CORCULUM: THE “HEART COCKLE” CLAM WITH AN UNUSUAL WAY OF OBTAINING ITS FOOD

Genus Corculum cardissa (Linnaeus, 1758) is commonly referred to as the “heart cockle” because of its “valentine” shape. This clam belongs to family Cardiidae. This clam, like its six other sister species, harbors the symbiotic dinoflagellate known as Symbiodinium in its mantle and gills. This dinoflagellate (also known as a zooanthellae) is used as food for the clam. In addition, this clam obtains oxygen (for physiological purposes) from this dinoflagellate. In turn, Corculum cardissa provides a “home” for the dinoflagellate.

The unusually shaped shell of C. cardissa is attached to the ocean floor by a few filaments. The underside valve of C. cardissa is flattened and also so thin that it is transparent, thereby allowing light to penetrate its shell, and this light is used for photosynthesis by the dinoflagellate. The upper valve of this clam is usually covered by filamentous algae or mud.

Kawagutii (1949) did the pioneering studies of the symbiotic relationship between Corculum and Symbiodinium. For more information about algal-bivalve photo-symbiosis, see Ohno et al. (1995).

An articulated (= both valves) specimen of Corculum cardissa: top, bottom, and left-side views, length 4.5 cm, width 4 cm.

A different specimen of Corculum cardissa with its valves separated: A) oblique bottom view of both valves (length 2.5 cm) showing the hinge. B) and C) perpendicular bottom views of left and right valves (length 2.5 cm, width 1.3 cm), respectively, showing the hinge.

References:

Kawagutti, S. 1949. Observations on the heart shell, Corculum cardissa (Linnaeus) and its associated zooxanthellae. Pacific Science 4(1):43–49. [online pdf free]

Ohno, T. , T. K. and T. Yamasu. 1995. The origin of algal-bivalve photo-symbiosis. Paleontology 38:1–21, 1 pl. [requires a subscription]

THE ANCIENT CYCAD PLANT

Cycads are plants that are often referred to as “living fossils” because they have a geologic time range of early Permian to recent. They belong to an ancient group of seed plants known as gymnosperms. Even though they resemble short, stout palms, they are not palms.

Two photos of the same cycad plant. The first image was taken in 2009, and the second image was taken in 2023, 14 years later. There has been noticeable growth but not much!

Starting in the Cretaceous, cycads gradually were replaced by angiosperm plants (the flowering plants). Today, there are 9 to 10 genera left of cycads and about 100 species. They are tropical to subtropical and very slow-growing. They require about 15 to 20 years to mature. Under the right growing conditions, they can reach up to 60 feet high, but many remain short/thick throughout their lives.

They are easy to grow and many gardeners cultivate them. They are, however, expensive to purchase because their forest habitats are vanishing. The fact that they are expensive is the reason why they are commonly stolen from gardens. 

Cycads are poisonous because they contain a toxin called cyasin. Cycad nuts are extremely poisonus, especially to dogs. Also, cycad fronds, which soon become stiff after sprouting in the middle of the plant, have very sharp tips that easily penetrate animal skin and cause slow-to-heal wounds. 

When the female cycads mature, they produce “flowers.” These  "flowers" have very specialized pollinators, usually a specific species of beetle. The seed-bearing cones are borne on nearby, separate male plants. When the males mature (it can take a long time), they produce, thick solid cones that resemble pine cones but are referred to as sarcotesta, which are brightly colored, fleshy, fruitlike structures. Enclosed within the sarcotesta, which is sugary, are kernels (nuts). 

This figure is of a sacrotesta “fruit” structure of the cycad Cycas circinalis, in the Washington, D.C. area. Image taken by Raul654 on May 7, 2005. This image is allowed to be used by GFDL Licensing, Creative Commons Attribution-Share Alike, courtesy of en.wikipedia.org  

Late Paleozoic and Mesozoic herbivorous reptiles (including some dinosaurs) undoubtedly fed on cycads. In fact, they co-evolved: both flourished during the Mesozoic, and when the dinosaurs died out at the end of the Mesozoic, cycads declined greatly. Dinosaurs most likely had color vision and the brightly colored sacrotesta of the cycads would have been easily detected. The kernels (nuts) within the sarcotresta could have just passed through a dinosaur’s digestive tract without releasing their poisonous interiors--as along as the kernels were swallowed whole and not chewed up. This technique of seed dispersal was very an effective means of widespread geographic dispersal of cycads (Mustoe, 2007). 

Hadrosaur dinosaurs had grinding teeth, thus they could not successfully feed on the cycad seeds without risking death. 

It should be mentioned that herbivore dinosaurs could have fed also on other land plants: ferns, ginkgo leaves, pine trees, lichens, club mosses, and Equisetum (see one of my previous posts).

