forests are found along much of the west coast of North America, and their presence is vital for the survival of fish, invertebrates, and many other species. Unfortunately, in recent years a double whammy has struck the ecosystems of Northern California and the West Coast: climate change has forced cold, nutrient-rich water off the coast, causing bull kelp to die, and overhunting has caused sea urchin populations to expand rapidly, destroying kelp forests.
