SAND-BURROWING Amphipods-- A North American Success Story by  John Foster V17#3 UN

 

The amphipod crustaceans variously called sand-burrowers, beach fleas, or sand-hoppers represent several families. They all occupy the littoral zone, either burrowed in the sediment or nestling in the strand line among debris. It is the bur­rowing variety, the family Haustoriidae, which is seldom seen. A casual observer may pick up a clump of algae or displace a washed-up piece of wood and it may ex­plode with jumping arnphipods, probably Orchestia or Talorclrestia, but a beach­comber will require a scoop and a screen to see the haustoriids, or sand-  burrowers.

These interesting and well adapted crusta­ceans are found in sandy environments both on wave-pounded beaches and in quiet inlets and channels. People who stroll the sandy beaches of the Atlantic and Gulf coasts will be curious about the role, behavior, and diversity of these obscure crustaceans.

The haustoriids are small to medium sized arnphipods, usually one to ten millimeters long. The family gets its name from the Latin word meaning "drawer of water." The significance of their name will become clear as their feeding and bur­rowing habits are considered. Within the family, there are variations in form. One subfamily, comprised of more slender forms such as Ampliporeia are equipped with mouthparts best adapted to "sand licking," that is, grazing upon the organic coating of the sand grains which compose the sediment. In Amplriporeia, and in similar American genera like Pontnporeia and Barthyporeia, the first segment of the first antenna is pronounced in such n way as to give the appearance of a prow. The members of this subfamily also show physical differences between males and females. This is known as sexual di­morphism.

The focus of this article is the other side of the family, the subfamily known as Haustoriinae.  It is composed of a more specialized group of amphipods with broad bodies and strong appendages for burrowing. Richly setose and spiny mouthparts, adapted for filtering in­terstitial water for food particles, are also characteristic. The general body architec­ture creates what amounts to a three-sided tunnel. The body trunk, known as the pereon, is made up of seven segments. It forms the top of the tunnel.  Connected to the pereon and projecting downward are seven pereopods, or legs, with broad­ened, plate-like segments. They form the sides of the tunnel. The next three seg­ments, known as the pleon, are equipped with a pair of pleopods, or swimmerets, per segment. The movement of the pleo­pods establishes a forward directed cur­rent of water over the gills (attached to the top, inner surface of the legs) and toward the mouthparts. An opposite, or posteriorly directed current, may be set up by movement of the broad lobes of the antennae. The ultimate destination of the food-bearing water is the mouthframe, comprised of a pair of mandibles, two pairs of maxillae, and a pair of maxilli­peds. The food particles are trapped on the setae and spines of the various parts and transferred to the mouth.

While most of the haustoriins look superficially similar, small differences in the form and function of the appendages, especially the mouthparts, separate them into genera and species. There is a distinct relationship between the structures of a genera or species and its place in the habitat. Before examining some of the various forms, a review of the difficult habitat of these crustaceans will be useful.

Animals dwelling in the sandy beach habitat require a high degree of adapta­tion for survival. The environment is usually unstable, depending on the loca­tion and wave-energy. It is usually highly oxygenated and comparatively silt free. Amphipod inhabitants have to cope with, in addition to the violence of wave stress, the grinding, abrasive action of moving sand, potentially rapid changes in tem­perature, salinity, and exposure, and the absence of any solid substrate around which to seek refuge. On the plus side, in addition to the presence of oxygen, there is a large food supply which is con­stantly replenished by rising tides and wave wash. Further positive points can be made for the intertidal sand habitat. While to us it seems uniform and some­times barren and hostile, to marine life it offers a wide variety of ecological niches with enough diversity to allow several sympatric species (those living in the same habitat) to coexist. The reverse beauty of the situation is that the various haustoriins occupying the habitat, although structur­ally similar, are sufficiently diverse to par­tition the environment to meet the needs of their own niche.

Evidence for evolutionary changes in the sand-burrowers is found initially in their truncate, broadly fusiform bodies, widened side plates, and powerful pereo­pods and pleopods for burrowing and pumping water. The most significant adaptation in structure which allows them to divide the resources of the habitat, and coexist, is found in the mouthparts. Dif­ferences in mouthparts reflect differences in ecological station. Whether a group is intertidal or subtidal (aquatic) depends, to a large degree, to the adaptation of their mouthparts for filter feeding.

Sand-burrowing amphipods may be generally divided into intertidal and sub­tidal forms. The haustoriins with the least modification of their mouthparts for filter feeding are subtidal. Examples are the genera Pseudohaustorius and Protohausorius. These, and others which occupy the subtidal zone exclusively, may be

found along high energy, wave-exposed beaches or shelly to muddy, partially sheltered flats, from the lower intertidal zone to depth of six or seven meters. They feed on organic materials extracted from the water along with microorganisms and small invertebrates. Both genera listed above are considered primitive, that is, they arc closest to the stock from which all other North American haustoriin genera have radiated.

