FWS Focus

Overview

Characteristics
Overview

This Texas-endemic freshwater mussel species occurs in the Colorado, Brazos and Trinity river basins of central Texas. Like many other freshwater mussels, the proposed threatened Texas fawnsfoot has a unique life cycle that requires the use of a host fish to transform the immature larva stage into a self-supporting juvenile mussel. What makes the Texas fawnsfoot even more unique is that the species is believed to utilize a self-sacrificing reproductive strategy when the females infests the molluscivore host fish with glochidia immediately before being ingested. The species is small in size with a compressed, elongate oval shell that is dull green, tan or yellow-brown with broken rays consisting of inverted chevrons, blotches or zig-zag patterns.

Scientific Name

Truncilla macrodon
Common Name
Texas fawnsfoot
FWS Category
Clams
Kingdom

Location in Taxonomic Tree

Identification Numbers

TSN:

Characteristics

Characteristic category

Physical Characteristics

Characteristics
Color & Pattern

The shell is dull green, tan or yellow-brown with broken rays consisting of inverted chevrons, blotches or zig-zag patterns. The interior shell surface, nacre, is typically white, with an occasional salmon or yellow tint.  

Size & Shape

The Texas fawnsfoot is a small to medium-sized freshwater mussel with a compressed, elongated oval shell. The beaks are elevated above the hinge line with three to six fine, single hoop ridges that sometimes may include one to two heavier ridges. The shell disc lacks sculpturing and has a slightly angular posterior ridge. Pseudocardinal teeth, which are molar-like structures located near the beaks on the interior surface, are compressed and triangular. The lateral teeth, two in the left valve and one on the right, are relatively thin.

Measurements
Adult length: Maximum of approximately 2.5 in (60 mm)

Characteristic category

Similar Species

Characteristics
Similar Species
Characteristic category

Food

Characteristics
Food

Like all other freshwater mussel species, Texas fawnsfoot are filter feeders that feed and survive on organic particulate matter, bacteria and algae that are filtered from the water column. Juvenile mussels live in the sediment and most likely feed interstitially rather than from the water column, using the relatively large muscular foot to sweep organic and inorganic particles found among the substrate into the shell opening.

Characteristic category

Life Cycle

Characteristics
Life Cycle

The Texas fawnsfoot, like all freshwater mussels, has an interesting life history. Males release sperm into the water column, which is taken in by the female fertilizing her eggs. The developing larvae are held by the female until they mature and are ready for release. The mature larvae, called glochidia, are released in proximity to a fish host and attach to the hostโ€™s gills or fins. It is believed that Texas fawnsfoot will frequently self-sacrifice and release mature glochidia immediately preceding being predated upon by the species host fish. Glochidia that fail to attach to a suitable host or attach to the wrong location will die. Attached glochidia encyst, or implant, into the host and develop into juvenile mussels over a period of weeks or months. Once fully developed, the juveniles excyst, or break away, from the host and settle on the stream bottom leaving their host relatively unharmed. 

Reproduction

Texas fawnsfoot are considered to be long-term brooders, or bradytictic, and are assumed to use freshwater drum (Aplodinotus grunniens) as a host fish. 

Life Span

No age at maturity information exists for this species, but similar species like Truncilla spp. and tribe Lampsilini, have life spans from 8 to 18 years and 4 to 50 years, respectively.

Characteristic category

Habitat

Characteristics
Habitat

The Texas fawnsfoot is found in flowing, medium to large-sized streams with substrates of mud, sand and gravel and are often found in bank habitats or occasionally backwater, riffle and point bar habitats that may serve as flow refugia during high water events.

River or Stream

A natural body of running water.

Geography

Characteristics
Range

The Texas fawnsfoot is endemic to Texas and is known to occur in the Colorado, Brazos and Trinity river basins across 28 counties in Texas. 

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Timeline

Explore the information available for this taxon's timeline. You can select an event on the timeline to view more information, or cycle through the content available in the carousel below.