The Fossil Cycad National Monument was created in 1922 in order to “protect” a Cretaceous (120 million years old) cycadeoid forest in the Black Hills of southwestern South Dakota. It was the third national monument specifically created to protect fossils in the United States. The first was the Petrified Forest National Monument in northern Arizona, and the second was the Dinosaur National Monument located on the Colorado and Utah border. 

Hundreds of specimens of a cycad-like cycadeoid, known as Pennettitales to paleobotanists, were exposed at the ground surface at the Fossil Cycad National Mounument. This made for easy collecting, and unfortunately, over-collecting and outright rampant theft resulted in all of the specimens “disappearing.” Thus, in 1957, the federal government deauthorized this national monument, and it no longer exists.

REFERENCES USED:

Mustoe, G.E. 2007. Coevolution of cycads and dinosaurs. The Cycad Newsletter, v. 30, no. 1, pp. 6–9.  [An excellent treatment of the topic, and the pdf is free].

en.Wikipedia.org 2023

THE WORLD'S MOST EXPENSIVE SEASHELL

One of the rarest sea shells in the world and currently the most expensive is the cowrie gastropod Sphaerocypraea incomparabilis (Briano, 1993). A single shell is worth $20,000! Three views of the shell are shown below: apertural, dorsal, and right-lateral side of a modern-day specimen of S. incomparabilis. Many cowrie shells are beautiful and eye-catching, but the distinctive coloration of the shell of this species is definitely memorable. 

The original genus name of this shell was Chimaeria. This cowrie, which ranges in size from 69 to 93 mm in length, lives in deep water in the Gulf of Aden, off the coast of Mozambique, but its exact location has not been established. The few known specimens are currently housed in museums. Most likely, more specimens will be found in the future, and, if so, the price will fall dramatically. That is what happened to the cone shell Conus gloriamaris, formerly one of the rarest and most expensive shell in the world, until the habitat of this gastropod was found in 1967 (see my previous post of June 30, 2023 entitled: “Three Famous Shells.”) 

Sphaerocypraea incomparabilis belongs to family Eocypraeidae; a family believed to have been extinct for the last 20 million years, until it was recognized in the fossil record (i.e., early Eocene of France). Thus, S. incomparabilis is a “living fossil.” 

BIVALVED GASTROPODS

This post is not in error! Bivalved gastropods have existed in the geologic past and still do today. Nearly all gastropods have either a single valve (can be internal) or have no valve all, but this post concerns two gastropod genera that HAVE TWO EXTERNAL VALVES. Both of these gastropods are “living fossils,” and they are comprised of the genera Berthlinia and Julia (both belong to family Juliidae). Both are opisthobranch (sacoglossan = “sap suckers”) snails with sluglike bodies, which like the valves, are green. This is because these gastropods live in clumps of green algae, graze on the algae, and incorporate the photosynthetic algal cells into their own body tissues for energy storage. This process is called kleptoplasty. Thus, the bodies of these two gastropods are also green. After death, however, the color of these translucent shells can easily fade. The larvae of both genera are planktonic (= floaters), therefore, both genera are geographically widespread. They are restricted, today like they were in the geologic past, to tropical waters.

BERTHLINIA:

A Recent Berthlinia limax (Kawaguti and Baba, 1959), showing mostly its left valve, 3 mm length, locality not known.

Berthlinia is found today in Japan, Australia, California, Hawaii, Jamaica, and the Indian Ocean region. The hinge of Berthlinia has no teeth, but faint corrugations are present. The left valve has a small spiral nucleus, which projects noticeably outward from the top of this valve. This nucleus (protoconch), which is the initial part of the left valve, is white and pearly. The animal living within the two valves of Berthlinia is sluglike and also green in color.  

Genus Berthlinia has a discontinuous geologic range of early Eocene (Ypresian Stage) to recent (Le Renard and others, 1996). Most of the Eocene specimens are found in Europe, especially the Paris Basin of France. The type species of this genus is of middle Eocene age and from France. 

There are a few other fossil genera/species of bivalve gastropods: most of them are Eocene in age or Miocene-Pliocene in age (Le Renard and others, 1996, table 1).

JULIA:

A living Julia exquisita Gould, 1862, showing mostly the exterior of its right valve, about 3 mm length, locality not known.

 

Exterior and interior views of a left valve (2.7 mm length) of living J. exquisita from Wailea Beach Hawaii. Gift from Bert Draper (deceased), March, 1987.

 Exterior and interior views of the opposing right valve from the same specimen of J. exquisita, immediately illustrated above.

Julia exquisita is found today in the Indo-West Pacific, including Hawaii, the Seychelles Islands (in the Indian Ocean), and the Kermadec Islands (off New Zealand) (Kay, 1979).

Genus Julia has a discontinuous geologic range of Oligocene?, Miocene, and early Pliocene (Le Renard and others, 1996) to recent. These specimens are found in Europe. 

