Meiofauna at Eagle Cove

The Meiofauna of Eagle Cove

Looking west toward the sandy beach of Eagle Cove

Looking east from the sandy beach to the rocky outcrop

Meiofauna: Introduction and Methods

At first glance, one might view a sandy beach as a barren and depauperate habitat. Such a view is undoubtedly biased by the scale at which humans operate. Small organisms living in these habitats are ubiquitous and diverse, but they are easily overlooked and therefore poorly appreciated. Community interactions that take place between grains of sand are less familiar and understandable than, for example, interactions in exposed parts of the intertidal. There is, in fact, no guide to the meiofauna of the Pacific coast. These animals are, however, dynamic and biologically interesting members of the Eagle Cove marine assemblage, and as alien in appearance and as quirky in behavior as anything that walks on land or does bellyflops in the harbor for whale-watching tourists.

Our team sampled sand from the beach using a small hollow cylinder core, outfitted with a plug at the top to prevent loss of sediment when the core was removed. We concentrated on sand at depths less than 12 cm. The small animals that live here cling tightly to it for protection and to avoid being washed away as the beach drains. To induce the animals to release from the sand grains, we mixed each sediment sample 1:1 with 1M MgC1₂, gave the capped container a hard shake, and decanted the suspended animals onto a filter. Each sample was then examined under a dissecting microscope, and animals were transfered with a small pipette to slides. Cover slips with clay feet were used to avoid crushing animals, and the animals were observed under a compound light microscope.

The sandy beach at Eagle Cove is inundated daily with water as the tide washes in, and is exposed as the tide moves out. As a result of direct solar exposure, a temperature gradient may develop as a function of depth into the sand. This gradient can become steeper as the sand remains exposed, though only on days with significant sun exposure (compare maximum temperatures on July 3rd vs. July 4th). Although time did not permit detailed sampling of the meiofauna, we hypothesize that different organisms will be found at different depths and at different times of day, depending on changes in physical conditions.

temperature fluctuation at different depths in the sand over 72 h

Is this a young sea cucumber? Its body wall is ciliated, and it also appears to have podia. Though appearances can be deceiving, especially in the world of larvae and juvenile invertebrates, this animal looks rather like a tiny Parastichopus, the large, rubbery, red sea cucumber.

Phylum Nematoda Nematodes (from the Greek "nema," or thread) are long thread-shaped worms with a characteristic sinusoidal wiggling motion. Nematodes are one of the few worm-shaped animals that lack circular muscles to constrict the body--hence, their only form of locomotion involves wriggling, a mechanism that works well for for moving between sand grains. At Eagle Cove, nematodes eclipsed all other animals in abundance--we had to bypass hundreds to find many of the other animal examples shown below.
Phylum Annelida: polychaete worms Annelid worms were also present in the sediment in relatively large numbers. These small worms are segmented, and equipped with parapodia which they use to swim, burrow, and crawl.
Kingdom Stramenopila : diatoms Living diatoms are very numerous in the sediments. They move around readily through the use of flagellae. Diatoms are encased in a test made of silica, carbonate, or protein. These animals are unicellular, and biflagellate at some point in their life. The tests have beautiful elaborations, which are better seen with something other than a simple light microscope.
Copepoda (Arthropoda: Crustacea: Maxillopoda) Copepods are one of the most ubiquitous forms of animal life in the open ocean, but they are also a common component of meiofaunal communities. Whereas copepods that dominate planktonic communities have long antennae used in swimming, meiofaunal copepods have shorter antennae and tend to move or crawl using shorter appendages.
Phylum Gastrotricha Gastrotrichs are all small and mostly meiofaunal. They may bear cuticular spines and always have a large field of cilia on the ventral surfaces (hence, the root of their name, from the Greek for "hairy-belly.") Like many interstitial organisms, gastrotrichs have a "duo-gland" adhesive system that allows them to temporarily fasten to sand grains using a cement gland, and then unfasten using a gland the produces the cement solvent. The adhesive glands are located in a pair of "toes" at the posterior end of the animal.

Mystery Animal #1:	Is this a young sea cucumber? Its body wall is ciliated, and it also appears to have podia. Though appearances can be deceiving, especially in the world of larvae and juvenile invertebrates, this animal looks rather like a tiny Parastichopus, the large, rubbery, red sea cucumber.
Mystery Animal #2:	Is this the same animal as Mystery Animal #1? While it is ciliated, it does not look like it has podia. The gut, or something large and gut-like is clearly visible in this strange creature. Another vermiform thing. The animal kingdom abounds with vermiform animals.
Mystery Animal #3:	This mystery animal bears cilia at both ends, which it uses to generate an alarming and visible current through the slit in the body wall on what is the upper portion of the screen. The animal is not moving - the changes result from focusing up and down to clarify which end is the head end
Mystery Animal #4:	Mystery animal 4 is another ciliated creature, shaped somewhat like an errant toucan head. It might be some kind of protist or body part, which does not narrow things down at all. It was very large and moved quickly and effectively using its cilia.
Mystery Animal #5:	Our final mystery animal is also vermiform. This particular worm looks unsegmented, though it may be an oligochaete. It may also be parasitic, judging by the sucker-like structure on the animal's dorsal (or ventral?) surface.