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. �'� Salt Marsh Harvest Mouse (Reithrodontomys raviventris raviventris) in the Redwood Shores Area* Perennial inhabitant of greater San Francisco Bay tidal marshes. Federal endangered species: 35 Federal Regi.rter 16047 — 16048 (13 October 1970). California endangered species Title 14 Caltfo�za Cod� ojRegulatton,r, � 670 5(�7 Tune 1971) Salt marsh harvest mice are members of the Cricetidae family, which includes field mice, lemmings, muskrats, hamsters, and gerbils (USFWS-Sacramento). There are two subspecies of salt marsh harvest mouse. The northern subspecies, Reithrvdontomy,r raviventn.r haliocoete.r, is found in Marin, Napa, Sonoma, Solano, and northern Contra Costa counues. The southern subspecies, R r. raviventri.r, occurs in San Mateo, Alameda, and Santa Clara counties, and in disjunct areas on the Marin peninsula. In the South Bay, populations of Kr. raviventrir primarily occur south of a imaginary line across the Bay from Redwood City to Hayward. The two subspecies show some differences in chromosome shape, which may indicate that "genetic isolating mechanisms are beginning to form benveen them" (Shellhammer, 2000). This report addresses the southem subspecies, Rr. raviventri.r. Description Salt mazsh harvest mice are among the smallest rodents in the U.S. T'heir bodies are 69 to 76 mm long (2.75 to 3 inches) and they weigh 8 to 12 grams (03 ounce), somewhat less than a nickel. The species name, Reithmdontomyr raviventrz.r, means groove-toothed mouse with a red belly (Shellhammer, 1998). Their backs are dark brown, their bellies and sides are pinkish-cinnamon to tawnj� (Cal/EPA DPR, on line). The mouse's tail can be bicolored and as long as or longer than the bodv. The Salt marsh harvest mouse is similar to the westem harvest mouse, but has darker ears and dorsal areas; tails that are thicker, less pointed, and more uniform in color; and frequendy darker bellies. R. r. raviventn.r has a cinnamon orange or rufous color on the venter. HBbitat. Salt marsh harvest mice require dense cover (LJSFWS-Sacramento). Preferred habitat is tidal and diked coastal salt marshes characterized by dense growth of pickleweed (Salicornia vir�inica). They are seldom found in cordgrass or alkali bulrush. Adjacent grasslands are also used by the mice, according to the Cal/EPA DPR, when new grass growth provides suitable cover. A Universit�� of Vermont Faxonomia report (Vermont 1996) describes preferred habitat for the mouse as having the following characteristics: • r�reas having 100 percent cover. • Depth of vegetation at summer maxunum of 30 — 50 cm. ■ Vegetation composed of 100 percent pickleweed to 50 percent pickleweed with other halophytes, pazticularly fat hen (Atnplexpatula) and alkali heath (Franklingrandfolia). ■ Few to no areas of salt grass (Dirticli.r .ipicata), brass buttons (Cotula coron opifolia), alkali bulrush, other Sczrpu.r species or Typha. • Large marsh areas. • No open ground or water bifurcating the vegetation. ' Minimal disruption. 10