Take a Walk Outside – Bimonthly Blog by Pete Slavin PhD

Blog #3 – The Emerald Elysia

The eastern emerald elysia, Elysia chlorotica, has my vote as the most interesting organism of the wetlands bordering the Delaware Bay. This sea slug is broad, flat, bright green and photosynthesizes like a leaf but is more closely related to a snail than to any plant.

Before I go any further let me digress and explain a little about photosynthesis. Photosynthesis is how green plants and algae and some microorganisms make their own food. Taking water (H2O) and carbon dioxide (CO2) from their environment and using light energy from the sun, plants are able to manufacture carbohydrate (C6H12O6) which they then use for energy to accomplish many metabolic functions. Animals, unable to make their own carbohydrates, eat plants or other animals and convert their prey’s carbohydrates to energy. This conversion of CO2 and H2O into carbohydrate is not done just anywhere in the plant but is confined to cellular structures called chloroplasts where the green pigments that do much of the work of photosynthesis are found.

1“Sea Slug is not a taxonomic term but is used to describe a number of groups of Mollusca including nudibranchs, sap-sucking sea slugs, bubble shells, sea hares and sea angels.

Way, way back in evolutionary history, about a billion years ago and about the time that plants diverged from animals, a cell with a nucleus ingested a photosynthesizing bacterium. Instead of digesting this bacteria and using its chemical constituents as nutrients, the bacteria remained viable and continued to produce carbohydrates. These bacteria were the first chloroplasts. The chloroplasts of green plants today are more bacteria-like than plant-like in their DNA and protein assembly structures, and are capable of reproducing independent of the plant cell’s replication. Over time the nucleated cell acquired the DNA that was then translated into proteins that supported the chloroplast. So here we have a symbiotic relationship, one in which both the plant and the bacteria-like chloroplast have benefited. O.K. Back to Elysia. The eastern emerald elysia lives in shallow pools on salt marshes. Its size is from an inch to several inches in length and it feeds on the long filaments (the cells are tube-like and connect end to end) of the alga Vaucheria litorea (sorry, it has no common name that I know of). The elysia bites off the end and sucks the contents out. Most of the alga’s contents are digested but the chloroplasts are sequestered in special cells of the highly branched digestive tract of the sea slug. Here they continue to photosynthesize for as much as a year. As a juvenile, elysia is a yellowish brown but turns a bright green as it accumulates chloroplasts. Adults can live months and through the winter on the food these chloroplasts produce.

AND this is only half the story! Photosynthesis supporting genes in Vaucheria’s chromosomes are also stolen by the elysia, incorporated into the sea slug’s chromosomes, and used to continue and maintain its carbohydrate production. This horizontal gene transfer, as opposed to vertical gene transfer from parents to their offspring is fairly common in bacteria but it is rarer in higher organisms though not unknown. Elysia is an hermaphrodite, meaning individual adults are both male and female. (Much more about hermaphrodites in later blogs.) In the spring, after the adult has laid its egg masses, all adults die of a virus that the individual carries with it throughout its life. This synchronous death even occurs in laboratory reared specimens. Elysia is uncommon and seems to be getting rarer but it is possible to spot them in the shallow pools in which Vaucheria lives. Both Elysia and Vaucheria can tolerate a broad range of salinities from seawater to brackish, almost freshwater.

Blog #2 – The Common Sea Star

One of the oddest feeding mechanisms in nature can be observed in the common sea star, Asterias forbesi, the most numerous of the Delaware Bay sea stars. To ingest its food the sea star everts its stomach (turns its stomach inside out) through its mouth, grabs its prey with the muscular stomach, and pulls the stomach with its meal back into its mouth. If this is a little hard to imagine, here’s a Youtube link where you can watch the process. https://www.youtube.com/watch?v=l6dnmLDu6Eg
It feeds on snails, barnacles and other small crustaceans, and bivalves (dead or alive) such as clams, oysters and mussels. Feeding on a live bivalve is a bit of an art for the starfish. One of the persistent explanations of how a starfish opens a bivalve is that a starfish releases an anesthetic that relaxes the adductor mussels causing the valves (shells) to gape open.

Asterias forbesi, the common sea star.Kennedy, Newell and Eble, in their 1996 book on the Eastern oyster mention this anesthetic but I have not been able to find the actual chemical structure or, indeed, if anyone has even isolated such a chemical. My search has admittedly been limited but I’m beginning to doubt the anesthetic’s existence. If any readers know that such an anesthetic has been isolated, please let me know.

