March 2011 Christina Ball Royal BC Museum Phil Lambert GLOSSARY FOR THE ECHINOIDEA The Echinoidea, similar to other echinoderm groups, have an ancient lineage dating back approximately 500 million years and includes the sea urchins, sand dollars and heart urchins. Today this globally distributed group comprises approximately 1000 species (Pearse et al. 2007). Nine species are known to occur in British Columbia (Lambert and Boutillier in press). Echinoids are found exclusively in the marine environment from the intertidal down into deep water and can be found on rocky, sandy or muddy substrates (Brusca and Brusca 1990). The echinoids have a variety of body shapes ranging from disc-like sand dollars to pyramidal deepwater species. While their morphology of the echinoids varies, the group shares several other characteristics. The echinoids have mutable collagenous tissue and a water vascular system. They also have a hard endoskeleton, called a test, that is made up of interlocking plates formed from fused ossicles. They have spines and pincher- like pedicellariae that attach to the outer surface of the test and a complex jaw structure called an Aristotle’s Lantern (Lambert and Austin 2007). There is also considerable colour variation within the Echinoidea. While colour can be a useful method for identifying otherwise similar species it is important to recognize that colour is a subjective trait. There can also be considerable colour variation within a species. Like all echinoderms the echinoids posses a unique tissue type called mutable collagenous tissue. This tissue can change rapidly, in less then a second to several minutes, from a rigid to a flaccid state. The change in physical state, from hard to soft or vice versa is not the result of muscle activation. Rather it is the result of chemical changes to the passive mechanical properties of the tissue itself. The current consensus is that in essence, mutable collagenous tissue is made up of discrete collagen fibrils that are organized into bundles (Wilkie 2002). Substances released by the juxtaligamental cells change the cohesion between the bundles causing the tissue to either harden or soften (Wilkie 2002). These changes are under the control of the nervous system. The water vascular system is a complex network of canals and reservoirs that uses hydraulic pressure and muscular action to operate the podia (Brusca and Brusca 1990). In echinoids the podia, or tube feet, are primarily used for locomotion, attachment and gas exchange (Brusca and Brusca 1990; Lambert and Austin 2007). The water vascular system opens to the surrounding sea water via a plate called the madreporite. The echinoids have a fairly obvious madreporite on the aboral surface. In all types of echinoid the hard test is formed from a series of interlocking plates that are themselves formed from fused calcareous ossicles. The two types of plates, ambulacral and interambulacral, run in longitudinal rows from the mouth to the anus. Rows of ambulacral plates alternate with rows of interambulacral plates. The ambulacral plates have small holes (pores) through which the tube feet extend (Brusca and Brusca 1990; Lambert and Austin 2007). The pores occur in pairs and the number and arrangement of the pore pairs is important for identifying specimens (figure 1). 1 March 2011 Christina Ball Royal BC Museum Phil Lambert On the outside of the test are tubercles to which the spines attach. At the center of the tubercle is a ball-like mamelon that is the articulation point for the spine. Mamelons may be either perforate (i.e. have a small hole in the middle) or not. The neck of the mamelon is either straight or undercut (Lambert and Austin 2007). A parapet surrounds the mamelon and can be either crenulated (i.e. wavy like a crown) or smooth and flat. Muscles surrounding each mamelon attach the spine to the test and control its movements. Both the tubercles and spines can be useful when identifying specimens. In cross section the spines are made up of a series of wedges (figure 1), the number of which is characteristic of a particular species. a b Figure 1. The arrangement of pore pairs in Strongylocentrotus franciscanus (a), and a cross section of a primary spine of S. fragilis showing the spine wedges (b. The echinoids also have pedicellariae, which attach to small tubercles on the outer surface of the test (Lambert and Austin 2007). The pedicellariae are used in defense, to clean debris off the surface of the test and to capture prey (Brusca and Brusca 1990; Lambert and Austin 2007). Pedicellariae are pincher- or jaw-like structures on flexible stalks (figure 2). There are three types of pedicellariae (ophicephalous, tridentate and globiferous, figure 2) and any given species might have two or more types. Within the different types of pedicellariae the shape of the three-part jaw and stalk can vary (Lambert and Austin 2007). The shape of the pedicellariae and its stalk can be important identifying features for some echinoids. Figure 2. Three types of echinoid pedicellariae: ophicephalous from Strongylocentrotus droebachiensis (a), tridentate from S. droebachiensis (b), tridentate from S. franciscanus (c), and globiferous from S. droebachiensis (d). Parts of a pedicellaria (e). 