Thursday, November 5, 2015

Life At the Water’s Surface: Brook Silverside Labidesthes sicculus by Don Orth

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I always enjoy observing fish, especially those that we can see from the surface.   One of these is the Brook Silverside, Labidesthes sicculus.  The fish would frustrate me during a phase when I was obsessed with quantifying fish numbers because these surface dwellers would easily jump over or swim through carefully anchored block nets (another violation of the closure assumption).   Brook Silverside is the most widespread member of the Atherinopsidae family in North America.  We pronounce the scientific name lah-beed-ess-theez  sick´-you-lus. The Atherinopsidae is the family of Neotropical Silversides that inhabit both marine and freshwaters of North, Central, and South America.  Based on fossil remains the family is of recent (Pliocene) origin.  There are 104 species in 13 genera in the Atherinopsidae.    Some silversides endemic to Mexico (Poblano spp.) are threatened species.  The most famous species are the California Grunnion, Leuresthes tenuis, Atlantic Silverside, Menidia menidia, and the Inland Silverside, Menidia beryllina.   These are small, very elongate fishes with body lengths 5-7 times the maximum body depth.   The silversides are small fishes and are important forage fish for all local piscivorous fishes.
Brook Silverside by Howard Jelks

The Brook Silverside species was described by Edward Drinker Cope  from a Michigan holotype in 1865. The genus name, Labidesthes was derived from the two words: labidos, forceps and esthio, eat.   It refers to the prolonged jaws, which form a short, depressed beak  (see photo). The species meaning of sicculus is less clear.   William Pflieger, in The Fishes of Missouri, interpreted Cope’s finding of specimens found in half-dried pools as meaning siccus (meaning dried).  Though this interpretation has been repeated in numerous subsequent references,  recently Christopher Sharf and Kenneth Lazaro explained that sicculus was more likely meant as an adjectival form of sicula, dagger, referring to the fish’s sharp snout and dagger-like shape.

The Brook Silverside is well camouflaged in near surface waters.  The fish, in life, is nearly transparent with pale shades of olive on the back and upper sides.   Dorsal scales are clearly outlined with melanophores.   A distinctive lateral silver band becomes broader in anterior portions and is underlain by black pigment.  Parts of the body, opercles, and underside of the head are silvery white with iridescent blue-green. Maximum size of the Brook Silverside is 11 cm (standard length). 

The range of Brook Silverside is usually described as Great Lakes region and south through the Mississippi Basin and Gulf Coastal Plain drainages.  Older references recognized a single species and an uncertainty over clinal variation or subspecies in Labidesthes.  However, Werneke and Armbruster (2015), after examining morphometric, meristic, and osteological data from numerous populations, concluded there are two valid species.   
Labidesthes sicculus is found in Gulf of Mexico drainages from the Brazos River East to the Pascagoula River, Mississippi River (absent in middle and upper Missouri River), and Great Lakes-St. Lawrence River (absent in Lake Superior). Labidesthes vanhyningi is found in Gulf Mexico drainages from the Neches River East around peninsular Florida North on the Atlantic Coast to the Pee Dee River, in the Mississippi River it is confined to lowland areas of the Lower Mississippi River.”

 Labidesthes sicculus (top) and Labidesthes vanhyningi (bottom).  In Labidesthes sicculus note the midlateral silver band is narrowest on caudal peduncle, broadening and fading anteriorly whereas it is not tapering anterior of first dorsal in Labidesthes vanhyningi.  The ratio of thoracic length to abdominal length is longer in Labidesthes sicculus. (Werneke and Armbruster 2015)

“Brook” is actually a misnomer -- the Brook Silverside more frequently inhabits open areas of rivers, lowland streams, lakes and reservoirs and avoids fast current and shallow waters.  It is highly adapted for living right at the water surface.  Brook Silverside are most associated with waters with low turbidity and fairly clean substrates or deep waters in weedy lakes and rivers.    The often occur in very large aggregations and make inshore-offshore migrations in the large schools.  

The body form of the Brook Silversides reveals its habits.  The flattened head is often in direct contact with the surface film.    The large eye indicates a visual feeding mode.  The large, beak-like terminal mouth indicates a specialized, near-surface predator.  A short s-shaped gut lacking pyloric caeca, indicates a carnivore.  A quick acceleration and a snap of the jaw results in rapid prey capture.  Most diet studies confirm that Brook Silversides eat planktonic crustaceans, small flying insects, and immature insects, such as the phantom midge Chaoborus. 

Close up of head of Brook Silverside, by Uland Thomas
They are often seen jumping out of water in seeking flying insect prey.   One  prey includes the dance flies (Empididae), which frequently hover in large swarms just above the surface.  Alvin Cahn made extensive observations on Brook Silverside in southern Wisconsin lakes.  Cahn estimated that Brook Silverside spends "most of their time within ten or twelve centimeters of the surface…" and “never under any conditions descends below the upper meter of water…. while nothing can drive the immature individuals more than a few centimeters below the  surface."     They have periods of very intense activity during the day and Hubbs (1921) observed that “adults at night were observed to lie quiescent as though asleep."   However, Cahn later observed periods of intense activity at night during full moon.   This video of a Brook Silverside does not capture the periods of intense surface feeding of large schools.   