More likely, the starfish is able to separate the valves by latching on to the valves with its suction-cupped tube feet on the bottom of its arms and pulling. When the valves open a tiny fraction of an inch, 0.1 mm, about the width of a period on this page, the starfish can then evert its stomach which squeezes into the gap and releases digestive enzymes. Even if the starfish cannot maintain this pulling pressure for long and the bivalve slams shut, the starfish will keep pulling until, little by little, the stomach is able to expand into the bivalve. Then digestion proceeds to the point that the bivalve relaxes and gapes open.

The common sea star’s predation on bivalves can be extensive. It is an important predator in the Chesapeake and Delaware Bays and even more important predator north of the Delaware Bay. Predation on oyster spat (young oysters) by the common sea star has sometimes exceeded 90% in Long Island Sound. In the days when many sailing vessels, Sharpies and Schooners, were dredging oysters from the Sound, the common sea star was so important a predator that oystermen would drag sea mops over the floor of the Sound. These mops consisted of hundreds of lengths of rope attached to a 10’ – 20’ long bar of steel. As these mops were dragged through the bottom of the sound, the sea stars would attach to or become entangled with the ropes. The crew would haul the mop to the deck, chop the sea stars in pieces and throw the remains overboard.

This, of course, backfired! Starfish are capable of regeneration and only 20% of a starfish is needed to grow into a whole new starfish. It reminds me so much of Mickey Mouse as “The Sorcerer’s Apprentice” in Disney’s film “Fantasia”. Mickey couldn’t stop the broom from carrying buckets of water so he chopped the broom into splinters and each splinter carried the buckets. The oystermen and clammers soon recognized the errors of their ways and rather than returning the pieces of starfish to the Sound, they sold them instead as a chicken feed supplement. Whether or not these efforts resulted in a reduction in predation and an increase in shellfish numbers has never been determined.

Strangely (to me) starfish and other Echinoderms (the phylum to which starfish belong) are more closely related to Chordates and therefore to us than they are to other invertebrates. Their biology, including their life cycle and unique water vascular system I hope to include in another posting. By the way, that little red disc, seen just off-center in the first picture, is an organ used to take water into the sea star’s water vascular system. (Sea Star’s have no blood.)

 

Blog #1 – Mile 0

My favorite stretch of beach in New Jersey is Cape May Point. It’s a great place to see dolphins and ghost crabs, whelk egg cases and sanderlings, and the thousands of other natural curiosities that draw me back there at any season. When there, I never fail to see someone in a T-shirt that reads “ Mile 0”, a reference to the beginning of the NJ Garden State Parkway.

But there’s another important Mile 0. If you were to draw a line from Cape May Point, NJ to Cape Henlopen, Delaware, you would define the boundary between the Delaware Bay and the Atlantic Ocean. From Mile 0 it is 330 miles north to the confluence of the East and West Branches of the Delaware River at Hancock, NY. The Delaware is the only major un-dammed river in the Eastern U.S. and it drains an area of over 13,500 square miles in five states (though only a few square miles of Maryland) . A river mileage system has been in use for the past fifty years to describe locations upstream of Mile 0.

Besides mile 0 there are two other important lines. One is visible and immovable, the other invisible and moveable. The immoveable one is called the Atlantic Seaboard Fall Line and occurs at the falls of the Delaware, Mile 133, just below the “Trenton Makes the World Takes” bridge. In a relatively short span the river drops about eight feet in elevation and separates the unidirectional flow of the river from the tidal portion of the river. The second, moveable line is the Salt Front. The Salt Front occurs when the concentration of the chloride ion reaches 250 milligrams per liter. This is based on drinking water quality standards. At times of low freshwater input, such as in periods of little rainfall, the Salt Front moves up the Delaware. Nineteen sixty-four was a drought year and by November of ’64 the Salt Front had moved up to River Mile 102, just north of the Benjamin Franklin Bridge. At the time of this writing in January, 2024 the Salt Front was at River Mile 54, south of where the C & D Canal enters the Delaware. Both the Fall Line and the Salt Line will impact the kinds and distributions of many organisms that live in these waters.

In the coming months we’ll be looking at the many creatures of the Delaware – who they are, where and how they live and how they interact with one another. My only purpose in writing these blogs is to share the joy and fascination that I’ve found exploring the ecology of the Delaware Bay and River.

I would greatly appreciate corrections, comments and questions.

Patrick “Pete” Slavin has lived within the Delaware River watershed most of his life. He holds a PhD in Ecology from Rutgers and has worked in public health, agriculture and academia.