2 March 2011 Christina Ball Royal BC Museum Phil Lambert The mouth is located on the side of the test facing the substrate. The anus is located opposite the mouth in the urchins, but shifted posteriorly in the sand dollars and heart urchins (Pearse et al. 2007). The area surrounding the anus is termed the periproct. All of the Echinoidea have a complex jaw structure called an Aristotle’s Lantern. It is shaped like an old fashioned lantern and is made up of five teeth, muscles and levers. The ends of the teeth protrude from the mouth (Brusca and Brusca 1990; Lambert and Austin 2007; Pearse et al. 2007). The echinoids use these teeth to cut up food, rasp algae off of rocks, and to build burrows. a b c Figure 3. Morphology of a typical sand dollar (a), heart urchin (b), and sea urchin (c). Most of the echinoids have two separate, though outwardly indistinct, sexes (Brusca and Brusca 1990; Lambert and Austin 2007). The regular echinoids have five gonads each leading into its own gonoduct. The irregular echinoids (e.g. sand dollars and heart urchins) have four gonads and a corresponding number of gonoducts (Brusca and Brusca 1990). Each gonoduct leads to a gonopore (opening) on the genital plate. The genital plates are located on the aboral surface near the anus (Lambert and Austin 2007). Eggs and sperm are released from the gonads and exit through the gonopores into the water where the sperm fertilize the eggs (Brusca and Brusca 1990; Lambert and Austin 2007). While in the water column the embryo develops into a bilaterally symmetrical larva. After 4-6 weeks the larva undergoes metamorphosis into a tiny juvenile urchin and settles onto suitable habitat (Lambert and Austin 2007). As an 3 March 2011 Christina Ball Royal BC Museum Phil Lambert alternative strategy a few echinoids carry and brood their young between their spines (Brusca and Brusca 1990). The Echinoidea inhabit a range of trophic niches. Most urchins are omnivorous grazers and scavengers that scrape algae and encrusting animals off hard surfaces. For example the familiar Purple Urchin consumes kelp and algae as well as encrusting animals and detritus (Lambert and Austin 2007). Some species use their teeth to build burrows in hard substrates. The animals scrape algae off the sides of the burrow or catch algae that drifts into the burrow (Brusca and Brusca 1990). Most of the irregular echinoids are detritivores. Some irregular urchins burrow into soft sediment and feed on organic particles. These species usually use their tube feet to search the sediment for food particles that they then pass to their mouths (Durham et al. 1980; Lambert and Austin 2007). Sand dollars are characterized by a highly modified Aristotle’s Lantern with non-protractible teeth (Lambert and Austin 2007). Most sand dollars are detritivores and particulate feeders that pick-up food particles from the substrate and pass them with their tube feet to the mouth (Telford and Mooi 1996). While most sand dollars are detritivores and particulate feeders, a few suspension feeders exist. For example Dendraster excentricus, a common sand dollar in BC, catches diatoms and other small food particles with its tube feet and then passes the food to the mouth (Francisco and Herka 2010). It can also use its pedicellariae to catch tiny swimming crustaceans (Brusca and Brusca 1990). In calmer waters this species is usually found with the anterior portion of the body buried in the substrate and the rest of the body protruding out of the substrate at an angle. In rougher waters the species tend to lie flat on the ground (Durham et al. 1980; Francisco and Herka 2010). In turn echinoids are predated upon by sea otters, crows, gulls, large fish, sea stars and large crabs (Lambert and Austin 2007; Snellen et al. 2007). Humans are among the most effective predators of sea urchins and can dramatically decrease their populations (Pfister and Bradbury 1996; Lambert and Austin 2007). Some small animals are parasites on urchins. For example parasitic copepods use their mouthparts to pierce the skin and suck out body fluids (Lambert and Austin 2007). Other animals, such as tube worms, scale worms, ostracods and small shrimp often hiding among the spines of urchins. The echinoids exhibit some interesting behaviour. Many urchins use their tube feet to hold small stones, shells and other objects on top of themselves. The exact reason for this behaviour is uncertain, but it is thought that it could confer several benefits (Lambert and Austin 2007).