Many Brook Silversides live just 15-17 months as they die sometime after the one and only spawning.   Therefore, it is important that spawning times are synchronized to ensure reproductive success.  The moon phases may be important cues that initiate spawning activity in the Brook Silversides.   Brook Silverside breed in spring (May-June) in shallow waters along shore.   There is no clear sexual dimorphism; however, pairs of silversides are seen swimming near shores in pairs as temperatures warm to 18°C.   Cahn discovered that the upper fish in a pair is male, the lower female.  As temperatures warm further (20-22°C) this swimming speed increases until the fish are travel in fast spurts, and often breaking the water surface.  Spawning aggregations may include many male and females.   The distance between the pair approaches 5 cm as the females continue to dart “this way and that way,” pursued by a trailing male.  Eventually, the female slows darting pace and allows the male to approach her side. The pair then swims toward the bottom and make repeated contacts with their abdomens. During the descent eggs are extruded and immediately fertilized by the attending male.  The female is spent after completing a single breeding, while the male ascends to the surface to pursue other females.   The eggs have a single gelatinous filament about six times the egg diameter.   The filament firmly attaches to the first thing it contacts.     Eggs hatch in 8-9 days at 24°C and the young silversides "wiggle" in attempt to their attempt to reach the water surface. 

Grier and his colleagues studied the Brook Silverside when ripe and stripped eggs from females with gentle pressure on the abdomen.  When they did this they observed many embryos with well-developed, pigmented eyes!    They only way for this to happen is if eggs were internally fertilized.  This is not what Alvin Cahn described.  Grier confirmed this via histological examination that revealed both sperm and developing embryos in the ovaries. Further observations revealed that the male Brook Silverside possesed a short genital palp immediately posterior to the anus, which presumably acts as an intromittent organ.  Therefore, the female Brook Silverside may release eggs, in different developmental stages, in open water or deposit them on aquatic vegetation and other substrates.

A) Scanning electron micrograph of ventral surface of male showing a short genital papilla;B) Ventral surface of female showing absence of a genital papilla. Grier et al. (1990).
Brook Silversides are not used as bait minnows – they are too fragile and thin bodied and do not survive well in bait buckets.  Similarly, they do not adapt readily to life in aquaria.   They are eaten by many piscivores, including Longnose Gar, Bowfin, Cisco, Yellow Perch, Smallmouth Bass, Largemouth Bass, Northern Pike, and sunfish.  But their small size (1-2 g) means large piscivores will not waste their time.  Water snakes, mudpuppies, and turtles are also likely to eat them.

The Brook Silverside is introduced to other waters via constructed canals or intentional introductions as prey fish.  For example, they were introduced as supplemental prey in Kentucky lakes in the 1960s.   Dams and industrial and agricultural pollution have extirpated many local populations.   Also, the Inland Silverside Menidia beryllina (synomous with Mississippi Silverside Menidia audens) was widely introduced as a prey fish in newly constructed reservoirs. Also, the non-native Inland Silverside was likely introduced to the Tennessee-Ohio River system via the Tennessee-Tombigbee Waterway, which connects Gulf Coast drainages to the lower Tennessee River.  Brook Silverside is virtually undetectable in reservoir electrofishing surveys after 14 years of co-occurrence with Inland Silverside. Declines in Brook Silverside were also observed in several other reservoirs after introduction of the Inland Silverside. 

Increase in abundance of the Mississipi Silverside (top) and concurrent decline in the Brook Silverside (bottom) in Tennessee River reservoirs from Simmons (2013).
The Brook Silveside is an excellent representative of life in the near-surface zone.  The large schools are fun to watch, especially the intense feeding and jumping to capture prey.   Only during breeding does the Brook Silverside experience life away from the water’s surface.    A most surprising finding was the discovery of internal fertilization in the Brook Silverside 125 years after the species was discovered.  Fascinating!

References

Cope, E.D.  1865. Partial catalogue of the cold-blooded vertebrata of Michigan. Proceedings of the Academy of Natural Sciences of Philadelphia  17:78–88.
Grier, H.J., D.P. Moody; and B.C. Cowell. 1990. Internal fertilization and sperm morphology in the brook silverside, Labidesthes sicculus (Cope). Copeia 1990(1):221-226.
Hubbs, C. L. 1921. An ecological study of the life-history of the fresh-water atherine fish Labidesthes sicculus. Ecology 2(4):262-276.
Keast, A., and D. Webb. 1966. Mouth and body form relative to feeding ecology in the fish fauna of small lake, Lake Opinicon, Ontario. Journal of the Fisheries Research Board of Canada 23(12):1845-1874.
Marsden, J.E., R.W. Langdon, and S.P. Good. 2000. First occurrence of the brook silverside (Labidsthes sicculus), in Lake Champlain, Vermont.  Northeastern  Naturalist 7(3):248-254.
Scharpf, C. & Lazara, K.J. (2014) The ETYFish Project. 27 August 2014—Cope headscratcher #6: Labidesthes sicculus. Available from: http://www.etyfish.org/names-of-the-week2014/ (accessed 04 November 2015)
Simmons. J.W.    2013.  Chronology of the invasion of the Tennessee and Cumberland River systems by the Mississippi Silverside, Menidia audens, with analysis of the subsequent decline of the Brook Silverside, Labidesthes sicculus.  Copeia 2013:292-302.
Strongin, K., C.M. Taylor, M.E. Roberts, W.E. Neill, and F. Gelwick. 2011.  Food habits and dietary overlap of two silversides in the Tennessee-Tombigbee waterway: The invasive Menidia audens versus the native Labidesthes sicculus. The American Midland Naturalist 166(1):224-233.
Werneke, D.C., and J.W. Armbruster. 2015. Silversides of the genus Labidesthes (Atheriniformes: Atherinopsidae).  Zootaxa 4032(5):535-550.    
Zimmerman, C. J. 1970. Growth and food of the brook silverside, Labidesthes sicculus, in Indiana. Transactions of the American Fisheries Society 99(2):435-438.

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