Robert Jan "Roy" van de Hoek, President
Ballona Institute
Los Angeles, California
June 17, 2014

Roy van de Hoek

          The research of Ross Pohlo 50 years ago in 1964 was focused on California seashore wetland soils between Pacific tides at places such as Tomales Bay, Bodega Bay, Mugu Lagoon, Newport Bay, Los Angeles beaches, San Diego beaches, and Estero Beach near Ensenada at Estero del Punta Banda. Interestingly, Estero Beach is a location that has been visited by distinguished line of marine biologists and naturalists including Charles Orcutt, Edward Ricketts, George MacGinitite, and Earl Segal. I had the good fortune visit Estero Beach as a student at CSUN in my marine biology courses. In my class at CSUN with Ross Pohlo called Marine Ecology class, we also had field trips to Malibu, Jalama Beach just north of Point Conception, and to Baja California, where we traveled to San Felipe and south to Coloradito on the shores of northern Gulf of California.
          Interestingly, Ross Pohlo recognized the substrate of intertidal mudflat, whether sandy or clayey, as soil which is very prescient and accurate for a scientist to have recognized. For example, botanists correctly recognize wetland substrate and salt marsh substrate as a soil with differing amounts of moisture but still exposed to the air directly with out water covering the surface for a portion of time either daily or seasonally. Thus intertidal mudflats which are exposed to direct air twice a day at low tide are indeed soils. I counted three times in this article that Ross used the word "soil" as a noun and one time he used "soil surface" where soil served as an adjective. Two additional times, "soil" was used as an adjective with a hyphen involved as in "water-soil interface" so that the total number times that the word "soil" was used in the article was 6 times on two pages. And the last word of the article by Ross is soil.
          There are a number of additional interesting features in the article by Ross. For example, Ross cites two European marine biologists that studied clams on the coast of mainland Europe. One of them is from the Netherlands named Ingovar Kristensen (1958), who studied the ecology of a cockle from the Dutch Wadden Sea. And the other scientist is from Germany named J. Jordan (1915). In addition, the research by Ross Pohlo was funded by two National Science Foundation grants (B-8755 and G-18767). The grant (B-8755) was a grant that was initially used to assist Ross 4 years earlier in 1960 in his doctoral research when he visited California from Chicago, in order to study clam ecology from a base of operations at Dillon Beach Marine Station of the University of the Pacific. While staying here, Ross Pohlo met the ditinguished marine biologist and oceanographer named Joel Hedgpeth, who was the Director of the Dillon Beach Marine Station. Dillon Beach is located near the north entrance to Tomales Bay. So Ross studied various species of clams in the soil of the intertidal mudflat at Tomales Bay, but also just to north at Bodega Bay, and in southern California at Mugu Lagoon.
MALACOLOGIA, Volume 1, Number 3: Pages 321-330