19 Items

Listing

Listing

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Critical Habitat

Listing

4d

4d

Critical Habitat

Listing

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Listing

Dec 15, 2009

Dec 15, 2009 Listing (Substantial)
90-Day Finding on Petitions To List Nine Species of Mussels From Texas as Threatened or Endangered Wโ€ฆ
  • Publication type: 90 day petition finding
Item 1

Listing

Oct 6, 2011

Oct 6, 2011 Listing (Warranted But Precluded)
12-Month Finding on a Petition To List Texas Fatmucket, Golden Orb, Smooth Pimpleback, Texas Pimplebโ€ฆ
  • Publication type: 12m petition finding
Item 2

Listing

Nov 21, 2012

Nov 21, 2012 Listing
Review of Native Species That Are Candidates for Listing as Endangered or Threatened; Annual Noticeโ€ฆ
  • Publication type: CNOR
Item 3

Listing

Nov 21, 2012

Nov 21, 2012 Listing (Warranted But Precluded: Resubmitted)
Review of Native Species That Are Candidates for Listing as Endangered or Threatened; Annual Noticeโ€ฆ
  • Publication type: 12m petition finding
Item 4

Listing

Nov 22, 2013

Nov 22, 2013 Listing
Review of Native Species That are Candidates for Listing as Endangered or Threatened; Annual Noticeโ€ฆ
  • Publication type: CNOR
Item 5

Listing

Nov 22, 2013

Nov 22, 2013 Listing (Warranted But Precluded: Resubmitted)
Review of Native Species That are Candidates for Listing as Endangered or Threatened; Annual Noticeโ€ฆ
  • Publication type: 12m petition finding
Item 6

Listing

Dec 5, 2014

Dec 5, 2014 Listing (Warranted But Precluded: Resubmitted)
Review of Native Species That Are Candidates for Listing as Endangered or Threatened; Annual Noticeโ€ฆ
  • Publication type: 12m petition finding
Item 7

Listing

Dec 5, 2014

Dec 5, 2014 Listing
Review of Native Species That Are Candidates for Listing as Endangered or Threatened; Annual Noticeโ€ฆ
  • Publication type: CNOR
Item 8

Listing

Dec 24, 2015

Dec 24, 2015 Listing
Review of Native Species That Are Candidates for Listing as Endangered or Threatened; Annual Noticeโ€ฆ
  • Publication type: CNOR
Item 9

Listing

Dec 24, 2015

Dec 24, 2015 Listing (Warranted But Precluded: Resubmitted)
Review of Native Species That Are Candidates for Listing as Endangered or Threatened; Annual Noticeโ€ฆ
  • Publication type: 12m petition finding
Item 10

Listing

Dec 2, 2016

Dec 2, 2016 Listing (Warranted But Precluded: Resubmitted)
Review of Native Species That Are Candidates for Listing as Endangered or Threatened; Annual Notificโ€ฆ
  • Publication type: 12m petition finding
Item 11

Listing

Dec 2, 2016

Dec 2, 2016 Listing
Review of Native Species That Are Candidates for Listing as Endangered or Threatened; Annual Notificโ€ฆ
  • Publication type: CNOR
Item 12

Listing

Oct 10, 2019

Oct 10, 2019 Listing
Review of Domestic and Foreign Species That Are Candidates for Listing as Endangered or Threatened;โ€ฆ
  • Publication type: CNOR
Item 13

Critical Habitat

Aug 26, 2021

Aug 26, 2021 Critical Habitat
Endangered and Threatened Wildlife and Plants; Endangered Species Status With Critical Habitat for Gโ€ฆ
  • Publication type: Proposed
Item 14

Listing

Aug 26, 2021

Aug 26, 2021 Listing (Threatened)
Endangered and Threatened Wildlife and Plants; Endangered Species Status With Critical Habitat for Gโ€ฆ
  • Publication type: Proposed
Item 15

4d

Aug 26, 2021

Aug 26, 2021 4d
Endangered and Threatened Wildlife and Plants; Endangered Species Status With Critical Habitat for Gโ€ฆ
  • Publication type: Proposed
Item 16

4d

Jun 4, 2024

Jun 4, 2024 4d
Endangered and Threatened Wildlife and Plants; Endangered Species Status With Critical Habitat for Gโ€ฆ
  • Publication type: Final
Item 17

Critical Habitat

Jun 4, 2024

Jun 4, 2024 Critical Habitat
Endangered and Threatened Wildlife and Plants; Endangered Species Status With Critical Habitat for Gโ€ฆ
  • Publication type: Final
Item 18

Listing

Jun 4, 2024

Jun 4, 2024 Listing (Threatened)
Endangered and Threatened Wildlife and Plants; Endangered Species Status With Critical Habitat for Gโ€ฆ
  • Publication type: Final
Item 19