References Cited:

Kay, E. A. 1979. Hawaiian marine shells. Reef and shore fauna of Hawaii: Section 4. Mollusca. Bishop Museum Press, Honolulu, Hawaii. 653 pp.

Kawaguti, S. and K. Baba. 1959. A preliminary note on a two-valved sacoglossan gastropod, Tamanovulva limax, n. gen., n. sp., from Tamano, Japan. Biological Journal, Okayama University, 5(3/4):177–184.

Keen, A.M. and A.G. Smith. 1961. West American species of the bivalved gastropod genus Berthelinia. Proceedings of the California Academy of Sciences, Fourth Series, 30(2):47–66.

Le Renard, J. and others. 1996. On Candinia (Sacoglossa: Juliidae), a new fossil genus of bivalve gastropods. Journal of Paleontology 70(2):230–235.  

BEAVERS

Beavers are rodents that belong to family Castoridae. They have large tails that are broad, flat, and scaly but mostly without hair. These animals are medium-dog size, thickset, and adapted for aquatic habitats, as evidenced by their webbed feet. Their small eyes (they have very poor eyesight) are protected by a nictitating membrane, which is a structure generally absent in mammals. They live in lodges or in the banks of large streams and rivers. They can also inhabit small lakes. They are herbivores and eat leaves, twigs, and bark. Today, one species (Castor canadensis) of beavers lives in North America (north of Mexico, especially in Alaska and Canada), and a second species (Castor fiber) lives in Europe and Siberia.

The geologic age range of rodents is mid Paleocene to recent, and they originated apparently in the northern part of the "Old World." During the Eocene, rodents began to diversify, and late in the Eocene, Old World beavers invaded Africa. During the late early Oligocene (30 million years ago [mya]), beavers reached North America as evidenced by fossils of Microtoheriomys articulataquaticus in Montana (news.osu.edu, 2022). Their migratory route was via Berginia #1. [See one of my previous blogs concerning the northern-hemisphere land bridge that existed between Alaska and Siberia during Eocene and Oligocene times]. 

By Miocene time, beavers had spread into Texas, and by late Miocene they had spread also into Nebraska. Also during the Miocene, one type of beaver made large helical burrows, known as the fossil Paleocastor = Deimonelix. These large fossil burrows, which are also known colloquially as “the devils’ corkscrew, are up to 8.5 feet (2.5 m) deep, with an entrance, a middle vertical or slightly tilted spiral, and a lower living chamber. 

Side view of a Paleocastor  [= Deimonenlix burrow], Miocene age, Nebraska.

About 2 million years ago, during the Pliocene, the beaver genus Castor was widespread in the USA. One species even lived in the Anza Borrego area of southern California (Jefferson and Lindsay, 2006).

During the Pleistocene and until about 10,000 years ago, a second genus Castoroides of beavers evolved and lived only in the USA and Canada. Castoroides ohioensis lived throughout much of USA and Canada, whereas C. dilophidus lived primarily in Florida and South Carolina. These beavers were “giants,” because they were moderate-bear size, but they had relatively small brain cases. Their height was about 6 feet tall, and their weight was probably about 150 to 250 pounds; thus, they represent the largest known beavers ever! They went extinct about 12,000 years ago. The modern-day adult beavers can weigh up to about 50 pounds and represent the second largest living rodent after the capybaras in South America.

Comparative sizes: a) of a modern beaver and (c) the giant Castoroides ohioensis versus a 6 foot tall human.

The above diagram shows a modern-day Castor canadensis skull (on the left) versus that of  a Pleistocene Castoroides ohioensis (on the right). Both skulls the eye sockets oriented "looking" upward. The colors used for the dentition (different types of teeth) are the same used in all of my previous posts on various recent and fossil examples of mammals, in case you might want to go back to those posts and view some previous skulls.

Based on current knowledge, beavers did not migrate into South America via the Panamanian land bridge (the GABI biotic interchange---see one of my previous blogs) during late Pliocene time.

How did they otherwise arrive is still a mystery? There are some who have postulated however that they most likely reached South America sometime during the middle Cenozoic, via floating on "rafts" of vegetation across the Atlantic Ocean (i.e., at least 600 miles of open ocean in the same area where hurricanes are born). This unsubstantiated concept has been “floated” since 1950.

 

In 1946, however, beavers were most definitely introduced by humans into southern Argentina (i.e., southern Patagonia area), in order to establish a fur trade in the region. The beavers have now spread throughout most of Tierra del Fuego and also the Brunswick Peninsula in mainland Chile. They have become very invasive to the local ecology, and the government is trying to undo this mistake via eradication of the beavers (en.Wikipedia.org).  

REFERENCES

Jefferson, G.T. and L. Lindsay, 2006. Fossil treasures of the Anza-Borrego Desert. The last seven million years. Sunbelt Publications, San Diego California. 394 pp.

news.osu.edu, 2022

enWikipedia.org