Ontogenetic Changes Of Form And Mode Of Life In Tresus nuttalli (Bivalvia: Mactridae)
Department of Biology
San Fernando Valley State College
Northridge, California
          There is a gradual change in mode of life during the ontogeny of Tresus nuttalli. Young individuals inhabit the upper, unstable layers of the substrate, and are more subject to being exposed by water currents, while larger animals may be buried to a depth of one meter and more protected. Burrowing times reveal that there is a progressive loss of burrowing ability in the larger forms. ..... The young active forms possess a tightly knit pattern of shell features which is associated with their greater burrowing ability. ..... The position of the large animal within the burrow is related to the change in body orientation. An upward pull is exerted on the shell when the siphons are being retracted. The shell is situated obliquely in the burrow, thus apparently offering greater resistance to that force.
          .... Kristensen (1957) described this phenomenon in Cardium edule L. and showed that burrowing ability is critical in juveniles and that it decreases with increasing size. Accompanying the reduction in burrowing activity, is a change in form from juvenile to adult cockles.
          Young Tresus nuttalli (Conrad, 1837) (=Schizothaerus nuttalli) occupy the upper unstable layers of sand while larger individuals may be buried to a depth of one meter. This paper deals with changes in form that are associated with this change in life habit.
          Forty-three Tresus nuttalli, ranging in length from 23 to 148 mm were collected from mud flats in Tomales Bay, California.
          This animal is a suspension feeder, having long united siphons which make contact with the water-soil interface. Larger individuals live in deep burrows where they rest on their antero-posterior axis is thus slightly inclined to the surface plane of the substratum.
When an unearthed animal is placed on the mud flat, it reburies itself in much the same manner as Jordan (1915) described for Mactra inflata. Tresus first probes the soil with the tip of the foot and repeats this action until the foot is deep in the substratum ..... Next, the posterior retractors contract, pulling the posterior edge downward until the long axis of the shell makes an angle of about 30 degrees with respect to the soil surface (Fig. 2, arrow 2). During both of these movements the animal is being drawn into the substratum towards the anchored foot.
          To assess relative speed of burial, the burrowing times of 14 Tresus were taken.
          Animals were placed under water (13oC) on the mud flat near the burrow from which they were removed. Burrowing time was calculated from the moment the foot probed the soil until the shell disappeared beneath the surface. ...Five specimens did not completely rebury themselves within a period of 2 hours, although they made an attempt. ..... Also, in contrast to large individuals, small forms begin burrowing shortly after being unearthed. Because animals around 60 mm long and over usually were not buried after 2 hours, this size was regarded as a convenient point to divide the individuals into 2 groups, those over and under 60 mm long. A total of 20 small and 23 large individuals was examined by quantitative and qualitative methods in order to illustrate differences in morphology that occur in the rapid (small) and slow (large) animals.

          In the early post-larval stages, many bivalves are bysally attached; thus they resist being swept away by water currents (Yonge, 1962). But they soon lose their byssus, and burrowing ability becomes their means of maintaining positional stability. The capability to rebury thus becomes critical for these juveniles clams. Tresus nuttalli follows this trend, in that the smaller forms are active and rebury themselves relatively quickly when unearthed. Larger forms live at great depths and are protected from disturbances. Associated with their deep burial is a gradual loss of burrowing activity and speed.
          It is conceivable that, if more features of the internal anatomy, such as the ctenidia or labial palps, were measured and the correlations determined, the large individuals might have higher correlations for these dimensions than the small forms. .... Thus the pattern and intensity of integration does change in the ontogenetic groups of T. nuttalli, and these variations are associated with alterations in life habits.
          ..... This figure illustrates that the long axis of large T. nuttalli is not perpendicular to the water-soil interface, and the least resistance dimension is not parallel to the long axis of the siphons. ..... To counteract this upward motion, the animal is so oriented in the burrow that more resistance is offered to the surrounding soil.
          The change in form that occurs during ontogeny is illustrated in Fig. 4. Young specimens have a more pointed, elongated end, and appear to be more streamlined. This shape is an advantage in burrowing, for the anterior edge is more wedge-like and helps penetrate the soil.

I wish to thank Drs. Ralph Johnson, E. W. Fager, and Charles R. Stasek, for critically reading this manuscript. This work was completed while the author was a Sverdrup Fellow in Oceanography at the Scripps Institution of Oceanography.

JORDAN, J. 1915. Uber die Art wei Mactra inflata sich den Sand einwuhlt. Sool. Jb. Abt. Zool. Physiol. 35: 289-300.
KRISTENSEN, I. 1957. Differences in density and growth in a cockle population in the Dutch Wadden Sea. Arch. Neerland, Zoologie, 12(3): 315-453.
STASEK, C. R. 1963. Orientation and form in bivalved Mollusca. J. Morphology 112 (3): 195-214.
YONGE, C. M. 1952. Studies on Pacific coast Mollusks. IV. Observation on Siliqua patula Dixon and on the evolution within the Solenidae. Univ. Calif. Publ. Zool. 55: 421 - 438.
YONGE, C. M. 1962. On the primitive significance of the byssus in the Bivalvia and its effect in evolution. J. Mar. Biol. Assoc., 42: 113-125.