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How do you differentiate the two species of ladyfish (Elopiformes: Elops spp.) from the western central Atlantic?

Ladyfish (rarely called ‘tenpounders’) are economically valuable and are landed throughout the southeastern United States in both commercial and recreational fisheries (Levesque 2011).  Ladyfish are common in coastal areas throughout most of the western central Atlantic Ocean, including the Caribbean and Gulf of Mexico (Smith and Crabtree 2002).  The genus Elops in the western central Atlantic was traditionally treated as a single species, E. saurus.  This has changed recently with the description of a new species, E. smithi, by McBride and co-workers (2010).  Here’s how to tell the two species apart:

Counts of myomeres (in leptocephala larvae) or vertebrae (in post-larval to adult specimens) are the only known distinguishing morphological characters (McBride and Horodysky 2004, McBride et al. 2010).  E. saurus has 79–87 (usually 81–85) myomeres or vertebrae versus 73–80 (usually 75–78) in E. smithi.  (Number of myomeres = number of vertebrae).The number of myomeres or vertebrae appears to be a good diagnostic character, despite the overlap in range, as this overlap occurred in only 2.9% of the 3,255 specimens examined by McBride et al. (2010).  In counts of leptocephalus pre-anal myomeres or vertebrae, there is no overlap (see following table).

Myomeres should be counted using a compound microscope at 40x magnification, beginning with the first myomere behind the head and ending with the group of three myomeres near the caudal peduncle (McBride and Horodysky 2004).  Vertebrae (in post-larval to adult specimens) can be counted using radiographs and a microscope or by filleting, steaming, scraping, and directly counting the vertebrae.  Vertebral counts include all centra between the proatlas to the urostyle (McBride and Horodysky 2004). 

There are seasonal and geographic recruitment differences between the two species that can help you determine the species by date of capture: E. saurus larvae are most often found during winter through spring versus summer through fall in E. smithi (McBride and Horodysky 2004).  Generally speaking, Elops along the northern U.S. Atlantic seaboard are most often E. saurus while Elops from the Caribbean basin are most often E. smithi (McBride and Horodysky 2004).

The following table presents morphological, seasonal, and geographic comparisons between E. saurus and E. smithi that can help in species-level determinations.  Although it is admittedly difficult to discern the two species from one another, particularly when field-identification is needed, this write-up and associated table should make it more straightforward.

ladyfishtable

Sources Cited:

Levesque, J.C.  2011.  Is today’s fisheries research driven by the economic value of a species?  A case study using an updated review of ladyfish (Elops saurus) biology and ecology.  Reviews in Fisheries Science 19(2):137–149.

McBride, R.S. and A.Z. Horodysky.  2004.  Mechanisms maintaining sympatric distributions of two ladyfish (Elopidae: Elops) morphs in the Gulf of Mexico and the western North Atlantic Ocean.  Limnology and Oceanography  49(4):1173–1181.

McBride, R.S., C.R. Rocha, R. Ruiz-Carus, and B.W. Bowen.  2010.  A new species of ladyfish, of the genus Elops (Elopiformes: Elopidae), from the western Atlantic Ocean.  Zootaxa 2346:29–41.

Smith, D.G. and R. Crabtree.  2002.  Tenpounders (ladyfishes).  Pp. 679–680.  In:  Carpenter, K.E. (ed.), FAO Species Identification Guide for Fishery Purposes:  The Living Marine Resources of the Western Central Atlantic.  Vol. 2:  Bony Fishes Part 1 (Acipenseridae to Grammatidae).  FAO, Rome, Italy.

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