A much more specialized group, using a habitat not suitable to less adapted groups, is comprised of the intertidal forms such as Neohaustorius, Haustorius, and L.epidactylus. They use the interstitial water to its maximum potential. The in­tertidal forms occupy the coarsest and least silty sands. They feed quite efficient­ly on the organic food in the water which is constantly renewed through tidal and wave activity. Neohaustorius is a genus which has not been collected to date in the Gulf of Mexico. The best known member of this genus is Neohaustorius schmiizi which has been studied extensively in North Carolina. This species does not at­tain the population densities of its relative Haustorius. It is normally found in den­sities about 100 individuals per square meter and occurs from Canada to the Atlantic coast of North Florida. Neohaus­torius schmitzi feeds on detritus and the bacterial film on the sand grains. Gut analyses have revealed small amounts of fine sand and diatoms in these amphipods. The diet of fine sand allows this species to obtain adequate nutrition because of the higher level of bacterial growth present. Haustorius, on the other hand, which oc­cupies the same habitat, includes minute interstitial invertebrates such as poly­chaete worms, nematodes, and copepods in its diet. It does so because its particular niche is found in coarser sands with generally less bacterial growth. Both species are high tide feeders. Croker, who did much of the early work on the diets of haustoriids, observed that within two to four hours after the sand was exposed by low tides, most animals examined had empty guts.

 

Lepidactylus appears to prefer quieter beaches. It is found in inlets, bays, and estuaries. In the Gulf of Mexico, this genus is very abundant in summer, at times occurring in densities of 1500 in­dividuals per square meter of sediment. A genus which also occurs in normally quiet, sheltered waters, in habitats similar

to that of Lepidactylus, is Acanthohaustorius.  It is so named because of the strong spine located on the last segment of the pleosome. It occurs, as at least eight separate species, from Maine to Texas in the midtide zone to shallow water. The  greatest concentrations  are found about  the mean low water level. In St. Andrew Bay, Florida, this genus is found on the sandy bayshore, sometimes in areas vege­tated with seagrasses .   In winter, when bay water temperatures drop into the 50°F range, Acanrhohaustorius is easily col­lected in the wet sand just at the low tide line or in very shallow water. In summer, when the surface water temperatures arc in the 80°F range, one must collect in waters one to four meters deep in order to locate any sizeable numbers. Acanrhohaustorius  apparently migrates with the seasons, at least in St. Andrew Bay.  Other members of the genus have been collected in deep waters (40 meters) off Long Is­land, in shell hash sands off the east coast of Florida, and in coarse coralline sands in the Florida Keys.

 

How can these various amphipods, which appear to use the same resources, occur sympatrically in the sandy beach habitat. The answer is complex, but to simplify, it can be said that they each oc­cupy a distinctive niche. An ecological niche is, simply stated, a particular com­bination of physical habitat character­istics, community roles, and behaviors which make the species distinctive within the ecological community. When similar species occur together, it must be assumed that there is a zone of non-overlap in their individual niches. There arc two proper­ties which allow this coexistence. First, mechanisms must exist which insure re­productive isolation. These may include spatial separation, habitat isolation, sea­sonal breeding isolation, and barriers to cross fertilzation. Secondly, the ability must exist to use the same resource as another species in a distinctive way. The haustoriids described above have accom­plished this adaptation, in part, through staggered breeding seasons. The subtidal species tend to breed in late winter and early spring while the intertidal species breed in late spring and early summer. In this formula, the young do not have to compete for food and space. Additionally, as we have seen, the haustoriins have divided the habitat by using different food resources and feeding strategies.

The success of American haustoriins is borne out by the fact that the North Amer­ican Atlantic coast possesses most of the known genera and the greatest number of endemic genera and species of the sub­family Haustoriinac. Dr. E. L. Bousfield of Canada, a world authority on haus­toriids, has pointed out in a 1970 paper on the evolution of the group, that the ability of the subfamily Haustoriinae to inhabit more landward and more estuarine areas relates to the increasing specializa­tion and efficiency of its filter feeding ap­paratus and its reproductive cycle, these features limit competition among the young of the sympatric family members, a process known as adaptive radiation. It occurs when the offspring of some com­mon ancestor, which has moved into a new environment, undergoes rapid evolu­tionary changes in the form and function of the body necessary to meet the demands of the new environment.

The sand-burrowers, while perhaps mundane to some, provide us with art evolutionary success story. The descend­ants of the ancestral stock haustoriin, thought to be closely related to Protohaurstorius, have become altered and bet­ter adapted to the process of expanding geographically. This expansion, which has been from the North Atlantic to the Gulf of Mexico, also includes the open­ing of new habitats, from wave-pounded outer beaches to quiet, estuarine bay­shores. Natural history has many glaring examples of short-lived, but flamboyant animal groups without the resilience of these adaptable, obscure sand-dwellers.

 

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                                                                      Haustorius               Acanrhohausrorious

9. Sand-Burrowing Amphipods        

1.  Where are amphipods found?

2.  Where would one find haustoriidi?

3.  How large are the haustoiids?

4.  What is “sand lickng”?

5.  Describe the pereon of sub-family Haustoriinae?

6.  How do the pleopods function?

7. Why are amphipod inhabitants of sandy beaches?

8.  How is the food supply replinished?

9. What is meant by “sympatric species”?

10.  Why would the mouth parts determine the ecology of these animals?

11. On what does Neohaustorius schmitzi feed?

12.  How are breeding seasons different in subtidal and intertidal species?

13.  What is an ecological niche?

14.  What did the increase in the efficiency filter feeding and reproduction of Haustoriinae bring about?Sand-